Wk 2/3 - GI(metabolism:''''() Flashcards
ATP
nucleotide
- nitrogen containing base attached to ribose sugar and 3 phosphates
- 3rd Pi hydrolysed off to make energy and form ADP
standard free energy and actual free energy of ATP
standard - -31kj/mole
actual - 60kj/mole
how efficient is ATP usage
40%
- so actual energy gain is about 24kj/mole
NADP
NICOTINAMIDE ADENINE DINUCLEUTIDE PHOSPHATE
- Nicotinamide ring reduced to dihydro-nicotinamide
- A hydrogen carrier
- Exists in an oxidised state and a reduced state
- Carries 2 electrons from 2 hydrogens
role of NADP
- Currency of reducing power (it’s reduced)
Pathways that reduce it
- > Pentose phosphate pathway
- This is a Glucose metabolism pathway
- Glucose is partially oxidised liberating CO2
how is reduced NADP re-oxidised
via biosynthetic pathways…
- fatty acid and cholesterol synthesis
- DNA synthesis
glucose energy store
- Small amount circulating in plasma
- Glucose can be used by all tissue
- But small concentration of glucose (not a lot of energy stored in circulating glucose)
- Glycogen stores glucose
- This is stored in Liver and muscle
glycogen energy store
- It can be mobilised very quickly
- It can be metabolised anaerobically (don’t need to increase breathing rate as no CO2 produced)
Disadvantages
- It’s hydrated
- Weight is limited by the fact ¾ of the weight will be water
- So stores of glycogen relatively small
tricylglycerol energy store
- Highly reduced
- Very high energy yield
- Not hydrated so no weight penalty
- Largest energy store in the body
Disadvantage
- Since it’s fully reduced it needs O2 to be metabolised
protein energy store
- When broken down it can be converted to intermediates which can be metabolised
- Either used to yield glucose or ketone bodies
- Not a very high energy output per gram
Disadvantage…
- There isn’t any storage protein
- All the protein is functional
- When you break down protein you Lose function (enzyme or plasma protein etc)
glycogen breakdown
glucose phosphate
- metabolized in the glycolytic pathway
AEROBIC directly metabolized to pyruvate and further oxidised to acetyl CoA and further in the mitochondria
ANAEROBIC
pyruvate reduced to lactate
tricylglycerol breakdown
- Main storage of fat
- Lipolysis releases free fatty acids into plasma
- These are taken up by tissues – muscle, heart, kidney
- And oxidised in the beta-oxidation pathway to acetyl-coenzyme A
- Fatty acids can’t be taken up by the brain
- Except in starvation when conc. Of free fatty acids rise acetyl CoA can be diverted to ketone bodies
- These are then circulated and used by tissue –(muscle, heart etc AND BRAIN)
- Ketone body production only in liver
protein breakdown
- Broken down by proteolytic enzymes to release amino acids
- 20 amino acids in proteins
- Each diff. breakdown pathways
- Some can be converted to glucose, called GLUCOGENIC
- Others can’t so end up as acetyl CoA, called KETOGENIC
- During starvation they can inc. prod. Of ketone bodies
acetyl- CoA
many fuels end up as this
- it’s the principal fuel in the terminal oxidation pathway
- also known as tricarboxylic acid cycle
- in this cycle acid groups completely oxidised to CO2 which generates a lot of ATP but requires O2
properties of mobilised glucose
- Circulates in plasma
- Conc. Maintained in tight levels -> glucose homeostasis
- If it gets low the brain notices this (25% of energy spent on brain)
- If it gets high – dangerous as glucose reactive (reacts with proteins and vasculature etc)
- So when glucose rises after feeding its brought down quickly by storage
- In diabetes glucose can get high
properties of mobilised fatty acids
- Not very soluble
- In plasma mostly bound to albumin
- During the fed state – free fatty acid in plasma pretty low
- Rises quickly during fasting (overnight or prolonged)
- Never gets above 2mmol/L
- And turned over very quickly
properties of mobilised ketone bodies
- Only one of thems a ketone
- Acetoacetate and 3-hydroxy butyrate derived from acetyl-CoA from fatty acid breakdown
- Conc. Low at fed state and increase during fasting
- Circulate and used as fuel by heart, muscle, kidney etc AND BRAIN
- High levels of ketone bodies vv dangerous as they’re strong acids so cause acid imbalance METABOLIC ACIDAEMIA
properties of mobilised amino acids
- 20 diff acids in varying conc.
- Overall conc. Doesn’t change vv much but balance between them may during fasting
properties of mobilised lactate
- Prod. By eg muscle anaerobically oxidising glucose or in RBC
- Circulates at low levels
- Used to fuel lots of tissues
- Anaerobic muscle exercise inc. concentrations
different types of muscle fibres - what are they for
type 1 - aerobic energy prod
type 2 - anaerobic energy prod.
creatine phosphate
- Creatine phosphate is a small energy store within the muscle
- Muscle relatively high conc. Of ATP
- Creatine phosphate higher than this
- Creatine phosphate has an attached phosphate group that can be transferred to ADP – RELEASING CREATINE
- As ATP is hydrolysed to ADP by myosin ATPase it can be re-phosphorylated back to ATP
fuels for the 2 diff. types of muscle
purely anaerobic (type 2)
- muscle ATP
- creatine phosphate
- muscle glycogen
purely aerobic (type 1)
- ATP, creatine-P, glycogen
- fatty acids (muscle and adipose tissue)
- plasma glucose (from liver glycogen and gluconeogenesis)
what stimulates glycogen breakdown?
glucagon, adrenalin, increase conc. of AMP
what are the intermediates between glucose and lactose in glycolysis
- glucose broken down to glucose-1-phosphate
- glucose-1-phosphate isomerized to glucose-6-phosphate
- which is broken down anaerobically to pyruvate
pyruvate reduced to lactate
1 molecule of glucose…
- generates 2 ATP
- yields 2 lactate
what are the steps of glycolysis from pyruvate if there’s O2 present
- pyruvate enters mitochondria to be oxidised to acetyl-CoA
- acetyl-CoA enters the TCA cycle
- generates ~30 ATP
what happens to lactate
- Enters plasma and circulates
- In some tissues converted back to glucose via gluconeogenesis (mostly LIVER), also kidney
- Essentially a reversal of glycolytic pathway
- Glucose then released into plasma and used by the muscle
- Requires 6ATP
distribution of fuels used during exercise
Start of exercise
- Most energy from muscle stores
- These are quickly depleted
then
- Energy supply taken over by non-esterified fatty acids in the plasma
- From mobilization of fat in adipose tissue
Then
- Plasma glucose plays increasing part
- From breakdown of liver glycogen and gluconeogenesis
Prolonged exercise
- NEFA principle fuel
- Ceiling of about 70—75% unclear why
- How quickly fatty acids can be transported???
control of metabolism during exercise
- During exercise ATP -> ADP
- Topped up by creatine phosphate
- This runs out
- ADP further utilised by a dismutation reaction catalysed by enzyme called adenylate kinase or myokinase to give AMP and ATP
- Essentially ATP broken down into ADP and then AMP
adenine nucleotide concentrations during exercise
- During exercise although ATP is used up the product is AMP
Consequence of this…
- AMP acts as a control feature
- Through the enzyme AMP-activated protein kinase
A lot of metabolic pathways controlled by phosphorylation/ dephosphorylation of regulatory enzymes
- Enzyme that transfers a phosphate from ATP to something else is a KINASE
- Other hormonal signals can activate phosphatase’s which remove the phosphates
AMP-protein kinase functions
- controls kinases
- also phosphorylates lots of regulatory enzymes
an inc. in AMP-protein kinase activates processes such as…
- glucose uptake
- glycolysis
- fatty acid oxidation
- mitochondrial biogenesis
these processes are needed during energy demand
- inhibits processes that happen normally during feeding
net protein utilization (NPU)
- Quantifies nutritional value of protein
- Fractional incorporation of amino acids into body protein
- A measure of ability of protein to sustain growth
Def.
(Wt. amino acids incorporated into protein) divided by (wt. amino acids supplied in diet)
Vegetables tend to have lower NPU
- Veg doesn’t have lower protein content
- But veg has diff. protein values
2 main types of amino acids
essential - cannot be synthesised within the body do must be supplied in the diet
non-essential - can be synthesised from other amino acids in the diet
list of essential amino acids
- Isoleucine
- Leucine
- Lysine
- Methionine
- Phenylalanine
- Threonine
- Tryptophan
- Valine
Some are a combo
- Arginine
- Histidine
what does it mean if a protein has a high NPU
it’s high in essential amino acids
kwashiorkor protein malnutrition
- Children having a body weight between 60-80% of that expected of that age
- Protein mal. Effects children more as they have high protein requirement
- Total energy intake is adequate but not enough protein
Leads to swollen bellies
- Oedema due to albumin deficiency
- Enlarged liver
- Muscle wasting
- Diarrhoea
marasmus protein deficiency
- More severe
- Body weight less than 60% of expected value
- Protein and calorie deficiency
degradation of extracellular proteins
- If become damaged they are degraded by endocytosis…
- Damaged cell binds to receptor
- Cell and receptor are Internalised (endocytosis)
- Form a closed compartment in the cell (endosome)
- Has a membrane with protein inside the membrasome
- Membrasome then fuse with lysosomes (degrative)
- Lysosomes have low ph
- Contain degradative enzymes
- Degrade captured protein
- This liberates amino acids inside the cell
degradation of intracellular proteins
- Recognised for degradation and tagged by attachment at c terminal by several molecules of UBIQUITON
- Label for degradation
- Degradation carried out by proteosome
- ATP dependant
- Protein enters cavity in proteosome and is degraded to release amino acids
- Ubiquitin is spared and reused
degradation of dietary proteins
- Degraded in digestive tract
- Starting in the stomach
- GASTRIC PIT
cells of the gastric pit
- Chief cells -. Produce degradative enzymes
- Parietal cells – lower ph in gastric lumen
- Mucus cells
parietal cells structure - apical side/ gastric lumen
so in the gastric lumen…
- high protein conc.
- ph 0.8
- acidified
gastric ATPase
- a proton pump that hydrolyses ATP
- pumps protons into gastric lumen in exchange for potassium ions
- makes the gastric lumen really acidic
parietal cells structure - basolateral side/ plasma
so on the plasma side…
- low protein conc.
- ph 7.4
- alkalified
anion transporter
- protons come from carbonic acid (formed by hydration of CO2)
- carbonic acid ionised to bicarbonate
- bicarbonate is v alkaline so its exported into plasma in exchange for chloride ions
drugs that affect parietal cells in the gastric pit
omeprazole and vonoprazan
- inhibit gastric ATPase
- used to treat gastric ulcers
why is the stomach acidified?
inhibits bacteria growth
denatures dietary proteins
- unfolded
- more easily hydrolysed by proteolytic enzymes in digestive tract
not essential though
- if secretion of hydrofluoric acid fails not a lot of consequences
- except vitamin B12 is uptake by intrinsic factor
- if don’t have a stomach you need injections of B12
what uptakes vitamin B12
intrinsic factor
whats the principal degrading enzyme in the stomach
pepsin
pepsin
- secreted by chief cells
- synthesis of degradative enzyme is hazardous for cell
- bc/ if enzyme was active it would degrade cell itself
- so enzymes are secreted as inactive precursors called ZYMOGENS
the excretion of pepsin
- Synthesis begins with protein made in cytoplasm -> lumen of endoplasmic reticulum -> golgi apparatus -> secretary vesicles
- Contained within vesicles
- Under appropriate stimulus these vesicles fuse with plasma membrane of cells and release pepsinogen into lumen of stomach
This is called REGULATED EXOCYTOSIS - HCl turns pepsinogen into pepsin activating it
how is pepsinogen activated
- when it encounters hydrogen ions in the stomach (low pH)
- causes spontaneous breaking of bond which changes structure of pepsinogen (activates it)
- pepsin can catalyse this process - AUTOCATALYSIS
function of pepsin
- Recognises large hydrophobic amino acid
- Doesn’t hydrolyse every bond in protein
- Pepsin degrades to fairly large peptides
- These are then further degraded by other enzymes
next stage of degradation after stomach/ pepsin
- further down the digestive tract
- content of stomach neutralised by pancreatic secretion
- pancreas secretes high levels of bicarbonate which lowers the pH
- contains a series of proteinases
- also degradative enzymes that work on fat and carbohydrate
cascade of activation of pancreatic proteases
like pepsin they’re secreted as inactive precursors
activation cascade…
- TRYPSIN IS KEY
- trypsinogen and enteropeptidase secreted into digestive tract
- enteropeptidase activates trypsinogen (changes to trypsin) and other proteinase precursors
- activation cascade triggered too early - degradation of cells (PANCREATITIS)
what inhibits pancreatic proteinases?
trypsin inhibitor
- binds to active trypsinogen to deactivate it
so first molecule of trypsinogen are deactivated
- later on inhibition swamped by activation and trypsin made
outcome of protein digestion
- Amino acids taken up into intestinal cells together with sodium ion
- By co-transporters which recognise diff. amino acids
- If peptides left over further degraded by peptonises or taken up and degraded by cells
- Amino acids leave cells and enter blood stream
- Free amino acids in the blood transported around and taken up by cells for protein synthesis
first stage of amino acid metabolism
transamination
- Catalysed by transaminases
- Amine group NH3 is transferred from amino acid onto oxoglutarate
- So amino acid is turned into an oxo-acid
- This carbon backbone/ oxo-acid is further metabolised (eg to glucose or fatty acids)
- amine group transferred to a diff. amino acid – GLUTAMATE
(so amino group removed and is present in glutamate)
stage 2 of amino acid metabolism
oxidative deamination
- Amino group removed from glutamate by glutamate dehydrogenase
- Oxidises the glutamate (by reduction of coenzyme NAD)
- Amino group liberated as ammonia
- In this process NAD is reduced to NADH
- ammonia enters urea cycle in the liver = urea
ammonia features
- Ammonia is toxic to central nervous system
- If ammonia builds up reaction stops
- Ammonia inhibits the TCA cycle
- So its important to remove the ammonia (removed as urea)
hows are amino acids excreted through urea?
Then excreted through urea as follows…
- Transamination reaction to form glutamate
- Glutamate can release its ammonia
- Make carbamyl phosphate
- Carbamyl phosphate incorporated into urea cycle
- Urea cycle only in liver – partly in mitochondria, and cytoplasm
what does ammonia form upon reaction with CO2
carbamyl phosphate
what happens when carbamyl phosphate reacts with ornithine in the urea cycle
- carbamyl phosphate transfers its amino group onto it to form citrulline
- Citrulline can receive another amino group from aspartate and cycle continues etc etc
where does carbamyl phosphate enter into the urea cycle?
- between ornithine and citrulline
how is amino acid transported from peripheral tissues to the liver
- it’s not glutamate that’s transported – it’s ALANINE
- Glutamate formed by transamination
- Can be transaminated with pyruvate to form alanine
- Alanine can then enter the plasma and circulate
- Be taken up by the liver
- And back-transaminated to glutamate
(a device for carrying amino groups from glutamate in tissues to glutamate in the liver) - So conc. Of alanine in the plasma rises when there’s peripheral amino acid breakdown
- Particularly during starvation
properties of urea
- Very soluble in water
- Electrically neutral – neither acidic nor base
- Contains 48% N by weight (protein contains ~16%)
- Synthesised in liver, not further metabolised
- Normal plasma conc. 2.5-7 mmol/l
- rises renal failure (uraemia)
- falls in liver cirrhosis (or deficiencies in the urea cycle enzymes)
- ammonia toxic to central nervous system
other routes of ammonia secretion (other than urea)
- directly through the kidneys
- in tissues ammonia liberated by glutamate dehydrogenase
- glutamine can transfer through the plasma to the kidneys
- where glutaminase takes the amino-group off again (or amido group) to form ammonia
- liberated directly into the urine through the kidneys
- system is important during starvation
- no intermediate formation of urea
oxo-acids
- if oxo-acid broken down into acetyl-CoA amino acid is ketogenic
- if oxo-acid… into TCA cycle intermediate it’s glucogenic
- bc/ it can be converted back to glucose
one-carbon metabolism
- glycine is not glucogenic or ketogenic it’s part of the one carbon pathway
One carbon pathway - single carbon fragments from amino acids are broken down and metabolised
- glycine, serine, histidine, tryptophan all contributes single carbons
- metabolized by attaching to tetrahydro-folic acid
- folic acid is a vitamin supplied by diet or bacteria
- reduced twice to be used as a co-enzyme to…
- dihydrofloric acid
- tetrahydrofolic acid
-tetrahydrofolic acid can be oxidised and reduced to methylene-tetrahedrafolic acid (further reduced to methyl-tetrahedrafolic acid)
one-carbon pathway important in synthesis of..
- purine (carbons come from one carbon metabolism)
- thymine (methyl group from…)
- methionine (methyl group from…)
so, essential in…
- DNA/RNA synthesis
- mitochondria synthesis
- etc
why is one-carbon metabolism dependant on B12?
- Vitamin B12 transfers methyl group onto methionine
- So if B12 deficiency then methyl-tetrahydrofolate accumulates
- Whole pathway seizes up
- In anaemia, cells in stomach fail to secrete intrinsic factor in stomach and vit. B12 not absorbed
- Overall effects DNA replication particularly in rapidly dividing cells
dihydrofolate reductase inhibitors\
- called anti-folate drugs
CYCLOGUANIL is an inhibitor of dihydrofolate reductase
- Component of antimalarial drug malerone
Methotrexate
- Anti-tumour drug
- Analogue of folic acid
- Inhibits dihydrofolate reductase
SULFONAMIDES (antibiotic)
- Works in bacteria
- Vv simple
- Sulfonamides mimic nitrogen groups where one-carbon atoms attach
- And so acts as an inhibitor of folic acid synthesis in bacteria
carbon links between 2 sugars can be…
below the plain of the ring -> alpha
above the plain of the ring -> beta
starch
- Major form of carb in the diet
- Principle storage polysaccharide in plants
- Glucose units joined together
2 components within starch…
Amylose
- 10-20%
- Formed by linking glucose units between carbon 1 and 4
- Alpha 1-4 link
- Straight chains
Amylopectin
- 80-90%
- Only glucose
- Straight chains with alpha 1-4 links
- Also contains branches
- Carbon 1 in one chain links to carbon 6 of another
enzymes in starch digestion
- alpha-amylase
- glucoamylase
- isomaltose
alpha-amylase
- In saliva (levels variable)
- Also secreted in the pancreas
- Hunter gatherers have lower levels than agriculture populations (evolutionary)
- Endoglycosidase – hydrolyses a(1-4) links
- Products are oligosaccharides
- A few sugars linked together
- Short and straight or branches
glucoamylase
- Present on luminal side of intestine wall
Deglycosylase -> hydrolyses a(1-4) links in…
- Oligosaccharides
- Trisaccharide’s
- Maltose
- Not alpha 1-6 links
isomaltose
- Present on luminal side of intestinal wall
- Hydrolyses a(1-6) link in maltose
what are the products of starch degeneration
maltose and isomaltose
maltose
- alpha 1-4 links -> hydrolysed by glucoamylase
- alpha 1-6 links -> hydrolysed by isomaltase
lactose
- contains galactose linked beta 1-4 to glucose
- won’t be hydralysed by amylase
- hydrolyzed by intestinal enzyme -> lactase (/ beta-galactosidase)
- hydrolyses lactose to galactose and glucose
lactose intolerance
- caused by low levels of lactase
- widespread natural sit.
- Lactose is a mil disaccharide
- Lactase expressed highly in young children but is normally shut down in adulthood
- So adults usually have low levels of lactase
- And so if adults drink milk etc it isn’t digested by small intestine, Instead ingested by large intestine by bacteria
- Causing gas, digestive problems, diarrhoea
- If adults are able to eat lactose, it’s due to a mutation
sucrose
- Contains glucose and fructose, linked alpha 1-2
digested by SUCRASE – intestinal enzyme
- Same complex as isomaltose – sucrase/isomaltase
- Quite large component of many diets
- Interest in artificial sweeteners which mimic sucrose
SUCRALOSE
- artificial sweetener
- Hydroxyl groups replaced by chloride
- Molecule cannot be hydrolysed by sucrase
- It’s excreted
- But tastes sweet
SACRASE breaks sucrose into glucose and fructose and these are individually taken up
- If there’s sucrose in the blood that’s a bad sign
- Maybe a stomach ulcer?
non hydrolyzable polysaccharides
- Roughage
- Plant polysaccharides so not attacked by mammalian digestive enzymes
- Degraded by bacterial enzymes a bit
active transport of glucose into the intestine
- Creation of sodium ion gradient by ATPase
- Gradient of sodium between intestine and inside of cell
- Source of energy for uptake of glucose
- 1 glucose and 2 sodium
- Sodium running down conc. Grad
- Glucose runs up
then. ..
- Glucose can run downhill to lower conc. in plasma
- Through uniporter – GLUT 2
oral rehydration therapy
- Combating loss of water by inc. conc. Of sodium ions in the body
- By exploiting the glucose transporter
- So use glucose and salt mixture
- Glucose promotes sodium ion uptake
- Which expands the plasma and retrieves water
levels of glucose after feeding
- glucose quickly rises following a meal
- this rise is detected by the endocrine pancreas -> secretes INSULIN
insulin rises very sharply and promotes uptake of glucose into…
- fat
- muscle
- insulin affects glucose utilization in the liver
- doesn’t directly impact glucose transport into the liver
increase in insulin also lowers the conc. Of NEFA (non-esterified fatty-acids) in the plasma
- because it inhibits lipolysis
- after feeding fatty acids dec. then gradually inc. As digestions completed
once glucose is in the plasma…
- circulates and is taken up by various types of cell
- these cells don’t have to carry out active transport
- as glucose conc. in plasma is higher than in the cell
- therefore, glucose runs straight in (passively)
types of glucose transporters
GLUT1 in body tissues
- Km value of 5 millimolar
- Km value = conc. To give half the maximum rate of transport
GLUT2 in liver, kidney, intestine, pancreas
- higher Km
- so, it keeps transporting glucose at high levels of plasma glucose
- continuous transport into cells as plasma conc. Inc.
GLUT3 in brain
- low Km
- close to saturation whatever the glucose plasma conc.
- Therefore glucose uptake in brain is pretty much continuous
- Good bc/ wouldn’t want brain to be dependent on rate of glucose production by digestion
GLUT4 in muscle adipose and heart
- Insulin responsive
- Insulin inc. amount of GLUT4 in these tissues
- Responsible for insulin dependent uptake after feeding
- (feeding inc. plasma glucose and insulin conc, and insulin inc. uptake of glucose)
SGLT1 in duodenum, jejunum, kidney
- Taking glucose up from extracellular space into the body
what happens to glucose once it’s taken up by liver cells
- Have an Uptake system designed to cope with high conc. Of glucose
- (Glucose up-taken in intestine and first tissue it meets through portal system is the liver)
Firstly
- PHOSPHORYLATED
- Phosphorylated on carbon 6 by phosphate on ATP
- Forms glucose-6-phosphate (starting point for metabolism)
- All tissues contain HEXOKINASE which catalyses this reaction
- Hexokinase has vv low Km for glucose
- Liver contains GLUCOKINASE
- Higher Km
- Unusual kinetics -sigmoidal saturation curve
- Can continue to work at vv high glucose conc
the metabolic fates of glucose
Converted to glycogen
- Involves isomerization of glucose-6-phosphate to glucose-1-phosphate
- Transfer reaction with UTP to make UDP-glucose (glucose attached to UDP)
- UDP-glucose is the precursor of glucose in glycogen
Glycolytic breakdown (glycolysis) - Glucose-6-phosphate converted to pyruvate or lactate (aerobic or anaerobic)
Pentose-phosphate pathway
- Generates 5 carbon sugars and reduced co-enzymes
glycogen
- A polymer of glucose
- Contains only glucose
- Same structure as starch
- Glucose chains linked alpha 1-4 (some alpha 1-6)
- End with free carbon-1 -> reducing end
- End with carbon-4 free -> non-reducing end
- Because of structure only one reducing end and then 1,6 branches and lots of non-reducing ends
- Non-reducing ends are where glucose is hydrolysed off or added
- Non-reducing end attached to a protein (GLYCOGENIN)
- Lots of branches (up to 12 layers) forming a big particle with a 40 nanometre diameter
- Contains ~50,000 glucose units
- ~2000 non-reducing ends so glucose can be added or hydrolysed off
what does glycogen synthase do
transfers glucose from UDP-glucose to glycogen
- activated by insulin
- insulin promotes glycogen synthesis in the liver and muscle
glycogen phosphorylase
- Activated by various hormones
- Adrenalin in muscle
- Glucagon in liver
- So these 2 inhibit glycogen synthesis and promote glycogen breakdown
Inhibited by INSULIN
- So insulin stimulates glycogen synthesis and inhibits glycogen breakdown
- Enzymes controlled by phosphorylation and dephosphorylation reactions
- Eg so adrenalin promote phosphorylation of glycogen phosphorylase (making it active)
glycolysis
starts as glucose 6-phosphate
phosphorylated to fructose bisphosphate
- (catalysed by phosphofructokinase) -> important control step in the pathway
Fructose bisphosphate split into 2 3-carbon phosphor-sugars
- Each of these oxidised and can generate 2 ATP
- Catalysed glyceraldehyde phosphate dehydrogenase
- Reduces NAD to NADH
- Incorporates phosphate into intermediate -> bisphosphoglycerate
- Which can transfer a phosphate to ADP and form ATP
Further along on pathway is intermediate phosphoenolpyruvate
- Which can be converted to pyruvate to generate ATP
balance of ATP in anaerobic glycolysis
- Each 3-carbon fragment produces 2 ATP
- So a glucose molecule produces 4 ATP
- Need 1 ATP for phosphofructokinase reaction
So if glucose comes from glycogen…
- No ATP needed to breakdown glycogen
- Consumes 1 ATP
- Overall net balance 3 ATP per glucose
If it comes from free glucose
- Another ATP is required for hexokinase/ glucokinase reaction
- So 2 ATP consumed, 4 produced
- Overall net gain 2 ATP per glucoseq
what happens to pyruvate
end-product of glycolysis
- pyruvate may be reduced to lactate
- by reoxidation by reduced NAD generated earlier in the pathway
- becomes self-sufficient de-mutation reaction
- > glucose converted to 2 lactates
- > with production of 2 ATPs
- pyruvate may also enter the mitochondria and be metabolised to CO2
- produces a lot more ATP
anaerobic glycolysis
Anaerobic conversion of glucose to lactate is important in muscle during anaerobic exercise
- So ATP can be generated quickly without inc. oxygen
- Also important in cells eg RBC that have no other kind of ATP production
gluconeogenesis
- Only takes place in the liver and kidney
- Replenishes plasma glucose from use of other precursors
- Brain has large daily demand of glucose
How does gluconeogenesis produce glucose?
- Pyruvate -> oxaloacetate -> phosphoenolpyruvate
- Consumes one ATP and one GTP (essentially 2 ATPs)
- The 2 phosphorylation reactions are also reversible
- Phosphate hydrolysed off
- Therefore, entire pathway from pyruvate to glucose is reversible
Except step from pyruvate to acetyl CoA is irreversible
- Acetyl coA cannot be converted to glucose (so fat can’t be converted to glucose)
precursors of glucose
- Lactate which comes from anaerobic glucose metabolism
- Glycerol from breakdown of triacylglycerol
- Gluconeogenic amino acids (during protein degradation or during starvation) Converted to intermediates eg pyruvate, oxaloacetate etc which can be converted to glucose
controls of gluconeogenesis
Hormonally
- Inhibited by insulin
- Promoted by glucagon
Glucagon inhibits forward action at pyruvate kinase
- Promotes fructose bisphosphotase
pentose-phosphate pathway background info
- Present in many types of cell
- Particularly Present in cells where fat/cholesterol synthesis are going on
- Liver, adipose tissue, mammary gland etc, RBC
- Produces reduced co-enzyme NADPH
- Functions in reductive biosynthetic pathways
- And anti-oxidative function
- Present in cytoplasm
pentose-phosphate pathway steps
- Glucose 6-phosphate oxidised to 6-phosphategluconate
(with the reduction of NADP to NADPH)
6-phosphategluconate oxidised by diff. dehydrogenase
- Produces 5-carbon sugar RIBULOSE 5-PHOSPHATE
- Loss of carbon dioxide
2 molecules of reduced NADP per glucose are produced
Ribulose 5-phosphate can be isomerised to other sugars
- It’s needed for DNA and RNA synthesis
Carbons are shuffled around to regenerate 6-carbon sugars and feed into beginning of pathway
Done through…
- TRANSKETOLASE -> takes 2 carbons from one sugar and adds them to another
- TRANSALDENASE -> moves 3 carbons
pentose-phosphate pathway products/ actions
Reduced NADP required for
- Reductive synthesis eg fat synthesis, cholesterol synthesis
Anti-oxidative
- In RBC lipid peroxidation is a hazard
- Lipids oxidised to peroxides by reaction with oxygen
peroxides are reduced by a tripeptide called GLUTATHIONE…
- this reduces peroxides to hydroxy compounds
- glutathione becomes oxidises to a disulphide
- the disulphide is reduced back to glutathione by reduced NADP
- so important function of ^ pathway in RBC is to protect against oxidative damage
glucose 6-phosphate dehydrogenase deficiency
- widespread deficiency of glucose 6-phosphate dehydrogenase
- most common genetic disorder in humans
- x-linked (so males effected more)
- in mutations glucose 6-phosphate dehydrogenase has LOW activity
- so haemoglobin gets cross linked and RBC undergone haemolysis
- exacerbated by drugs etc antibiotic and broad beans
- name of this deficiency if FAVISM
metabolism of fructose
- from hydrolysis of sucrose
- sucrose isn’t taken up in intestine, but fructose is
- fructose can be taken up by cells
- rarely phosphorylated by hexokinase and straight into glycolytic pathway
- but glucose competes and so not vv. much
- in liver there’s a special enzyme for fructose phosphorylation -> FRUCTOKINASE
- phosphorylates fructose to fructose 1 - phosphate
- then metabolized by glycolysis
- 3-carbon sugars fed into glycolytic pathway
- Fructose escapes control on glycolytic pathway
- Which is why high fructose diets lead to fat
deficiency of fructose 1-phosphate aldolase
- Build-up of fructose 1-phosphate
- Leading to fructose intolerance
So inhibitions of… - Glycogen breakdown
- Gluconeogenesis
- Oxidative phosphorylation
Resulting in… - Hypoglycaemia and lactic acidaemia
metabolism of galactose
- Metabolised in liver by galactokinase
- Which phosphorylates galactose to galactose 1-phosphate
- Which is then epimerized to glucose 1-phosphate
- By first being transferred to UDP
- And then an epimerase acting on UDP-galactose
- Converting it to UDP-glucose
alcohol metabolism
- Most ethanol oxidation goes on in the liver
- Ethanol oxidised by NAD to ethanal
- Ethanal oxidised to ethanoic acid
- This is convertible to acetyl CoA
- Precursor of fatty acids or ketones
- Or oxidised in TCA cycle
aldehyde dehydrogenase
2 isoforms of this enzyme
- 1 in mitochondria (low Km value)
- 1 in cytoplasm (high Km value)
- Widespread mutations in mitochondrial one
- Inactivates it
- Means alcohol doesn’t oxidate all the way to ethanoic acid
- results in build-up of high conc. Of ethanal
- side effects of nausea etc…
- ALCOHOL INTOLERANCE
disulfiram/ antabuse
- Used to inhibit aldehyde dehydrogenase
- Used as aversion therapy to treat alcoholism
- Bc if you drink on this ethanal builds up leading to discomfort
other alcohols metabolised
- Alcohol dehydrogenase is a non-specific enzyme
- Oxidise a lot of alcohol (some toxic)
Eg methanol
- yields formaldehyde
- Reactive with nucleic acids etc – DAMAGE
Eg ethylene glycol (found in antifreeze)
- Oxidised to glyoxal and onto oxalic acid
- Oxalic acid precipitates calcium in the kidneys – DAMAGE
Metabolism of toxic alcohols can be inhibited
- By infusing ethanol to act as a competitive substrate
- Or drug called fomepizole (inhibits it)
types of double bonds in fatty acids
- Exist as cis or trans isomers
- Naturally occurring ones (made in body) are always cis
- products can contain trans ones (these may be bad for you)
fatty acid properties and storage
- Fatty acids are insoluble in water
- But they’re membrane permeant
- Stored as esters with glycerol – TRIACYLGLYEROLS
tricylglycerols
- Main storage form of fat
- 95% of fat in diet is triacylglycerol’s (as non-esterified fatty acids taste bad)
- At temp. of body they’re liquid
- Digestion begins in intestine
- But in stomach they’re transferred into lipid droplets
digestion of tricylglycerols
- In stomach digested by pancreatic enzyme -> pancreatic lipase
- the droplets have to be further broken down
- Broken down by BILE SALTS
- action of pancreatic lipase releases 2 fatty acids
- Leaving one in the middle – that’s monoacylglycerol
- both of these taken up by the intestine
bile salts
- Synthesised in liver
- Stored in the gallbladder
- Reach digestive tract through the bile duct
- Detergents are amphipathic in structure (hydrophilic on outside and hydrophobic on inside)
inhibitor of pancreatic lipase
orlistat
- inhibits triacylglycerol breakdown
- useful in treatment of weight
digestion of tricylglycerol
- by pancreatic lipase to fatty acids and monoacylglycerol
- these are taken up by intestinal cells
- 10% pass through portal system to liver
- Majority of them are re-esterified to triacylglycerol and transported around the body as particles called chylomicrons
chylomicrons contain
- Triacylglycerides
- Phospholipids
- Proteins
- Cholesterol
- Fat-soluble vitamins – A, D, E, K
secreted from intestinal cells via lymphatic system
- enter blood via thoracic duct
- chylomicrons are a member of plasma lipoproteins
chylomicrons general info
secreted from intestinal cells via lymphatic system
- enter blood via thoracic duct
- chylomicrons are a member of plasma lipoproteins
- hydrophobic at centre
- hydrophilic pn outside
circulate in the plasma
- about 500nm in diameter
- formed only in the intestine
- rapidly broken down
- half-life 5 mins
degradation of chylomicrons
- Picks up apoprotein C2
- This activates the enzyme that degrades them -> LIPOPROTEIN LIPASE
- This enzyme exposed on outside of endothelial cells
- It hydrolyses the triacyclglycerols in chylomicrons all the way to free fatty acids and glycerol
Free fatty acids taken up by cells
- Glycerol remains in plasma and taken up and metabolised by liver
- Chylomicrons converted to remnants containing cholesterol and fat soluble vits.
- Taken up by liver
synthesis of fatty acids
First reaction catalysed by acetyl-CoA carboxylase
- Important control point for synthesis and degradation
- Acetyl CoA is carboxylated to malonyl-CoA
- CO2 from bicarbonate
- Require ATP
- Intermediate is the enzyme with a bound CO2
- Bound to prosthetic group Biotin (a vitamin)
regulation of acetyl-CoA carboxylase
- Exists in active and inactive forms
- The inactive form is a phosphorylated form
activated by…
- insulin
- high blood glucose
- high ATP production (so body can store molecules for later)
inactivated by…
- glucagon
- AMP-actiavted protein kinase (inc. AMP)
- Long-chain fatty acyl-CoA
fatty acid synthase pathway
1) Priming -> attaching acetyl group from acetyl-CoA to a cysteine sidechain (acyl-carrier protein) of fatty acid synthase
2) Loading -> transfer of a malonyl group from malonyl-CoA onto another sulfhydryl group (a vitamin prosthetic group on the enzyme)
- Enzyme now loaded with an acetyl group and a malonyl group attached as vio-ester
3) Next reaction is a condensation reaction
- Acetyl group transferred onto the malonyl group
- CO2 is lost which drives the equilibrium of the reaction
- Left with a 4 carbon oxo-acid attached to phosphopantetheine
4) Carbonyl group reduced in 2 steps (eliminating water)
- 1-> firstly to a hydroxy acid
- 2-> 4-carbon saturated fatty acid
- Reduction carried out by co-enzyme NADP (generated by pentose-phosphate pathways)
5) 4-carbon fatty acid elongated
- Transferred back to initial cysteine
- Enzyme reloaded with another malonyl-CoA
- Process repeated
fatty acid synthase complex can convert acetyl CoA to…
- Long-chain fatty acids which are saturated (even no.s of carbons)
- End product is 16-carbon saturated fatty acid
- Separate systems can elongate these further
regulation of malonyl-CoA to fatty acids (from acetyl-CoA)
Insulin
- Acetyl-CoA likely to come from glucose
- uptake of glucose into fat and muscle controlled by insulin
Glucagon
- inhibits glucose oxidation to pyruvate in the glycolytic pathway
- (pyruvate enters mitochondria for oxidation to acetyl-CoA)
Malonyl-CoA
- inhibits uptake of fatty-acyl CoA into mitochondria
- so inhibits fatty acid oxidation
de-saturation of fatty acids
- Fatty acid synthase system makes saturated fatty acids
- Formation of double bond involves the removal of 2 hydrogens
- (These are used to reduce oxygen to water)
- Other oxygen atom is reduced by reduced NAD
- (So that desaturation requires enzymes, oxygen and reduced NAD)
4 different desaturases - where do they insert bonds
- Which introduce double bonds at carbons 4,5,6 or 9
- Insertion of double bond starts at carbon 9
- Other desaturases work closer to carboxyl groups
- Introduction of double bonds past the carbon 9-10 junctions isn’t possible
- This means some acids that we require which have double bonds at places other than 9 are ESSENTIAL
- We can’t make them so they’re important in the diet
examples of essential fatty acids
Linolenic which is a precursor for arachidonic acid
- which makes important molecules e.g. prostaglandins
omega-6 fatty acids
- typically present in vegetable oils e.g. sunflower oil
- recommended input is ~10 grams per day
- big reservoir of fatty acid in the adipose tissue
- so that if it fails in the diet
- deficiency takes a while to develop
mobilization of tricylglycerol
- triacylglycerol stored in adipose tissue is hydrolysed by hormone sensitive lipase
- pancreatic lipase in digestive system
- lipoprotein lipase digests chylomicrons in the circulation
intracellular lipase activated by… - adrenalin - glucagon - growth hormone inhibited by… - insulin method… - takes one fatty acid of triacylglycerol - other lipases break it down further - eventual product is glycerol + free fatty acids
free fatty acids
- Low solubility
- Capable of diffusing through biological membrane
- Doesn’t happen fast enough for rate of metabolism required
- Transported bound to serum albumin
- Total conc. Of free fatty acid rises to 0.6mmol/L during starvation
- Very rapid turnover
- Half-life is a few minutes
- Enter cells with the aid of transporters
fatty acid transport into cells - what do they attach to
- Once they’re in cells to prevent leakage they’re attached to coenzyme A
- Sequestered by protein called fatty-acid binding protein
- Then co-enzyme A is attached in a process called fatty acid activation
- (Requires ATP)
- So fatty acids are taken up by cells and become acyl-CoA
- (Long chain fatty acids esterified onto coenzyme A)
cholesterol
- 27 carbons
- Many functions
- Component of membranes
- Precursor for bile acids and bile salts
- Precursor of steroid hormones
- Biosynthesis occurs in the liver mostly
- Starts with acetyl CoA
cholesterol synthesis regulated by
Level of HMG-CoA reductase
- Biologically regulator of this is AMP-activated protein kinase
- Phosphorylation inhibits this
- No. of drugs used to inhibit HMG-CoA reductase
- Statins
intermediates in cholesterol synthesis
acetyl-CoA acetoacetyl-CoA HMG-CoA Mevalonate Squalene cholesterol
production of bile acids
- Cholesterol is hydrophobic
- Bile salt Has to be made more hydrophilic to be used as a detergent
First step in this…
- converted to 7 hydroxycholesterol by 7-alpha hydroxylase
- Further hydroxylation’s and shortening of side chain
- makes cholic acid and chenodeoxycholic acid (primary bile acids)
at this point they are hydrophilic on one side and hydrophobic on other side
- then conjugated with glycine and taurine and go on to make secondary bile acids
what are the 2 primary bile acids
Cholate and chenodeoxycholate
what are the 2 amino acids that attach to the 2 primary bile acids
taurine and glycine
- this forms esters with the side chain = BILE SALTS
the 4 primary bile salts info
4 bile salts formed by addition of sidechains to cholate and chenodeoxycholate
- 2 corresponding to each
- They are stored in the gall bladder
- Bile salts enter digestive tract -> intestine
In intestine meet bacteria which modify bile salts
- Hydrolyse to conjugated amino acids
- Reduce them
2 more compounds formed -> LITHOCHOLATE & DEOXYCHOLATE
- Called secondary bile acids
what does bile contain
bile salts
free cholesterol
drugs used to lower plasma cholesterol
Cholestyramine
- A resin which absorbs bile acids
- Traps bile acids in the intestines
- Can’t be reabsorbed and so is excreted
Plant sterols
- Inhibit the uptake of cholesterol in the intestine
- Eg benecol in synthetic margarine
- Lower level of chol. In plasma
Statins
- Inhibit de-novo cholesterol synthesis
- Inhibit HMG-CoA reductase
- But… when chol. Levels start to fall HMG-CoA reductase is upregulated
- (body attempts to overcome statin)
- Statin also upregulates receptors for LDL
- So rate of uptake of cho. From plasma into tissues also inc.
- Red. Of 30-60% in plasma chol. Levels
Fibrates
- Lower triacylglycerol levels in the plasma
- Effect cholesterol levels
- Inc. rate of uptake of LDL into the liver
transport of fatty acids into mitochondria by carnitine
Outer membrane of mitochondria isn’t a permeability barrier
- Contains protein pores – porins
- So acyl-CoA can easily enter space between the membranes
- Cannot cross inner membrane
- Its impermeable to
- So acyl group transferred from acyl-CoA onto diff. molecule -> CARNITINE
- Catalysed by enzyme on outer membrane -> CARNITINE PALMITOYL TRANSFERASE 1
Acyl- carnitine then imported through inner membrane in exchange for free carnitine
- Once inside acyl group transferred from carnitine back onto co-enzyme
inhibition of transfer of acyl-CoA onto carnitine
This transport step is the rate limiting step in fatty acid oxidation
- Transfer for acyl group catalysed by enzyme that’s inhibited by MALONYL-COA
- Malonyl-CoA is synthesized by carboxylation of acetyl-CoA
- Process under hormonal control
- Malonyl-CoA is a precursor for fatty acid synthesis and inhibitor of fatty acid oxidation
pyruvate dehydrogenase complex
- it’s intra-mitochondrial (so pyruvate must enter mitochondria via a transporter)
Overall reaction of this enzyme is the oxidative decarboxylation of pyruvate
- Oxidised to an acetyl group with loss of CO2
- NAD is reduced to NADH
- Acetyl group formed is esterified onto co-enzyme A
- Making acetyl-CoA
subunit prosthetic groups of pyruvate dehydrogenase
Pyruvate dehydrogenase subunit
- Thiamine pyrophosphate (vit. B1)
- Catalyses 1st step
Transacetylase
- Lipoic acid (a prosthetic group)
Dehydrogenase – which reoxidises the enzyme
- Flavin adenine dinucleotide (vit B2)
thiamine deficiency
Thiamine deficiency occurs as a dietary deficiency called beri-beri, and wernickes neurological sympyomd
Also common in alcoholics as alcohol inhibits uptake of vitamin B1 and its processing into thiamine pyrophosphate
Symptoms include
- Tremor
- Paralysis
Alcoholism also induces hypoglycaemia crises
- As it inhibits gluconeogenesis
- If alcohol hypoglycaemia its nec. To check vit. B1 level
pyruvate dehydrogenase regulation
- By phosphorylation/dephosphorylation cycle
- Dedicated kinase which phosphorylates it and thereby DEACTIVATES IT
- Kinase activated by…
- NADH
- ATP
- Acetyl-CoA
- These are products of pyruvate oxidation
- So pyruvate dehydrogenase products inhibits its activity (negative feedback)
Pyruvate dehydrogenase phosphatase
- Takes phosphate off ACTIVATES the enzyme
- Activated by…
- Calcium
- Insulin
- So insulin stimulates pyruvate oxidation
pyruvate pathway…
- So pyruvate enters mitochondria and is oxidised to acetyl-CoA
- Then incorporated into TCA cycle
- A cyclic pathway in the matrix of the mitochondria
All enzymes in solution in mitochondria
- Apart from succinate dehydrogenase
- Bc it’s soluble in water
- Part of inner mitochondrial membrane
where/how does acetyl CoA enter the TCA cycle
- by reaction with oxaloacetate to form citrate
what are the 3 dehydrogenase which reduce NAD (to NADH) in the TCA cycle
- Isocitrate dehydrogenase
- Oxoglutarate dehydrogenase
- Malate dehydrogenase
what dehydrogenase reduces ubiquinone in the TCA cycle
- succinate dehydrogenase
- ubiquinone is a co-enzyme (involed in fatty oxidation)
how is GTP generated in the TCA cycle
- Succinyl-CoA to succinate
- Linked to GTP formation
- This is called substrate level phosphorylation
the oxidation of acetyl-CoA in the TCA cycle
Acetyl-CoA is completely oxidised as it goes around the cycle
- 2 points at which CO2 is lost
- > Isocitrate dehydrogenase
- > Oxoglutarate dehydrogenase
So, acetyl-CoA (2 carbons) reacts with oxaloacetate (4 carbons) to form citrate
- Loses 2 carbons on the way round to return to oxaloacetate
ketone body production
- Produced in the liver
- In times of starvation
- When there’s considerable fat breakdown
- Then acetyl-CoA formed by beta-oxidation builds up
- Instead of being oxidised in the TCA cycle a lot of it’s diverted into ketone prod.
- Takes part in mitochondria
- First intermediate is HMG-CoA
Which is broken down into…
- Acetoacetate
- Reduced to 3-hydroxybutyrate
- And acetone
These circulate in plasma
- Can be taken up by many tissues and oxidised as fuel (even by brain)
acetoacetate circulation
- Acetoacetate can be spontaneously decarboxylated
- not an enzymic reaction (it’s just not vv stable)
- product is acetone which isn’t further metabolized
acetone. ..
- it’s membrane permeant and volatile
- so when there are high conc. Of circulating ketones, you can smell acetone on the breath of the person
- this happens during starvation when conc. Of ketones rises to ~8 mmol/L in the plasma
- or in type 1 diabetes when fat breakdown becomes unregulated and happens to great extent
- conc. Of ketones can become vv high
- 10,20,30, 50 mmol/L
- Since they are strong acids they cause METABOLIC ACIDAEMIA
mitochondrial ultrastructure
Infoldings of membrane to inc. surface area -> CHRISTAE
2 membranes
Outer membrane – permeable for proteins but not small molecules
- Has pores in it called porins
Inner membrane – more impermeable
- Carriers to transport molecules
- Electron transport chain components are integral membrane proteins to it
Space between membranes -> MITOCHONDRIAL MATRIX
- Certain proteins in here
- All oxidative enzyme of TCA cycle are inside this space
electron transport chain
- A series of carriers that are able to pass electrons from one to another
Carrier A
- Becomes reduced by accepting electrons from an electron donor
Reduced form of A reduces oxidised form of next form -> carrier B
Then reduced form of B reoxidised by transfer of electrons to the final electron acceptor
Movement of electrons from carriers of lower affinity for electrons to carriers with higher affinity for electrons
- This is called redox potential
- So carriers on left have lower redox potential than those on the right
prosthetic groups in the electron transport chain
- These are prosthetic groups
- Attached to proteins either
- Covalently
- Or by strong non-covalent bonds
1) Flavoproteins
- Have a flavin prosthetic group
- Flavin derived from vitamin B2 (or riboflavin)
- 2 forms of prosthetic group…
- Flavin mononucleotide (FMN)
- Flavin adenine dinucleotide (FAD)
- Flavin can accept 2 hydrogens and become reduced
- Ie it’s a hydrogen carrier
2) Iron-sulfur clusters
- Prosthetic group is formed by a cluster of iron and sulphur atoms
- And side chains of cysteines in the protein
- Can accept electrons
- Acts as a single-electron carrier
- One iron goes from the ferric to the ferrous form
ubiquinone
- A co-enzyme
- Free in mitochondrial inner membrane
- part of the electron transport chain
Not derived from vitamin
- It’s synthesised
- The pathway that synthesises it starts of as the same pathway that synthesises cholesterol
- Maybe why statin drugs can have effects on muscle activity?
It’s very hydrophobic
- Dissolved in the inner mitochondrial membrane
- Acts as a hydrogen carrier
cytochrome C
- Have an iron atom co-ordinated by a series of 4 5-membered ring
- Called HAEM
- Diff. types of haem depending on sidechains of the rings
- They may be covalently or non-covalently attached to proteins
Cytochrome C - Simplest and smallest cytochrome in mitochondria
- Has haem covalently attached to cysteine side chains
- Molecular weight about 12,000
- transfers electrons from carrier III to IV in electron transport chain
Haems act as electron carriers because the iron can exist in ferrous or ferric forms
- Ferric form accepts electron and becomes ferrous
inhibitors of mitochondrial complexes
Rotenone
- Plant-derived poison
- Used as an insecticide/ fish poison
- Inhibits complex 1 by binding to it and blocking the elctron transport chain
Atovaquone
- Inhibits complex 2
- Component of anti-malaria drug malatone
Cyanide and CO
- Inhibit complex 4
- Block electron transport to oxygen
- Vv toxic
how is ATP made
- Made by complex 5 -> ATP synthase
- Doesn’t have a redox prosthetic group
- Works through a proton circuit in the mitochondrial membrane
- Electron transport chain translocate proteins out of the mitochondrial matrix
- Setting up an electro-chemical gradient of protons
This has 2 components…
- A small diff. in pH ~1.5 units
- pH inside mitochondria is higher than outside
- as proton conc. Inside is reduced
- there is a membrane potential of ~150 millivolts
- arises bc/ protons are pos. charged
- so electron transport along the chain translocate out these protons
- setting up an electrochemical gradient of protons
- used as of energy for ATP synthesis
- bc protons return through ATP SYNTHASE which drives synthesis of ATP
proton circuit through mitochondria
Electron transport chain in the inner membrane
- Oxidizing reduced NAD
- Reducing oxygen to water
- Blockable via
- Rotenone, antimycin, cyanide
- Electron transport translocates protons out of mitochondrial matrix
- And in the circuit, they then return through ATP Synthase
- Inhibited by oligomycin
Drives synthesis of ADP + Pi to ATP
when can ATP synthesis go wrong
- Depends upon inner membrane being completely impermeable to protons
- If impermeability breaks down then protons short-circuit
- Protons are pumped out and they come back in without the ATP being made
- Energy then released as heat
- Occurs if there’s mitochondrial damage
unicouplers
- Eg 2, 4 dinitrophenol used as explosives in WWII
- Lipid-soluble weak acids
- Bind protons on the outside, diffuse through membrane, release them inside
- Energy from oxidation is not conserved as ATP – instead released as heat
also occurs in brown fat of hibernating animals
yield of ATP from the proton circuit
P/O ratio = number of molecules of ATP produced per atom of oxygen reduced
- E.g. Mol. Of ATP per 2 electrons moving along the chain
In terms of proton circuit theory -> CHEMIOSMOTIC THEORY
- No. of protons from reducing an oxygen divided by number of protons you need to make an ATP
- For oxidation of NADH by oxygen 10 protons translocated
- ATP synthase makes 3 ATPs from translocation of 8 protons
- So number of protons per ATP ~8/3 -> ~3
- Take into account the proton required for phosphate import
yield of ATP for NADH oxidation
- 10 protons come out
- ~4 protons are needed to make an ATP
- So P/O ratio is about 2.5 (3)
yield of ATP for succinate oxidation
- No. of protons translocated is 6 (from reoxidation of ubiquinone)
- So P/O ratio is 6/(3 + 1)
- About 1.5 (2)
differences in metabolism in stavation vs trauma
basal metabolic rate inc. during trauma but dec. during starvation
nitrogen balance dec. during trauma but inc. during starvation
