MEH Carb metabolism Flashcards
How much glucose is present in blood at any one time?
5mM
Name 4 types of sugar disorder/disease
Galactosaemia
Fructose intolerance
Lactose intolerance
Diabetes
What level of glucose in the blood is considered too high (e.g. in diabetes)
≥ 7mM
Why can sugars be metabolised using less energy than for fats?
As they are partially oxidised so require less O2 for complete oxidation
Where is glycogen synthesised in the body? What bonds does it contain?
Liver and skeletal muscle
alpha-1,4 and alpha 1,6 (branching) glycosidic bonds
Why do RBC have an absolute requirement for Glucose?
No mitochondria so need glucose to undergo anaerobic glycolysis for energy
Why do neutrophils have an absolute requirement for glucose?
Because they use their O2 for other processes - respiratory burst to kill pathogens
Why do the innermost cells of the kidney medulla have an absolute requirement for glucose?
Because they have a high demand for O2 but O2 in the blood is low when it gets here, so need anaerobic glycolysis
Why does the lens of the eye have an absolute requirement for glucose?
Poor blood supply of O2 as needs to be transparent so replies on anaerobic glycolysis
Does the CNS have an absolute requirement for glucose? What else can it use?
No but it prefers it
Can supplement with ketone in times of starvation but needs time to adapt.
What happens to GLUT4 and glucose uptake into tissues in the presence of insulin?
Insulin triggers uptake of glucose into adipose and skeletal muscle via GLUT4 - upregulation of receptors on membrane
Where in the body is the main site of fructose and galactose metabolism?
Liver
What is glycolysis driven by the need for? What is it driven by in the liver?
Driven by the need for energy. In the liver driven by glucose delivered
How many ATP produced in glycolysis? What size Carbon intermediates does it produce per glucose molecule?
2 (makes 4 but used 2 in the investment stage)
3C and 6C intermediates
Is glycolysis oxidation or reduction of glucose?
Oxidation
Is glycolysis in the cytosol or mito?
Cytosol
Does glycolysis occur in all tissues?
Yes
How can glycolysis be anaerobic?
With an enzyme pyruvate dehydrogenase
What is the difference between Hexo and Glucokinase?
Hexo - low affinity and inhibited by glucose-6P
Gluco - in liver - high affinity and not inhibited by product
What are 3 main enzymes of glycolysis and what products do they form? Are they reversible?
1) Hexokinase/Gluco Glucose –> Glucose 6P
2) PFK –> Fructose 6P to Fructose 1,6BisP
3) Pyruvate kinase –> phosphoenolpyruvate to pyruvate
Which enzyme in glycolysis is a key control enzyme?
PFK
Why so many stages of glycolysis?
1) Gives versatility
2) Some parts reversible
3) Allows fine control
4) Chemical reactions easier in smaller parts
Why are stages 1, 3, 10 of glycolysis irreversible?
As they have a large -ve gibbs free energy
Why is step 3 the committing step of glycolysis?
As step 2 fructose 6P can be reversibly changed to glucose6P that can go onto other pathways. But fructose 1,6bisP cannot be converted back - so the only way is glycolysis
Why can glucose 6P not cross the plasma membrane but glucose can?
As glucose has been phosphorylated it is now negatively charged so can’t cross membrane
What happens after fructose 1,6bisP in glycolysis?
the 6C is converted into 2 3C intermediates that are interchangeable
Is there loss of CO2 in glycolysis?
No
When Glucose is oxidised - where do the electrons go?
to carrier molecule NAD+ that is reduced to NADH - then contains reducing power.
Is glycolysis exergonic or endergonic overall?
Exergonic
What is substrate level phosphorylation? What are the 2 net ATP a result of?
It is phosphorylation that is not couple to oxidation - is quicker. 2 net ATP in glycolysis are produced in this way by adding Pi to ADP
To convert pyruvate to glucose, which steps need to be bypassed? What is this called?
3 steps
10, 3, 1 (pyruvate kinase, PFK, hexo/glucokinase)
Called gluconeogenesis
What is FDG and what can it be used for clinically?
Radioactive hexokinase, can be used to scan for cancers as they have increased rates of glycolysis
How is PFK a key regulator of glycolysis? Which two ways?
1) Allosteric - inhibited by increased ATP, stimulated by high AMP
2) Hormonal - inhibited by glucagon, stimulated by insulin
Which hormone stimulates gluconeogenesis?
Glucagon
4 ways that glycolysis is regulated via the 3 main enzymes? How is glycolysis regulated by high/low energy signals?
PFK - 3 ways explained in another Q
Hexokinase - product inhibition by glucose 6P
Pyruvate Kinase - stimulated by high insulin
Metabolic regulation - High NADH or low NAD+ is a high energy signal that inhibits glycolysis
Name 2 important intermediates in glycolysis - what enzymes are used?
1) Glycerol phosphate - glycerol-3 phosphate dehydrogenase
2) 2,3 BPG - bisphosphoglycerate mutase
What are these to glycolysis intermediates important for?
- Glucose phosphate
- 2,3 BPG
Glucose phosphate - important for lipid biosynthesis in adipose and liver
2,3 BPG - promotes release of O2 at tissues in RBCs
Does lipid synthesis in adipose and liver require glycolysis?
Yes adipose
No liver - can also phosphorylate directly from glycerol
Once NAD+ is reduced to NADH what is needed to regenerate it? Why is this relevant in RBCs for example?
Needs O2 to regenerate back to NADH
RBC don’t have mito so can regenerate NAD+ back to NADH so need lactate dehydrogenase to complete glycolysis
What would happen if all NAD+ was reduced to NADH hypothetically?
Glycolysis would stop when all NAD+ is converted to NADH in step 6 of glycolysis.
What reaction does lactate dehydrogenase catalyse? IS this reversible? Where in the body can use lactate as a direct fuel? Where else can lactate be used and how?
Pyruvate + NADH to Lactate + NAD+
Yes reversible using same enzyme e.g. in heart
In liver and kidney for gluconeogenesis that can then be used in glycolysis
In low O2 conditions what happens to pyruvate in glycolysis?
Converted to lactate instead of stage 4 of metabolism that requires O2
When would gluconeogenesis in the liver and kidney be impaired (3)
Liver disease
Vitamin deficiency
Enzyme deficiency
What is normal, hyperlactaemia and lactic acidosis levels in the blood?
Normal below 1mM
Hyper - 2-5
Lactic acidosis about 5mM and reduction in pH
Which 3 enzymes are involved in galactose metabolism so it can either enter glycolysis or be stored as glycogen? Can any or all result in galactosaemia if deficient?
1) Galactokinase (Galactose –> galactose 1P)
2) Uridyl transferase (Galactose 1P to glucose 1P)
3) UDP - galactose epimerase (Galactose 1P to UDP galactose)
Any of these can result in galactosaemia - uridyl transferase is the worst one
What happens if fructokinase or aldolase are missing in humans? What is the treatment?
1) Fructo - essential fructosuria - no clinical signs - fructose in urine
2) Aldolase - fructose intolerance - Fructose 1P accumulates in liver and leads to liver damage.
Remove fructose from diet
Where does the pentose phosphate pathway come off from glycolysis? What is it used for? What is the rate limiting enzyme?
From Glucose 6P
Essential for biosynthesis as creates NADPH, also for GSH maintenance and detoxification reactions
Rate limiting enzyme is G6PDH
What else is made from the pentose phosphate pathway and what is it’s use? What can the 5C sugars in pentose phosphate pathway be used for?
Also creates Ribose - 5-P used to make DNA and RNA, nucleotides and coenzymes.
5C sugars can be converted to 6C and 3C sugars that are then fed back into glycolysis.
Is ATP synthesised in the pentose phosphate pathway? is CO2 produces?
No ATP is not
Yes CO2 is produced unlike glycolysis
What is the role of glutathione (GSH)?
Protective role against oxidative damage in cells.
What happens in G6PDH deficiency?
Get low NADPH, low glutathione, cells prone to oxidative stress, particularly RBCs, become aggregated - Heinz bodies - results in haemolytic anaemia
How does pyruvate enter stage 3 metabolism (TCA cycle)? What enzyme is used? Is this reversible and what are the implications of this? Where in the cell is this?
It does not enter directly - is metabolised first by pyruvate dehydrogenase. Is irreversible (irreversible loss of CO2) so you cannot convert Acetyl Co A to pyruvate to glucose. In mito matrix.
What happens in pyruvate dehydrogenase deficiency?
Lactic acidosis - pyruvate gets converted to lactic acid instead
How is PDH regulated?
Allosterically by acetyl co A
Stimulated by insulin (fed state)
Stimulated by ADP and inhibited by ATP and NADH
Why are there no known genetic defects in the TCA cycle?
As they would be lethal
Does the TCA cycle require oxygen?
Yes
What is the main aim of TCA cycle in terms of breaking Acetyl Co A
To break the C-C bond in acetyl co A, and oxidise the C atoms to CO2. The H+ and e- released from acetate are transferred to NAD+ and FAD
What does TCA cycle produce?
2 ATP (GTP)
2CO2
Precursors for biosynthesis - amino acids, haem, glucose, fatty acids
Is oxidative - produces NADH and FADH2
What are two important irreversible(rate limiting) steps of the TCA cycle and why are they irreversible? What enzymes catalyse these steps? Are they oxidative? Where are the reducing equivalents?
Isocitrate (C6) to C5 - by isocitrate dehydrogenase
C5 to C4 - alpha-ketoglutarate dehydrogenase
Both irreversible as release CO2
Both oxidative as produce reducing equivalents in the form of NADH
Are the rate limiting steps in the TCA cycle activated by high or low energy substrates.
Activated by low energy substrates - ADP and inhibited by low energy substrates ATP and NADH
How can the TCA cycle produce precursors for many different things?
Because it produces carbon skeletons that can then go onto be synthesised into different things:
Amino acids, fatty acids, haem, glucose
Is TCA cycle catabolic anabolic or both? Explain why
Both as breaks acetyl co A to release energy but also the carbon intermediates are used in biosynthesis of other molecules
Where is the energy stored ready for ATP synthesis in the electron transport chain?
In high energy bonds of carrier molecules NADH and FADH2
What are the two processes of the electron transport chain?
1) Electrons on NADH and FADH2 transferred through a series of carrier molecules to oxygen releasing energy in steps
2) Oxidative phosphorylation used to drive ATP synthesis
Where in the mitochondria does the ETC occur?
Inner mitochondrial membrane
What happens in the ETC?
Electrons are transferred from NADH and FADH2 through a series of carrier molecules releasing energy. Some of this energy is used to pump H+ ions out of inner membrane via the proton translocating complex - which creates an electrochemical gradient (p.m.f - proton motive force). This EC grad is then used to drive protons back into the matrix across the inner membrane producing ATP
Does ATP synthase work in both directions?
Yes normally in pumps H+ out of matrix, but in ETC it is used in reverse to drive ATP synthesis
How is the ETC and ATP synthesis kept tightly coupled?
Because H+ have to enter via ATP synthase due to the inner mitochondrial membrane being impermeable
What is the difference in the roles of the proton translocating complex and ATP synthase?
PTC - pumps H+ out of inner membrane creating a gradient
ATP synthase - drives H+ back into matrix synthesising ATP
What happens with O at the end of the electron transport chain?
O binds with 2 H+ that have been released in the ETC from NADH and FADH2 to create H2O.
Which produces more energy NADH or FADH2? How much do they produce? How many proton translocating complexes do each use?
NADH has more energy
NADH per 2 moles NADH - 5 ATP
FADH2 per 2 moles FADH2 - 3 ATP
NADH uses all 3, FADH2 only uses 2
The higher the p.m.f the (more/less) ATP produced. Why?
More. As more H+ will drive back through to the matrix through ATP synthase producing more ATP.
What is the substrate for ATP synthase? What happens when ATP is high and ADP is low?
ADP
When ATP is high and ADP is low there is no substrate so inward flow of H+ stops - no ATP produced.
Which or both of ETC and oxidative phosphorylation are controlled by mitochondrial [ATP]?
Both
What does cyanide do? Is it dangerous?
Inhibits oxidative phosphorylation by blocking O2 accepting electrons in the ETC - prevents ATP - lethal
What do uncouplers do?
They increase the permeability of the mitochondrial inner membrane to protons - dissipate the p.m.f. so no drive for ATP synthesis
Name 2 inhibitors of ETC
Cyanise CO
Name 3 uncouplers
Dinitrophenol
Dinitrocresol
Fatty acids
What happens in oxidative phosphorylation diseases?
Genetic defects in proteins encoded by mtDNA reduce ETC and ATP synthesis
What does efficiency of oxidative phosphorylation depend on?
Coupling
How does brown fat produce heat roughly?
Uses uncouplers - fatty acids that allow more energy to be released as heat
What uncoupling protein does brown adipose contain and what is the process of ETC from ATP uncoupling?
Contains Thermogenic (UCP1) - in response to cold, NA activates:
1) Lipase - releases fatty acids from triacylglycerol
2) Fatty acid oxidation –> activate UCP1
3) UCP1 transports H+ back into mitochondria
4) Uncouples ETC from ATP synthesis so extra energy released as heat
Which produces more ATP substrate level or oxidative phosphorylation?
Oxidative
