Energy production - Carbohydrates Flashcards
Carbohydrates
- general formula (CH2O)n
- contain an aldehyde or keto group
- contain multiple hydroxyl groups
what happens to excess carbohydrate in the diet
- converted to glycogen for storage
- converted to triacylglycerols for storage in adipose tissue
types of carbohydrates
- monosaccharide = single sugar units (glucose, fructose, galactose)
- disaccharides = 2 units (maltose, sucrose, lactose)
- oligosaccharides = 3-12 units
- polysaccharides = 10-1000s units (starch, glycogen, cellulose)
physico-chemical properties of sugars
- hydrophilic - water soluble, attract water, don’t readily cross cell membranes
- partially oxidised - need less oxygen than fatty acids for complete oxidation
structure of glucose
- α has hydroxyl group on same side
- β has hydroxyl groups on opposite sides
glycogen
- polymer of glucose found in animals
- joined by α-1,4 and α-1,6 glycosidic linkages
- highly branched
- synthesised in liver and skeletal muscle
starch
- polymer of glucose found in plants
- mixture of amylose (α-1,4 linkages) and amylopectin (α-1,4 and α-1,6 glycosidic linkages)
- hydrolysed to release glucose and maltose in GI tract
cellulose
- structural polymer of glucose found in plants
- joined by β-1,4 linkages to form long linear polymers
- human GI tract doesn’t produce the enzyes to hydrolyse β-1,4 linkages so cellulose can’t be digested
- major part of essential dietary fibre
glucose requirements of tissues
- blood glucose concentration normally held relatively constant
- all tissues can metabolise glucose but some have an absolute requirement (RBC, neutrophils, kidney medulla cells, lens of eye)
- rate of glucose uptake depends on [blood glucose]
- min amount is 180g/day
- CNS prefers glucose as fuel (use ketone bodies in times of starvation)
overview of carbohydrate catabolism
- stage 1 - metabolism of dietary carbohydrates
- stage 2 - metabolism of glucose in tissues
- stage 3 - tricarboxylic acid cycle (TCA cycle)
- stage 4 - oxidative phosphorylation
overview of stage 1 catabolism
- breakdown complex molecules to building block molecules for absorption into circulation
- extracellular - GI tract
- short pathways
- break C-N and C-O
- no energy produced
overview of stage 2 catabolism
- glycolysis
- breakdown of building blocks into metabolic intermediates (organic precursors)
- oxidative (release of reducing power and energy)
- intracellular (cytosolic and mitochondrial)
- C-C broken
overview of stage 3 catabolism
- tricarboxylic acid/Kreb’s cycle
- mitochondrial
- oxidative (requires NAD+ and FAD)
- some energy produced
- acetyl converted to 2 CO2
- precursors for biosynthesis
overview of stage 4 catabolism
- oxidative phosphorylation
- mitochondrial
- electron transport chain
- converts reducing power (NADH + FADH2) to ATP
- requires oxygen as final electron acceptor
- large amounts of energy produced
stage 1 - metabolism of dietary carbohydrates
- dietary polysaccharides hydrolysed by glycosidase enzymes
- salivary amylase - glucose, maltose + dextrins
- duodenum and jejunum - pancreatic amylase
- small intestine - disaccharidases attached to brush border membranes of the epithelial cells
- lactase, sucrase, glycoamylase, isomaltase - release glucose, fructose + galactose
lactose intolerance
- low level of lactase so lactose not digested
- lactose persists into colon where bacteria breaks it down
- lactose in colon lumen increases the osmotic pressure of contents
- draws water in lumen causing diarrhoea and dehydration
- colonic bacteria produces hydrogen, carbon dioxide and methane gases causing bloating and discomfort
primary lactase deficiency
- absence of lactase persistence allele
- highest prevalence in northwest europe
- only in adults
secondary lactase deficiency
- caused by injury to small intestine - damge epithelial lining
- gastroenteritis, coeliac disease, Crohn’s disease, ulcerative colitis
- occurs in infants and adults
- generally reversible - epithelial cells recover
congenital lactase deficiency
- extremely rare
- autosomal recessive defect in lactase gene
- cannot digest breast milk
absorption of monosaccharides (glucose, galactose and fructose)
- actively transported into absorptive cells lining gut
- facilitated diffusion via GLUT2 into blood supply
- facilitated diffusion via GLUT1 - GLUT5 from blood into tissues
how are monosaccharides actively transported into intestinal epithelial cells
- Na+ K+ pump maintains a sodium gradient within the epithelial cell
- pumps 3Na+ into blood and 2K+ into cell using ATP
- Na+ diffuses down it’s concentration gradient into cell via the co-transporter SGLT1
- brings glucose and galactose into cell with it
Glucose transporters (GLUTs)
- GLUTs have different tissue distribution and different affinities for glucose
- can be hormonally regulated e.g. insulin regulates GLUT4 in skeletal muscle and adipose tissue
- GLUT1 = fetal tissues, erythrocytes, blood-brain barrier
- GLUT2 = kidney, liver, pancreatic beta cells, small intestine
- GLUT3 = neurons, placenta
- GLUT4 = adipose tissue, striated muscle
- GLUT5 = spermatazoa, intestine
pathways glucose can enter in tissues (stage 2)
- glycolysis
- pentose phosphate pathway
- conversion to glycogen for storage
- conversion to other sugars
glycolysis
- 10 enzyme-catalysed steps
- cytoplasm
- generates ATP, NADH, building block molecules, useful intermediates
- pyruvate is the end product
- Glucose + 2Pi + 2ADP + 2NAD+
→ 2pyruvate + 2ATP + 2NADH + 2H+ + 2H2O
overview of glycolysis
- 6C molecule phosphorylated using 2ATP
- 6C cleaved into 2 3C molecules
- 3C molecules oxidated to produce pyruvate, 2NADH and 2ATP
phase 1 of glycolysis (steps 1-3)
- phosphorylation of glucose to **glucose-6-phosphate **(hexokinase + ATP)
- glucose-6-phosphate to fructose-6-phosphate
- fructose-6-phosphate phosphorylated to fructose-1,6-bisphosphate (phosphofructokinase-1 + ATP)
why is phosphorylation of glucose important (step 1 glycolysis)
- makes sugar anionic so prevents it crossing plasma membrane
- increases reactivity of sugar so it can be metabolised by several pathways
- allows substrate level phosphorylation
phase 2 of glycolysis (steps 4-10)
- cleavage of 6C molecule into 2 3C molecules
- interconvertible 3C units
- **2x NADH **produced from NAD+
- substrate level phosphorylation producing 2ATP
- substrate level phosphorylation producing 2 pyruvate and 2 ATP (pyruvate kinase)
enzymes of glycolysis
- hexokinase (glucokinase in the liver) - step 1
- phosphofructokinase (key control enzyme) - step 3
- pyruvate kinase - step 10
why are there so many steps in glycolysis
- chemistry is easier
- efficient energy conservation
- versatility
- fine control
key features of glycolysis
- 6C or 3C molecules
- no loss of CO2
- some C3 intermediates used for cell functions
- glucose oxidised to pyruvate and NAD+ reduced to NADH
- exergonic process with –ve ∆G value
- all intermediates phosphorylated by substrate level phosphorylation
- net yield of 2 moles of ATP
important intermediates of glycolysis
glyceraldehyde 3-P ↔ dihydroxyacetone phosphate
- glycerol 3-phosphate dehydrogenase converts it to glycerol phosphate
- triglyceride and phospholipid biosynthesis in adipose and liver
1,3-bisphosphoglycerate
- bisphosphoglycerate mutase converts it to 2,3-bisphosphoglycerate
- regulator of haemoglobin oxygen affinity in RBCs
why is lactate produced
to regenerate NAD+ for step 6 of glycolysis
- some cells dont have mitochondria to perform electron transport chain e.g. RBCs, eye lens
- supply of oxygen is inadequate so anaerobic respiration during exercise or pathological situation
how is lactate produced
- lactate dehydrogenase (LDH) reduces pyruvate to lactate
- Pyruvate + NADH + H+ ↔ lactate + NAD+
- lactate transported to liver, kidney and heart for breakdown to pyruvate
- converted to glucose (liver and kidney) or oxidised to CO2 (heart)
plasma lactate concentration
- normally constant <1mM
- determined by relative rates of production, utilisation and disposal
- elevations seen in physiological and pathological conditions
hyperlactaemia
- blood lactate 2-5mM
- below renal threshold
- no change in blood pH due to buffering capactiy
lactic acidosis
- blood lactate above 5mM
- above renal threshold
- blood pH lowered due to overcoming buffering capacity
- marker of severe illness
how is fructose metabolised
- sucrose hydrolysed by sucrase to glucose and fructose
- metabolised in liver
- fructokinase converts fructose to fructose-1-phosphate
- aldolase converts to glyceraldehyde-3-phosphate
- joins step 6 of glycolysis
clinical importance of fructose metabolism
essential fructosuria - fructokinase missing
- fructose in urine
- no toxic symptoms
fructose intolerance - aldolase missing
- fructose-1-P accumulates in liver
- liver damage
- remove fructose and sucrose from diet
how is galactose metabolised
- Galactose + ATP → Glucose 6-phosphate + ADP
- galactose → galactose-1-phosphate by galactokinase
- galactose-1-phosphate → glucose-1-phosphate by galactose-1-P uridyl transferase
- UDP-glucose ↔ UDP-galactose by UDP-galactose 4-epimerase because UDP-glucose acts catalytically to form glucose-1-phosphate
- glucose-1-phosphate → glucose-6-phosphate
- joins step 2 of glycolysis
3 enzymes of galactose metabolism
- galactokinase
- galactose-1-P uridyl transferase
- UDP-galactose 4-epimerase
what is galactosaemia
- deficiency in the enzymes involved in galactose metabolism so unable to utilise galactose
- galactokinase deficiency = accumulation of galactose
- transferase deficiency = accumulation of galactose and galactose-1-P
- galactose is reduced to galactitol using NADPH and aldose reductase
- treatment = no lactose diet
effects of galactosaemia
- galactose to galactitol depletes NADPH available for lipid production, GSH regeneration and reduction of disulphide bonds
- lens of eye damaged due to -S-S- bonds and glycosylation of lens proteins, leads to cataracts
- raised intra-ocular pressure **(glaucoma) **could cause blindness
- accumulation of galactose-1-P causes damage to liver, kidney and brain
- oxidative stress due to reduced GSH regeneration
allosteric regulation of glycolysis
- at the irreversible steps 1, 3 and 10
- activator or inhibitor binds at site that isn’t active site
- covalent modifications like phosphorylation or dephosphorylation
phosphofructokinase (step 3) regulation
allosteric (muscle)
- inhibited by high [ATP] and high citrate
- stimulated by high [AMP] and high fructose-2,6-bisphosphate
hormonal (liver)
- inhibited by glucagon
- stimulated by insulin
hexokinase (step 1) regulation
- product inhibition by glucose-6-phosphate
- high [G-6-P] reduces activity of hexokinase
- negative feedback
pyruvate kinase (step 10) regulation
- hormonal activation
- stimulated by high insulin:glucagon ratio
- high glucose = insulin released = activates enzyme
- enzyme dephosphorylation
stages of pentose phospate pathway
- phase 1: glucose-6-phosphate oxidised and decarboxylated by glucose-6-phosphate dehydrogenase using 2NADP+
- phase 2: non-oxidative reactions convert 5C sugar to 3C and 6C intermediates of glycolysis (fructose-6-P and glyceraldehyde-3-P)
functions of pentose phosphate pathway
important source of NADPH
- reducing power for biosynthesis
- maintenance of GSH levels in RBCs
- detoxification mechanisms
produces 5C sugar ribose
- synthesis of nucleotides
- DNA and RNA
regulating pentose phosphate pathway
- rate-limiting enzyme = glucose-6-phosphate dehydrogenase (G6PDH)
- controlled by NADP+:NADPH ratio
- NADP+ activates and NADPH inhibits
G6PDH deficiency
- X linked gene defect
- point mutations in G6PDH gene
- NADPH levels insufficient to prevent damage
how does G6PDH deficiency cause haemolysis
- decreased G6PDH activity limits amount of NADPH
- NADPH required to convert oxidised glutathione back to active reduced form
- lower levels of reduced glutathione leaves cell susceptible to oxidative damage
- RBCs particularly affected since pentose phosphate pathway is only source of NADPH and they’re oxygen carriers
- haemoglobin become cross-linked by disulphide bonds from oxidative damage and form insoluble aggregates called Heinz bodies
- premature destruction of RBCS - haemolytic anaemia
chemicals that can reduce levels of NADPH
- antimalarials, sulphonamides, glycosides in broad beans
- cause acute haemolytic episodes
summary of glycolytic pathway regulation
allosteric regulation
- product inhibition of hexokinase by G-6-P
- PFK stimulated by AMP and inhibited by ATP
hormonal activation
- PFK and pyruvate kinase stimulated by high insulin:glucagon ratio
metabolic regulation
- high [NADH] or low [NAD+] causes product inhibition of step 6
what is acetyl coA
coenzyme A covalently bound to acetyl group
pyruvate dehydrogenase (PDH)
- multi-enzyme complex catalysing pyruvate → acetyl coA
- requires various coenzymes (FAD, thiamine pyrophosphate, lipoic acid) supplied by B vitamins
- requires NAD+
- link reaction is irreversible
- activated by pyruvate, CoA, NAD+, ADP, insulin
- inhibited by acetyl-CoA, NADH, ATP, citrate
tricarboxylic acid (Krebs) cycle
- occurs in mitochondria
- acetyl CoA enters and is combines with oxaloacetate to produce citrate
- requires NAD+, FAD and oxaloacetate
- breaks C-C bonds in acetate
- generates reducing power from oxidation of acetyl-CoA
- generates intermediates for biosynthetic reactions
- tightly coupled with to ETC so needs oxygen
- produces 6x NADH, 2x FADH2, 2x GTP
allosteric regulation of TCA cyle
regulated by ATP:ADP and NADH:NAD+
isocitrate dehydrogenase
isocitrate to α-ketoglutarate
- stimulated by ADP
- inhibited by NADH
α-ketoglutarate dehydrogenase
α-ketoglutarate to succinyl-CoA
- inhibited by NADH, ATP, succinyl-CoA
major interconversions in TCA cycle
- precursors for glucose, amino acids, haem, and fatty acids
- replacement of intermediates from pyruvate carboxylase
pyruvate + CO2 + ATP + H2O → oxaloacetate + ADP + Pi + 2H+
oxidative phosphorylation
- NADH and FADH2 are re-oxidised
- electrons are donated to electron transport chain within mitochondrial membrane
- release energy as they travel down energy levels
- combine with oxygen at end of chain, to form water
- energy used to pump protons from matrix to intermembrane space through proton translocation complexes
- creates potential difference called proton motive force (electrochemical gradient)
- protons move back to matrix through** ATP synthase**
- drives phosphorylation of ADP to ATP
use of reducing power in ATP synthesis
- electron transport - electrons in NADH and FADH2 transferred through carrier molecules to oxygen releasing free energy
- ATP synthesis - free energy used to drive ATP synthesis by ATP synthase
electron transport
- four highly specialised protein complexes I-IV
- complexes I, II and III also act as proton translocating complexes
- translocating complexes transform chemical bond energy of electrons to electro-chemical potential difference of protons
- greater the chemical bond energy, greater the p.m.f, more ATP made
- NADH electrons have more energy than FADH2 so NADH uses all 3 PTCs but FADH2 uses 2 PTCs
ATP synthesis
- 2 moles of NADH produces 5 moles of ATP
- 2 moles of FADH2 produces 3 moles of ATP
oxidative phosphorylation vs substrate level phosphorylation
oxidative
- requires membrane associated complexes (inner mitochondrial membrane)
- energy coupling occurs indirectly through generation and subsequent utilisation of proton gradient
- cannot occur in absence of oxygen
- major process for ATP synthesis in cells requiring lots of energy
substrate level
- requires soluble enzymes (cytoplasmic and mitochondrial matrix)
- energy coupling occurs directly through formation of a high energy of hydrolysis bond (phosphoryl-group transfer)
- can occur to limited extent in absence of oxygen
- minor process for ATP synthesis in cells requiring lots of energy
coupling between ET and ATP synthesis
when [ATP] is high
- [ADP] is low so ATP synthase stops as lack of substrate
- prevents transport of protons into matrix
- [H+] in intermembrane space increases to a level preventing more protons being pumped
- electron transport stops
inhibition of oxidative phosphorylation
- under anaerobic conditions
- block electron transport so prevents acceptance of electrons by oxygen so no p.m.f. so no oxidative phosphorylation
- lethal - irreversible cell damage
- e.g. cyanide, carbon monoxide
uncoupling of oxidative phosphorylation
- uncouplers increase permeability of inner mitochondrial membrane to protons
- protons can re-enter matrix without driving ATP synthesis
- processes are uncoupled and potential energy of p.m.f is dissipated as heat
- ET continues, no ATP synthesis, excessive amount of heat
- e.g. dintirophenol, dinitrocresol, fatty acids
uncoupling proteins (UCP)
- UCP 1-5
- allow **leak of protons **through inner mitochondiral membrane, reducing p.m.f., inhibiting ATP synthesis
- uncouple ETC and ATP synthesis to generate heat
- UCP 1: brown adipose
- UCP 2: widely distributed (linked to diabetes, obesity, metabolic syndrome and heart failure)
- UCP 3: skeletal muscle, brown adipose and heart (modifying fatty acid metabolism and protecting against ROS damage)
UCP 1 in brown adipose tissue
non-shivering thermogenesis so mammals to survive in cold environments
- noradrenaline released in response to cold
- stimulates lipolysis which releases fatty acids from triacylgycerol
- β-oxidation of the fatty acids forms NADH and FADH2, driving ET and increasing p.m.f
- activates UCP1
- protons re-enter matrix without driving ATP synthesis, dissipating p.m.f as heat
ATP synthesis from glucose
net total = 32 moles ATP per mole of glucose
