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MEH > Energy production - Carbohydrates > Flashcards

Energy production - Carbohydrates Flashcards

1
Q

Carbohydrates

A
  • general formula (CH2O)n
  • contain an aldehyde or keto group
  • contain multiple hydroxyl groups
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2
Q

what happens to excess carbohydrate in the diet

A
  • converted to glycogen for storage
  • converted to triacylglycerols for storage in adipose tissue
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3
Q

types of carbohydrates

A
  • monosaccharide = single sugar units (glucose, fructose, galactose)
  • disaccharides = 2 units (maltose, sucrose, lactose)
  • oligosaccharides = 3-12 units
  • polysaccharides = 10-1000s units (starch, glycogen, cellulose)
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4
Q

physico-chemical properties of sugars

A
  • hydrophilic - water soluble, attract water, don’t readily cross cell membranes
  • partially oxidised - need less oxygen than fatty acids for complete oxidation
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5
Q

structure of glucose

A
  • α has hydroxyl group on same side
  • β has hydroxyl groups on opposite sides
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6
Q

glycogen

A
  • 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
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7
Q

starch

A
  • 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
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8
Q

cellulose

A
  • 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
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9
Q

glucose requirements of tissues

A
  • 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)
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10
Q

overview of carbohydrate catabolism

A
  • stage 1 - metabolism of dietary carbohydrates
  • stage 2 - metabolism of glucose in tissues
  • stage 3 - tricarboxylic acid cycle (TCA cycle)
  • stage 4 - oxidative phosphorylation
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11
Q

overview of stage 1 catabolism

A
  • 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
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12
Q

overview of stage 2 catabolism

A
  • glycolysis
  • breakdown of building blocks into metabolic intermediates (organic precursors)
  • oxidative (release of reducing power and energy)
  • intracellular (cytosolic and mitochondrial)
  • C-C broken
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13
Q

overview of stage 3 catabolism

A
  • tricarboxylic acid/Kreb’s cycle
  • mitochondrial
  • oxidative (requires NAD+ and FAD)
  • some energy produced
  • acetyl converted to 2 CO2
  • precursors for biosynthesis
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14
Q

overview of stage 4 catabolism

A
  • oxidative phosphorylation
  • mitochondrial
  • electron transport chain
  • converts reducing power (NADH + FADH2) to ATP
  • requires oxygen as final electron acceptor
  • large amounts of energy produced
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15
Q

stage 1 - metabolism of dietary carbohydrates

A
  • 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
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16
Q

lactose intolerance

A
  • 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
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17
Q

primary lactase deficiency

A
  • absence of lactase persistence allele
  • highest prevalence in northwest europe
  • only in adults
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18
Q

secondary lactase deficiency

A
  • 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
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19
Q

congenital lactase deficiency

A
  • extremely rare
  • autosomal recessive defect in lactase gene
  • cannot digest breast milk
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20
Q

absorption of monosaccharides (glucose, galactose and fructose)

A
  • actively transported into absorptive cells lining gut
  • facilitated diffusion via GLUT2 into blood supply
  • facilitated diffusion via GLUT1 - GLUT5 from blood into tissues
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21
Q

how are monosaccharides actively transported into intestinal epithelial cells

A
  • 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
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22
Q

Glucose transporters (GLUTs)

A
  • 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
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23
Q

pathways glucose can enter in tissues (stage 2)

A
  • glycolysis
  • pentose phosphate pathway
  • conversion to glycogen for storage
  • conversion to other sugars
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24
Q

glycolysis

A
  • 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
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25
Q

overview of glycolysis

A
  • 6C molecule phosphorylated using 2ATP
  • 6C cleaved into 2 3C molecules
  • 3C molecules oxidated to produce pyruvate, 2NADH and 2ATP
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26
Q

phase 1 of glycolysis (steps 1-3)

A
  1. phosphorylation of glucose to **glucose-6-phosphate **(hexokinase + ATP)
  2. glucose-6-phosphate to fructose-6-phosphate
  3. fructose-6-phosphate phosphorylated to fructose-1,6-bisphosphate (phosphofructokinase-1 + ATP)
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27
Q

why is phosphorylation of glucose important (step 1 glycolysis)

A
  • 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
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28
Q

phase 2 of glycolysis (steps 4-10)

A
  1. cleavage of 6C molecule into 2 3C molecules
  2. interconvertible 3C units
  3. **2x NADH **produced from NAD+
  4. substrate level phosphorylation producing 2ATP
  5. substrate level phosphorylation producing 2 pyruvate and 2 ATP (pyruvate kinase)
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29
Q

enzymes of glycolysis

A
  • hexokinase (glucokinase in the liver) - step 1
  • phosphofructokinase (key control enzyme) - step 3
  • pyruvate kinase - step 10
30
Q

why are there so many steps in glycolysis

A
  • chemistry is easier
  • efficient energy conservation
  • versatility
  • fine control
31
Q

key features of glycolysis

A
  • 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
32
Q

important intermediates of glycolysis

A

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

33
Q

why is lactate produced

A

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

34
Q

how is lactate produced

A
  • 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)
35
Q

plasma lactate concentration

A
  • normally constant <1mM
  • determined by relative rates of production, utilisation and disposal
  • elevations seen in physiological and pathological conditions
36
Q

hyperlactaemia

A
  • blood lactate 2-5mM
  • below renal threshold
  • no change in blood pH due to buffering capactiy
37
Q

lactic acidosis

A
  • blood lactate above 5mM
  • above renal threshold
  • blood pH lowered due to overcoming buffering capacity
  • marker of severe illness
38
Q

how is fructose metabolised

A
  • 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
39
Q

clinical importance of fructose metabolism

A

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

40
Q

how is galactose metabolised

A
  • 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
41
Q

3 enzymes of galactose metabolism

A
  • galactokinase
  • galactose-1-P uridyl transferase
  • UDP-galactose 4-epimerase
42
Q

what is galactosaemia

A
  • 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
43
Q

effects of galactosaemia

A
  • 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
44
Q

allosteric regulation of glycolysis

A
  • 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
45
Q

phosphofructokinase (step 3) regulation

A

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

46
Q

hexokinase (step 1) regulation

A
  • product inhibition by glucose-6-phosphate
  • high [G-6-P] reduces activity of hexokinase
  • negative feedback
47
Q

pyruvate kinase (step 10) regulation

A
  • hormonal activation
  • stimulated by high insulin:glucagon ratio
  • high glucose = insulin released = activates enzyme
  • enzyme dephosphorylation
48
Q

stages of pentose phospate pathway

A
  • 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)
49
Q

functions of pentose phosphate pathway

A

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

50
Q

regulating pentose phosphate pathway

A
  • rate-limiting enzyme = glucose-6-phosphate dehydrogenase (G6PDH)
  • controlled by NADP+:NADPH ratio
  • NADP+ activates and NADPH inhibits
51
Q

G6PDH deficiency

A
  • X linked gene defect
  • point mutations in G6PDH gene
  • NADPH levels insufficient to prevent damage
52
Q

how does G6PDH deficiency cause haemolysis

A
  • 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
53
Q

chemicals that can reduce levels of NADPH

A
  • antimalarials, sulphonamides, glycosides in broad beans
  • cause acute haemolytic episodes
54
Q

summary of glycolytic pathway regulation

A

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

55
Q

what is acetyl coA

A

coenzyme A covalently bound to acetyl group

56
Q

pyruvate dehydrogenase (PDH)

A
  • 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
57
Q

tricarboxylic acid (Krebs) cycle

A
  • 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
57
Q

allosteric regulation of TCA cyle

A

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

57
Q

major interconversions in TCA cycle

A
  • precursors for glucose, amino acids, haem, and fatty acids
  • replacement of intermediates from pyruvate carboxylase
    pyruvate + CO2 + ATP + H2O → oxaloacetate + ADP + Pi + 2H+
58
Q

oxidative phosphorylation

A
  • 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
59
Q

use of reducing power in ATP synthesis

A
  • 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
60
Q

electron transport

A
  • 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
61
Q

ATP synthesis

A
  • 2 moles of NADH produces 5 moles of ATP
  • 2 moles of FADH2 produces 3 moles of ATP
62
Q

oxidative phosphorylation vs substrate level phosphorylation

A

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

63
Q

coupling between ET and ATP synthesis

A

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

64
Q

inhibition of oxidative phosphorylation

A
  • 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
65
Q

uncoupling of oxidative phosphorylation

A
  • 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
66
Q

uncoupling proteins (UCP)

A
  • 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)
67
Q

UCP 1 in brown adipose tissue

A

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

68
Q

ATP synthesis from glucose

A

net total = 32 moles ATP per mole of glucose

MEH (26 decks)

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