Energy production - Carbohydrates Flashcards
(70 cards)
1
Q
Carbohydrates
A
- general formula (CH2O)n
- contain an aldehyde or keto group
- contain multiple hydroxyl groups
2
Q
what happens to excess carbohydrate in the diet
A
- converted to glycogen for storage
- converted to triacylglycerols for storage in adipose tissue
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)
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
5
Q
structure of glucose
A
- α has hydroxyl group on same side
- β has hydroxyl groups on opposite sides
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
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
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
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)
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
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
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
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
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
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
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
17
Q
primary lactase deficiency
A
- absence of lactase persistence allele
- highest prevalence in northwest europe
- only in adults
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
19
Q
congenital lactase deficiency
A
- extremely rare
- autosomal recessive defect in lactase gene
- cannot digest breast milk
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
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
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
23
Q
pathways glucose can enter in tissues (stage 2)
A
- glycolysis
- pentose phosphate pathway
- conversion to glycogen for storage
- conversion to other sugars
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
25
overview of glycolysis
- 6C molecule phosphorylated using 2ATP
- 6C cleaved into 2 3C molecules
- 3C molecules oxidated to produce pyruvate, 2NADH and 2ATP
26
phase 1 of glycolysis (steps 1-3)
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)
27
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**
28
phase 2 of glycolysis (steps 4-10)
4. **cleavage** of 6C molecule into 2 3C molecules
5. interconvertible 3C units
6. **2x NADH **produced from NAD+
7. substrate level phosphorylation producing **2ATP**
8. substrate level phosphorylation producing **2 pyruvate and 2 ATP** (pyruvate kinase)
29
enzymes of glycolysis
- hexokinase (glucokinase in the liver) - step 1
- phosphofructokinase (key control enzyme) - step 3
- pyruvate kinase - step 10
30
why are there so many steps in glycolysis
- chemistry is easier
- efficient energy conservation
- versatility
- fine control
31
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
32
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
33
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
34
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)
35
plasma lactate concentration
- normally constant <1mM
- determined by relative rates of production, utilisation and disposal
- elevations seen in physiological and pathological conditions
36
hyperlactaemia
- blood lactate 2-5mM
- below renal threshold
- no change in blood pH due to buffering capactiy
37
lactic acidosis
- blood lactate above 5mM
- above renal threshold
- blood pH lowered due to overcoming buffering capacity
- marker of severe illness
38
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
39
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
40
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
41
3 enzymes of galactose metabolism
- galactokinase
- galactose-1-P uridyl transferase
- UDP-galactose 4-epimerase
42
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**
43
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
44
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
45
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
46
hexokinase (step 1) regulation
- product inhibition by glucose-6-phosphate
- high [G-6-P] reduces activity of hexokinase
- negative feedback
47
pyruvate kinase (step 10) regulation
- hormonal activation
- stimulated by high insulin:glucagon ratio
- high glucose = insulin released = activates enzyme
- enzyme dephosphorylation
48
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)
49
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
50
regulating pentose phosphate pathway
- rate-limiting enzyme = **glucose-6-phosphate dehydrogenase (G6PDH)**
- controlled by **NADP+:NADPH ratio**
- NADP+ activates and NADPH inhibits
51
G6PDH deficiency
- X linked gene defect
- point mutations in G6PDH gene
- NADPH levels insufficient to prevent damage
52
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**
53
chemicals that can reduce levels of NADPH
- antimalarials, sulphonamides, glycosides in broad beans
- cause acute haemolytic episodes
54
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
55
what is acetyl coA
coenzyme A covalently bound to acetyl group
56
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
57
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**
57
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
57
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+
58
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**
59
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
60
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**
61
ATP synthesis
- 2 moles of NADH produces 5 moles of ATP
- 2 moles of FADH2 produces 3 moles of ATP
62
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
63
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**
64
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
65
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
66
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)
67
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**
68
ATP synthesis from glucose
net total = **32** moles ATP per mole of glucose
