14-32: METABOLISM (GLYCOLYSIS, TCA) Flashcards
1
Q
Control of metabolic pathways
A
- change rate of pathway to meet the needs of the cell
- allows adaptation to environment
- 3 main ways:
1. change amount of enzyme (slow because has to start by changing rate of transcription)
2. change activity of enzyme (main way; by allosteric regulation -v. fast; or by reversible covalent modification ie. (de) phosphorylation- fast)
3. availability of substrate
2
Q
allosteric regulation
A
- allosteric enzymes undergo a conformational change as a result of binding of a regulatory molecule at a site distinct from BS
- changes structure of active site which increases or decreases affinity for S
3
Q
allosteric enzyme example
A
- PFK1: phosphofructokinase I
- converts Fru-6-P to Fru-1,6-bisP
- hydrolyses a molecule of ATP to transfer P group
- main regulatory step in glycolysis
4
Q
reversible covalent modification
A
-phosphorylation by protein kinase using ATP
-dephosphorylation by phosphoprotein phosphatase using water to hydrolyse
-enzymes have OH group on Ser or Thr residues that can be phosphorylated
-controlled by hormones:
e.g. insulin (released from pancreas when blood sugar is high)
glucagon (released from pancreas when blood sugar is low)
adrenaline (released from adrenal glands to fight/flight)
5
Q
glycolsysis
A
- phosphorylation: Glu to Glu-6-P by hexokinase using ATP molecule
- isomerization: glu-6-P to fru-6-P by phosphoglucoisomerase; move carbonyl from C1 to C2, aldo to keto sugar
- phosphorylatiom: Fru-6-P to Fru-1,6-bisP using ATP for group transfer reaction
- C-C cleavage: glyceraldehyde-3-P
- oxidation by NAD/phosphorylation using Pi: NAD accepts 2e (reduced); enzyme glyceraldehyde-3-P DH produces 1,3-bisphosphoglycerate
- ATP production #1: group transfer P from C1 to ADP to generate 3-phosphoglycerate by phosphoglycerate kinase
- P is moved: from C3 to C2 by phosphoglycerate mutase
- Dehydration: water removed from 2-phosphoglycerate to give phosphoenolpyruvate by enolase; activates P group for transfer to ATP
- ATP production #2: unstable enol transfers P to ADP to generate ATP by substrate-level phosphorylation; converted to more stable ketone pyruvate
6
Q
control of glycolysis by allosteric regulation
A
- at irreversible steps: hexokinase reaction, PFK1 reaction (first committed step), pyruvate kinase reaction
- ATP inhibits PFK1; AMP stimulates it; decreases affinity for substrate
- PFK1 has different isozymes in liver and muscle; same catalytic mechanism but have different regulatory sites
- muscle: low pH inhibits PFK1
- liver: citrate inhibits PFK1; fru-2,6-bisP stimulates PFK1
7
Q
substrate level phosphorylation
A
- occurs in glycolysis at steps catalysed by phosphoglycerate kinase and pyruvate kinase
- need compound with higher phosphoryl transfer potential than ADP
- hydrolysis of ATP has large -ve ^G; free energy is released
8
Q
control of glycolysis by phosphorylation
A
- in liver
- pyruvate kinase regulated by reverse phosphorylation
- phosphorylated form is less active
- phosphorylated by protein kinase A (PKA) stimulated by glucagon hormone when blood glucose is low
- when blood glucose is high, P is taken off by phosphoprotein phosphatase
9
Q
importance of producing lactate from pyruvate
A
- catalysed by lactate DH enzyme
- reduction using NADH; oxidation back to pyruvate using NAD+
- red blood cells: generate NAD+ for glycolysis; do not have mitochondria so rely on glycolysis for ATP production in glyceraldehyde-3-P DH reaction; no mitochondria no ETC to oxidise NADH back to NAD+
- in rapidly contracting muscle, [O2] is low so ox.ph cannot provide ATP so anaerobic glycolysis is used; regenerates NAD+ so glycolysis can take place
- lactate released into blood and transported to liver; liver cells are well oxygenated; ox. ph in mitochondria is used to generate ATP and NAD+; lactate in liver is oxidised back to pyruvate and used to synthesise glucose via gluconeogenesis
- part of cori cycle Glu is released back to blood taken by muscle and red blood cells for glycolsysis
10
Q
Pyruvate dehydrogenase reaction
A
- links glycolysis to TCA cycle
- occurs in mitochondrial matrix
- PDH large enzyme complex of 3 enzymes + 5 cofactors
- oxidative decarboxylation reaction (1. decarboxylation 2, oxidation 3, transfer to CoA)
- pyruvate (CH3-C=O,COO-) to acetyl coA (CH3-C=O,S-CoA) + CO2
- NAD+ is reduced to NADH
11
Q
NAD+ and NADH
A
- NAD+ is an oxidizing agent
- it is reduced to NADH
12
Q
Coenzyme A
A
- acts as an activated carrier of acyl groups (particularly acetyl)
- derived from pantothenic acid (Vit B5)
- CoA = adenine + ribose-3-P + phosphopantotheine
- has very reactive thiol group (SH) which reacts w/carboxylic acid to form thioester; hydrolysis of thioester bond has large -^G so acetyl group from CoA can be readily transferred to other molecules
13
Q
TCA cycle net reaction
A
- newly generates 3x NADH and 1x FADH2 in ox. phosphorylation
- 1x NADH enter in cycle generates 2.5 ATP
- 1x FADH2 enter in cycle generates 1.5 ATP
- reduced cofactors can carry e into ETC for ATP generation
- no new synthesis of C (oxaloacetate); 2C comes in, 2 comes out
14
Q
TCA cycle
A
- CONDENSATION by citrate synthase
- acetyl CoA + oxaloacetate = citrate
- methyl group of acetyl CoA to CH2 in citrate
- needs H2O; releases CoA-SH + H+ - ISOMERISATION by aconitase
- citrate to isocitrate
- reposition OH to set up decarboxylation in next step - OX. DECARBOXYLATION by isocitrate dehydrogenase
- isocitrate to a-ketoglutarate
- lose 1C as CO2
- NAD+ accepts 2e to form NADH - OX. DECARBOXYLATION by a-ketoglutarate dehydrogenase
- a-ketoglutarate to succinyl CoA
- CoA-SH comes in; 1C is lost as CO2
- requires NAD+ - SUBS-LEVEL PHOSPHO by succinyl CoA synthetase
- succinyl CoA to succinate
- CoA is hydrolysed; energy of thioester bond hydrolysis provides driving force for GTP/ATP synthesis from GDP/ADP + Pi - OXIDATION by succinate dehydrogenase
- succinate to fumarate
- introduces double bond
- uses FAD as e acceptor - HYDRATION REACTION by fumarase
- fumarate to malate
- water is added across double bond; introduces OH into malate - OXIDATION by malate dehydrogenase
- malate to oxaloacetate
- oxidation of OH to C=O
- uses NAD+
- regenerates oxaloacetate; available to react w/another acetyl CoA and cycle continues
15
Q
TCA building blocks to synthesise other molecules
A
- citrate used to synthesise fatty acids and sterols
- a-ketoglutarate and oxaloacetate important precursors for AAs
- oxaloacetate precursor for nucleotides
- succinyl CoA used to make porphyrins and haem
