W2 Glycolysis and Metabolic Fates of Pyruvate

Question 1

step 1 of glycolysis

Answer 1

transfer of phosphate from ATP to glucose to form glucose 6P via hexokinase

phosphorylation reaction is irreversible under physiologic conditions due to high negative free energy

involved nucleophilic attack of C6-OH of glucose on electrophilic terminal phosphate of ATP

Question 2

step 2 of glycolysis

Answer 2

isomerization reaction of glucose 6P to fructose 6P catalysed by phosphoglucose isomerase

Question 3

why is isomerization reaction necessary

Answer 3

the “moving” of carbonyl from C1 to C2 creates a new primary alcohol function at C1 > becomes easily phosphorylated

activation of C3 > facilitating C-C bond cleavage in step 4 of glycolysis, instead of producing two- and four-C fragments

Question 4

step 3 of glycolysis

Answer 4

phosphorylation of fructose 6P to fructose 1,6-biphosphate by phospho-fructokinase (PFK-1)

C1-OH of F6P acts as nucleophile > attacks electrophilic gamma phosphate

reaction is thermodynamically and kinetically irreversible

Question 5

step 4 of glycolysis

Answer 5

fructose 1,6-bisP cleaved into two phosphorylated 3C compounds, G3P and DHAP by aldolase

reaction is reversible: aldol cleavage for forward reaction and aldol condensation for reverse reaction

Question 6

what is a schiff base

Answer 6

an imine, nucleophilic addition product between amin and carbonyl group

Question 7

mechanism for fructose 1,6-bisP aldolase reaction

Answer 7

nucleophilic attack on keto carbon at C2 with lysine elipson amino group in active site, facilitated by protonation of carbonyl oxygen by an active site > carbinolamine > dehydration > protonated schiff base

reto-aldol reaction cleaves protonated schiff base > examine + GAP > enamine protonated to give another protonated schiff base > hydrolysed to give 2nd product, DHAP

Question 8

step 5 of glycolysis

Answer 8

isomerization of DHAP to G3P by isomerase

Question 9

step 6 of glycolysis

Answer 9

triose phosphate dehydrogenase oxidises aldehyde group of G3P into enzyme-bound carbonyl group > transfers electrons to NAD+ to form NADH

oxidation step dependent on cysteine residue at active site of enzyme> forms high energy thioester bond > accepts inorganic phosphate > forms 1,3-biphosphoglycerate > substrate level phosphorylation (formation of high energy phosphate bond)

Question 10

step 7 of glycolysis

Answer 10

phosphate from high energy phosphate bond in step 6 transferred from ADP to form ATP by phosphoglcyerate kinase > produce 3-phosphoglycerate

Question 11

step 8 of glycolysis

Answer 11

phospho-glyceromutase moves phosphate from C3 in 3-phosphoglcyerate to C2 > produce 2-phosphoglcyerate

Question 12

step 9 of glycolysis

Answer 12

enolase removes water from 2-phosphoglcyerate > phosphoenol-pyruvate (PEP)

Question 13

step 10 of glycolysis

Answer 13

pyruvate kinase converts PEP to pyruvate

(enolphosphate bond is a high-energy bond > transfer of phosphate to ADP by pyruvate kinase energetically favourable)

Question 14

products of glycolysis

Answer 14

2 pyruvate + 2 H2O
net gain of 2 ATP
2 NADH

Question 15

steps in glycolysis that are irreversible

Answer 15

1, 3, 10

Question 16

regulatory sites in glycolic pathway

Answer 16

hexokinase and phosphofructokinase-1 major regulatory enzymes in skeletal muscle

pyruvate dehydrogenase determines whether pyruvate is converted into lactate or acetyl coenzyme A

Question 17

regulation of PFK-1

Answer 17

usual levels of ATP > saturate substrate-binding site > ATP bind to ATP allosteric site > decrease glycolysis

low levels of ATP > high levels of AMP > AMP bind to allosteric activator site > increase affinity of enzyme for fructose 6-P > increase glycolysis

Question 18

3 common metabolic pathways glucose 6-phosphate can enter

Answer 18

glycolysis, pentose phosphate pathway and glycogen synthesis

Question 19

function of glycolysis

Answer 19

main: provide ATP as energy source

in addition: generates precursors for biosynthetic pathways

Question 20

process of anaerobic respiration

Answer 20

after glycolysis, pyruvate acts as electron carrier > reduced by NADH to lactate > NAD+ regenerated

reaction catalysed by lactate dehydrogenase

only 2 ATP molecules produced per glucose oxidised

Question 21

fate of lactate

Answer 21

lactate released from cells taken up by liver, heart and skeletal muscle > oxidised back to pyruvate

Question 22

what is cori cycle

Answer 22

the cycling of lactate and glucose between peripheral tissues and the liver

Question 23

2 reactions required in anaerobic conversion of pyruvate to ethanol

Answer 23

pyruvate decarboxylase catalyse irreversible decarboxylation of pyruvate (requires Mg2+ and TPP) into acetaldehyde

NADH reduces acetaldehyde to ethanol, catalysed by alcohol dehydrogenase > NAD+ regenerated

Question 24

which steps in glycolysis are nadh and atp produced

Answer 24

1 & 3: atp CONSUMED

6: 2 NADH produced

7 & 10: 4 atp PRODUCED (2 at each step)