Johnson - Metabolism (glycolysis) Flashcards
what is the first step in glycolysis and which enzyme catalyses it?
Glucose + ATP —> glucose-6-phosphate + ADP + H+
catalyzed by hexokinase
what is the second step in glycolysis and which enzyme catalyses it?
G6P —> F6P catalyzed by phosphoglucose isomerase
what is the third step in glycolysis and which enzyme catalyses it?
F6P + ATP —> F1,6BP + ADP + H+
catalyzed by phosphofructokinase
what is the fourth step in glycolysis and which enzyme catalyses it?
F1,6BP —> DHAP + GAP
catalyzed by aldolase
what is the fifth step in glycolysis and which enzyme catalyses it?
DHAP —> GAP
triose phosphate isomerase
what is the sixth step in glycolysis and which enzyme catalyses it?
G3P + NAD+ + Pi —> 1,3BPG + NADH + H+
catalyzed by glyceraldehyde 3-phosphate dehydrogenase
what is the seventh step in glycolysis and which enzyme catalyses it?
1,3BPG + ADP —> 3PG + ATP
catalyzed by phosphoglycerate kinase
what is the eighth step in glycolysis and which enzyme catalyses it?
3PG —> 2PG
catalyzed by phosphoglycerate mutase
what is the ninth step in glycolysis and which enzyme catalyses it (including its cofactor)?
2PG —> PEP + H2O
enolase and Mg2+
what is the tenth step in glycolysis and which enzyme catalyses it?
PEP + ADP + H+ —> pyruvate + ATP
pyruvate kinase
where does glycolysis take place?
in the cytosol
is O2 required for glycolysis?
no, but oxidation takes place (e- added or lost)
is ATP more or less stable than ADP + Pi?
why is this?
ATP less stable
1) -ve phosphate group repel each other
2) entropy is increased (as there are 2 molecules produced from 1)
3) water stabilises ADP + Pi (more interactions between them)
4) free Pi stabilised by resonance structures (isomerism due to e- movement)
does the hydrolysis of ATP require energy?
no, it releases energy (under cellular conditions: deltaG = -57kJmol-1)
how is ATP used to drive +deltaG reactions?
coupling mechanisms, -ve deltaG reactions drive unfavourable reactions (net free energy must be less than 0 for this to happen)
describe the free energy changes involved in the formation of G6P from glucose.
Pi transfer from ATP to glucose has v high Ea in absence of hexokinase.
Uncoupled ATP hydrolysis and Pi transfer to glucose is slow and has a +deltaG
Hexokinase lowers the Ea barrier for direct transfer of the Pi group and avoids hydrolysis
how does hexokinase catalyse the formation of G6P?
glucose binding to hexokinase causes conformational change
the hexokinase binding cleft closes (excludes H2O)
this favours direct Pi transfer and not the unwanted side reaction of ATP hydrolysis by water.
describe the mechanism of action of hexokinase
hexokinase AS contains Asp which deprotonates C6 ‘OH’ of glucose
deprotonated O:{-} acts as nucleophile and attacks last (gamma) Pi group of ATP
Pi group trasnferred directly to glucose without ATP hydrolysis by water
why does the first step of glycolysis happen? (glucose + Pi —> G6P)
G6P contains -ve Pi group meaning it cant cross the cellular membrane. also keeps cellular [glucose] low, which promotes glucose uptake
why is triose phosphate isomerase (TIM) used in step 5 of glycolysis?
TIM suppresses formation of toxic intermediate methyl glyoxal from the enediol intermediate (which is the faster reaction in free solution)
how does TIM catalyse step 5 of glycolysis?
stops toxic intermediate formation by movement of a 10 a/a loop region over the AS, blocking exit of the enediol until GAP is formed. means it enediol can’t form toxic methyl glyoxal
how many molecules of pyruvate are formed per glucose? why is this?
2 molecules of pyruvate, this is because glucose is 6C sugar and forms 2x 3C sugars. This means that steps 6-10 are done twice.
describe the 2 step in the oxidation of GAP to form 1,3BPG involving NAD+
oxidation is a 2 step, enzymatically-coupled process:
1) energetically favourable formation of aldehyde to acid
2) energetically unfavourable formation of acyl-phosphate
describe the mechanism of action of glyceraldehyde 3-phosphate dehydrogenase
G3PG Cys sidechain reacts with aldehyde in GAP to form hemithioacetal
His side chain removes H{+} from hemithioacetal facilitating its ox. by NAD+ (forms thioester intermediate)
NADH released
Pi group attacks thioester –> 1,3BPG
what is substrate level phosphorylation?
the formation of ATP by direct transfer of a Pi group from a high phosphoryl transfer compound to ADP
what does having a comparatively high phosphoryl transfer potential mean?
a molecule that has a stronger tendency to donate Pi group than another molecule
briefly describe the mechanism of phosphoglycerate kinase
the AS cleft closes around its substrates, excluding water
what is the significance of 2PG being isomerised to PEP?
PEP and 2PG have the same metabolic potential energy but PEP is a high-phosphoryl transfer potential compound and therefore releases more potential energy than 2PG when its Pi group is removed.
what are the final products of glycolysis?
2 x pyruvate
2 x ATP (4 made, 2 used)
2 x NADH
what happens after glycolysis under anaerobic conditions?
further ox of pyruvate can’t occur
this means glycolysis wont proceed for long due to the build up of NADH which cant be recycled in oxidative phosphorylation
therefore NADH used to reduce pyruvate to lactate/ethanol
