Lecture 31 Pyruvate Fates

why fermentation exists

body has a limited pool of NAD+ and NADH has to be turned back into NAD+ to keep glycolysis running which is done when oxygen is present bc ETC recycles NADH to NAD+

if no oxygen

there is not ETC so it cant recycle NADH to NAD+ but glycolysis still needs NAD+ so pyruvate undergoes fermentation to recycle NADH back to NAD+

fermentation: what gets reduced and oxidized

pyruvate is reduced to lactate and NADH is oxidized to NAD+ to keep glycolysis going

glucose H:C ratio

12:6 which is 2

pyruvate H:C ratio

1.33

lactate C:H ratio

2

pyruvate and glucose C:H ratio

fermentation does not produce a net oxidation beyond glucose level overall but pyruvate itself is more oxidized than glucose and CO2 is highly oxidized

Alcoholic fermentation in yeast

step 1: Pyruvate → acetaldehyde + CO2

step 2: acetaldehyde → ethanol

step 1: Pyruvate → acetaldehyde + CO2

decarboxylation reaction that starts with 3 carbons in pyruvate and end with 2 carbon acetaldehyde and 1 carbon in CO2

only in yeast

enzyme used in step of alcohol fermentation in yeast

pyruvate decarboxylase

step 2: acetaldehyde → ethanol

aldehyde is more oxidized than ethanol so acetaldehyde is being reduced and NADH is oxidized to NAD+ by enzyme alcohol dehydrogenase

Lactic acid fermentation

pyruvate → lactate, pyruvate gets reduced bc it has a ketone and lactate has a alcohol and NADH is oxidized to NAD+ by the enzyme lactate dehydrogenase

redox rxn

Quick recap

alcoholic fermentation step 1 = decarboxylation

enzyme = pyruvate decarboxylase

enzyme is present only in yeast

Then shifts to the aerobic pathway

Pyruvate → acetyl-CoA: the pyruvate dehydrogenase complex reaction

occurs in presence of oxygen in the mitochondrial matrix where dehydrogenase enzymes oxidizes their substrates

rxn is oxidative decarboxylation

similarities b/t alcoholic fermentation in yeast and lactic acid fermentation

occur when there’s no O2, regenerate NAD+, and both make 2 ATP per glucose from glycolysis

difference b/t alcoholic fermentation in yeast and lactic acid fermentation

lactic acid fermentation happen in muscle cells where NADH gives electrons to pyruvate and produces lactate glucose → 2 lactate + 2 ATP

alcoholic fermentation happen in yeast where pyruvate loses CO2 and acetaldehyde gets reduced by NADH and produces ethanol and CO2

Pyruvate → acetaldehyde → ethanol +CO2

PDH overall reaction

pyruvate + NAD+ + coenzyme A → acetyl-CoA + NADH + CO2 + H+

Pyruvate dehydrogenase complex organization

3 enzymes and 5 coenzyme

Coenzymes help enzymes carry out their function

PDH complex enzyme 1:

pyruvate oxidative decarboxylase and permanently bound to TPP (coenzyme)

PDH complex enzyme 2:

dihydrolipoyl transacetylase that transfers the acetyl group

coenzymes are 'lipoic acid / lipoamide — permanently bound and coenzyme A — diffusing substrate

PDH complex enzyme 3:

dihydrolipoyl dehydrogenase that oxidizes reduced lipoamide back toward original state

coenzymes FAD — permanently bound and NAD+ — diffusing substrate

Permanently bound coenzymes

Must be restored to original state within the complex

TPP, lipoic acid, and FAD

Diffusing coenzymes

Not restored inside this same complex

TPP (thiamin pyrophosphate) characteristics

Coenzyme for enzyme 1 that has a special thiazolium ring that contains a sulfur and nitrogen, it also has a acidic proton

Acidic proton in TPP

if TPP loses the proton the negative charge on carbon would get stabilized by the nearby positive nitrogen in the ring

TPP’s role

acts as a carbon carrier and the ring acts like an electron sink, it helps stabilize the reactive intermediate formed after attack on pyruvate

lipoic acid (Lipoamide)

coenzyme for enzyme 2 that is a carrier of both electrons and carbons, has a disulfide bond as the active site, and can shift between: oxidized disulfide form and reduced dithiol form

lipoic acid (Lipoamide) mechanics

Because it has a long flexible arm, it can swing/react between sites: pick up acetyl/electrons, transfer acetyl to CoA, and transfer reducing equivalents onward

Big PDH mechanism summary step 1

Enzyme 1

• pyruvate undergoes oxidative decarboxylation

• forms TPP-bound intermediate

• CO2 released

Big PDH mechanism summary step 2

• lipoamide accepts intermediate/reducing equivalents

• acetyl group eventually transferred to CoA

• forms acetyl-CoA

• lipoamide becomes reduced

Big PDH mechanism summary step 3

• reduced lipoamide must be reoxidized

• FAD participates

• then NAD+ is reduced to NADH