Biochemistry 1 Chapter 16 : Citric Acid Cycle

cellular respiration

Process that releases energy by breaking down glucose and other food molecules in the presence of oxygen

what need to happen to Pyruvate before the citric acid cycle can start?

it needs to be converted to acetyl coa

What enzyme complex converts pyruvate to acetyl CoA by oxidative decarboxylation?

pyruvate dehydrogenase complex (PDH)

Acetyl coa structure

Oxidative decarboxylation

oxidation reactions in which a carboxylate group is removed, forming carbon dioxide

when pyruvate is converted to acetyl CoA is it oxidized or reduced?

Oxidized

How many enzymes are part of the pyruvate dehydrogenase complex (PDH)?

3 enzymes, given abbreviations E1 (pyruvate dehydrogenase), E2 (dyhrdeoxyllipoyl transacytylace) and E3 (dihydrollipoyl dehydrogenase)

What does pyruvate dehydrogenase (E1) do?

Decarboxylation: Cleaves the bond of pyruvate, releasing a molecule of CO₂.

Intermediate Formation: Binds the remaining two-carbon fragment (hydroxyethyl group) to the TPP cofactor.

Acyl Transfer: Helps transfer the acetyl group onto the lipoyl-lysine swinging arm of the E2 component

What are the benefits of having 3 enzymes in one complex?

- everything can be regulated as one unit instead of as multiple

-intermediates dont need to travel far so there are less side reactions and its faster

What does dihydrolipoyl transacetylase (E2) do?

hydroxyethyl-TTP interacts with lipoamide and CoA to form acetyl CoA. TTP is regenerated.

What does dihydrolipoyl dehydrogenase (E3) do?

Catalyzes the reoxidization of lipoic acid to allow the process to restart by reducing FAD to FADH2

When this FADH2 is later oxidized it is used to reduce NADH

What cofactors are involved in citric acid cycle?

Coenzyme A

Lipoic Acid

NAD+

TPP

FAD

What does coenzyme A do?

Transfers acetate between molecules

What does NAD+ do in PDH?

substrate for E3 and accepts a hydride/ is reduced by FADH2

What does TPP do?

decarboxylates pyruvate

What does FAD do?

It accepts electrons doing redox reactions (is reduced)

To what enzyme is TPP bound?

E1

To what enzyme is Lipoic acid bound?

E2

Which cofactor is like a swinging arm that interacts with both E1 and E2?

Lipoic acid

What enzyme is coenzyme A bound to?

Disociably bound to E2

Where is FAD in the PDH complex?

Bound to E3

What is released when PDH converts pyruvate to acetyl CoA

CO2 and NADH

Is the conversion of Pyruvate into Acetyl CoA reversible?

No, its irreversible because its favorable to lose CO2 when pyruvate is converted

citric acid cycle (CAC or TCA cycle)

A chemical cycle involving eight steps that completes the metabolic breakdown of glucose molecules to carbon dioxide;

Where in the cell does the citric acid cycle occur?

Mitochondria

Step 1 of CAC

Acetyl CoA + Oxaloaetate ---> Citrate + CoASH

Enzyme is Citrate Synthase

IRREVERSIBLE, Product Inhibition

Cofactors inout and output in step 1 of CAC when acetyl CoA is turned into citrate

Input: H2O

output: H2O, HSCoA

Step 2 of CAC

citrate converted to isocitrate by aconitase

REVERSIBLE

What is the intermediate that citrate goes through before becoming isocitrate?

Aconitate

What does lipoic acid do?

covalently linked to lysine on E2; accepts the carboanion from E1 as an acetyl group

Step 3 of CAC

isocitrate undergoes oxadative decarboxylation to give a-Ketogluterate

(IRREVERSIBLE)

Enzyme used to convert isocitrate to a-ketogluterate

Isocitrate dehydeogenase

What is the cofactor input to convert isocitrate to a-ketogluterate, and what are the cofactor output(s)?

Input: NAD+

Output: NADH and CO2

step 4 CAC: oxidative decarboxylation

alpha-ketoglutarate ----> Succinyl CoA

Enzyme is alpha-ketoglutarate dehydrogenase

NADH Created

Irreversible

Enzyme used to turn a-ketogluterate into succinyl-CoA

alpha-ketoglutarate dehydrogenase

What cofactors are used and released in step 4 of the CAC when a-ketogluterate is turned into succinyl-CoA

Input: NAD+, Coenzyme A

output: NADH, CO2, H+

Step 5 of CAC(substrate level phosphorylation)

succinyl-CoA turned into succinate

enzyme: succinyl CoA synthase

reversible

Step 6 of CAC (Oxidation of an alkane)

succinate turned into fumarate

Enzyme that turns succinate into fumarate

Succinate dehydrogenase

What cofactor is input and output in turning succinate into fumerate?

input: FAD

output: FADH2

Why is FAD used instead of NAD+ in the conversion of succinate to fumarate in the citric acid cycle?

the redox potential of NAD is too low, so FAD is used instead. FAD also is more tunable

Step 7 of the CAC (Hydration)

conversion of fumarate to malate

what enzyme catalyzes the conversion of fumarate to malate

fumerase

what molecule is used in the hydration of fumerate to malate

H2O

step 8 of the CAC (oxidation of an alcohol to a ketone)

conversion of malate to oxaloacetate

What enzyme catalyzes the conversion of malate to oxaloacetate?

malate dehydrogenase

what cofactors are input and output in the conversionof malate to oxaloacetate?

input: NAD+

output: NADH

how many moles of CO2 do you get per pyruvate after the citric acid cycle? How about per glucose?

2 Moles per pyruvate, 4 moles per glusse

how many moles of NADH are produced by the citric acid cycle per mole pyruvate?

3 mol

How many moles of FADH2 are made by the CAC per mole pyruvate?

1 mol

How many moles of ATP(of GTP) are made by the CAC per mole pyruvate?

1 mol

what signs down regulate (deactivate) the citric acid cycle?

signs of high energy

what signs upregulate (activate) the citric acid cycle

signs of low energy

NAD+

CoA (substrate activation)

does insulin increase or decrease PDH complex activity?

increase

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