Pyruvate dehydrogenase and the TCA cycle

Question 1

Outline an equation for the link reaction and outline some features of this reaction

Answer 1

pyruvate + NAD+ +Co A–> acetyl CoA + NADH +CO2 (irreversible, rate limiting and tightly regulated allosterically, covalently and hormonally)
Controls entry pyruvate to TCA cycle

Question 2

What enzyme catalyses this reaction?

Answer 2

Pyruvate dehydrogenase- multienzyme complex (3 enzyme complexes), 5 coenzymes and 4-10 million Daltons (huge)

Question 3

Outline 3 enzymes within PDH

Answer 3

• E1- pyruvate- decarboxylase- shell surrounding E2
• E2- dihydrolipoyl transacetylase- internal core
• E3- Dihydrolipoyl dehydrogenase- shell surrounding E1

Question 4

5 coenzymes of PDH

Answer 4

• thiamine pyrophosphate (TPP)- B1
• Lipoamide- lipoic acid (10 C Fatty acid)
• CoA- pantothenic acid (B5)
• FAD+- riboflavin (B2) prosthetic group
• NAD+- niacin (B3)

Question 5

Outline mechanism of PDH

Answer 5

1) decarboxylation of pyruvate- need TPP- bound to E1 subunit- We end up with 2 C hydro ethyl TPP and CO2
2) Then oxidised lipoamide (long 10C fatty acid chain) binds with hydroxyethyl TPP and Hydroxyethyl–> acetyl group hence acetyl lipoamide formed
3) Acetyl group is linked to CoA forming to acetyl CoA and lipoamide reduced
4) Now we need to reform oxidised lipoamide
5) Reduced lipoamide reacts with FAD regenerating oxidised lipoamide and producing FADH2
6) Covalently bound to PDH so can’t take electrons to ETC
Therefore donates electrons to NADH

Question 6

Explain how PDH is regulated

Answer 6

Activated in dephosphorylated from

• insulin, Ca activates phosphatase which dephosphorylates PDH so in active form (REM activates glycogen synthase also as XS glucose so need to store and make ATP)
• ATP, NAD, acetyl CoA activates kinase forming phosphorylated inactive form PDH (if XS ATP pyruvate can be used for many other processes e.g make fa/ aa)
• Pyruvate inhibits kinase keeping PDH in the active dephosphorylated form

Question 7

Outline a medical problem associated with lack thiamine

Answer 7

Beri- beri: damage to PNS and weakened muscle–> CV disorders
(as not produce active LDH so little ATP)
- problem in far east where rice main component of diet as thiamine (B1) only found in husk- removed
- mercury and arsenite poisoning- binds to lipoamide on E2 subunit- cant make PDH and ATP
- vitamin deficiencies- riboflavin (FAD) and niacin (NAD)
- genetic defect with PDH gene- leading to deficiency- lacticacidemia, less ATP, more alanine

Question 8

Outine what NAD+ and FAD+ can accept as electron carriers

Answer 8

NAD+ +2H+ +2e–> NADH + H+

FAD+ +2H+ +2e- –> FADH2

Question 9

Outline steps in TCA where NADPH and CO2 is formed

Answer 9

isocitrate to alpha ketoglutarate via isocitrate dehydrogenase
alpha ketoglutarate to succinyl CoA via alpha ketoglutarate dehydrogenase

Question 10

Outline steps in TCA cycle where GTP is formed

Answer 10

succinyl CoA to succinate via succinate synthase

Question 11

Outline steps where FADH2 is formed

Answer 11

succinate to fumarate via fumarase

Question 12

Explain how the TCA cycle is controlled

Answer 12

isocitrate dehydrogenase inhibited by NADH and ATP activated by ADP
alpha ketoglutarate inhibited by succinyl CoA , NADH and ATP.

Question 13

What is the overall reaction for each molecule of acetyl CoA that enters the TCA cycle

Answer 13

3X NADH, 1 x FADH2, 2 x CO2, 1 X GTP

Question 14

Where does the link reaction and TCA cycle take place

Answer 14

Matrix of mitochondria