Johnson - Metabolism (Link and Krebs)

Question 1

what is the first step in link and which enzyme catalyses it?

Answer 1

Pyruvate +TPP —> hydroxyethyl-TPP + CO2

catalysed by pyruvate decarboxylase

Question 2

what is the second step in link and which enzyme catalyses it?

Answer 2

Hydroxyethyl-TPP and lipoamide form acetyl-dihydrolipoamide. This reacts with CoA forming acetyl CoA and a reducing lipoamide. This is catalysed by dihydrolipoyl transacetylase

Question 3

what is the third step in link and which enzyme catalyses it?

Answer 3

The reducing lipoamide is oxidised (donates 2H+ and 2e-) by FAD (bound to enzyme dihydrolipoyl dehydrogenase) forming FADH2. FADH2 is oxidised by NAD+ forming NADH + H+ (NAD+ holds 2e- and 1H+)

Question 4

what is the first step in krebs and which enzyme catalyses it?

Answer 4

Acetyl-CoA + H2O + oxaloacetate —> citrate + CoA
(2H from acetyl-CoA and 2H from H2O to make citrate)
catalysed by citrate synthase

Question 5

what is the second step in krebs and which enzyme catalyses it?

Answer 5

citrate —> isocitrate

aconitase (aconitate intermediate)

Question 6

what is the third step in krebs and which enzyme catalyses it?

Answer 6

isocitrate —> alpha-ketoglutarate + CO2
(uses NAD+)
isocitrate dehydrogenase

Question 7

what is the fourth step in krebs and which enzyme catalyses it?

Answer 7

alpha-ketoglutarate + CoA —> CO2 + succinyl-CoA
(uses NAD+)
alpha-ketoglutarate dehydrogenase

Question 8

what is the fifth step in krebs and which enzyme catalyses it?

Answer 8

succinyl-CoA + H2O + ADP + Pi —> ATP + succinate + CoA

succinyl-coA synthetase

Question 9

what is the sixth step in krebs and which enzyme (and cofactor) catalyses it?

Answer 9

Succinate ---> fumarate
succinate dehydrogenase (uses FAD)

Question 10

what is the seventh step in krebs and which enzyme catalyses it?

Answer 10

fumarate + H2O —> Malate

fumarase

Question 11

what is the eighth step in krebs and which enzyme catalyses it?

Answer 11

malate ---> oxaloacetate 
malate dehydrogenase (uses NAD+)

Question 12

describe the structure of the pyruvate dehydrogenase complex

Answer 12

made of 3 enzymes; pyruvate decarboxylase (PD), dihydrolipoyl transacetylase & dihydrolipoyl dehydrogenase. approximately 60 polypeptide chains

Question 13

what is the key linkage in acetyl-CoA?

Answer 13

thioester linkage, the hydrolysis of which is energetically favourable.

Question 14

which part of acetyl-CoA is oxidised in krebs?

Answer 14

acetyl fragment

Question 15

how does the pyruvate dehydrogenase complex increase efficiency?

Answer 15

uses substrate channeling instead of products diffusing to next active site they are passed (increases reaction rate and efficiency as well as decreasing unwanted side reactions) the lipoamide cofactor ‘fishes’ for hydroxyethyl substrate (carries it from e1 to e2 to e3)

Question 16

are fats more or less reduced than sugars? what is the significance of this upon oxidation?

Answer 16

fats are more reduced than sugars therefore they generate more energy when oxidised

Question 17

what does a lipase do?

Answer 17

breaks down fats into glycerol and 3 f/a’s

Question 18

how does glycerol enter glycolysis?

Answer 18

glycerol —> DHAP

Question 19

how do fatty acids enter glycolysis?

Answer 19

f/a beta-oxidation
f/a’s activated by linkage to Acetyl-CoA (requires ATP –> AMP + Pi) and then transported into mitochondria. 4 enzymes then oxidise the f/a-CoA one carbon unit at a time. The -ve deltaG reactions are coupled to NADH and FADH2 formation

Question 20

describe the types of functional groups and linkage that must be present in order for C-C cleavage to occur

Answer 20

C-C cleavage between:
adjacent carbons, with a carbonyl next to one of the carbons
an alpha-hydroxyketone (hydroxyl on alpha C)