Metabolism 4- ATP production II

Question 1

Describe the important features of acetyl-coA

Answer 1

The thioester bond is a high-energy linkage, so it is readily hydrolysed, enabling acetyl CoA to donate the acetate (2C) to other molecules.

Question 2

Describe the first reaction of the TCA cycle

Answer 2

Oxaloacetate (4C) is converted into citrate (6C). reaction is catalysed by citrate synthase. The thio-ester linkage of acetyl CoA allows the acetyl group to be readily donated to oxaloacetate.

Question 3

Describe the second reaction of the TCA cycle

Answer 3

Isomerisation of citrate to give isocitrate. Catalysed by aconitase. OH group moved from 3C to 2C.

Question 4

Describe the third reaction of the TCA cycle

Answer 4

isocitrate (6C) undergoes oxidative decarboxylation and is converted into a-ketoglutarate (5C) by isocitrate dehydrogenase. One molecule of NADH is produced.

Question 5

Describe the fourth reaction of the TCA cycle

Answer 5

a -ketoglutarate (5C) undergoes oxidative decarboxylation and is converted into succinyl-CoA (4C). HS-coA produced from the first reaction donates H atom to form NADH. The reaction is catalysed by a-ketoglutarate dehydrogenase complex.

Question 6

Describe the fifth reaction of the TCA cycle

Answer 6

succinyl-CoA (4C) is converted to succinate (4C). CoA is displaced by a phosphate molecule which is subsequently transferred to GDP to form GTP. Reaction is catalysed by succinyl CoA
synthetase. Water is needed to hydrolyse thioester bond.

Question 7

In which tissues is GTP produced.

Answer 7

G-SCS, the GTP forming version is found predominantly in tissues that catalyse anabolic reactions e.g. liver
An alternative isoform of succinyl CoA synthetase (A-SCS) is found in skeletal and cardiac muscle and catalyses the same reaction but generates ATP from ADP.

Question 8

Describe the sixth reaction of the Krebs Cycle

Answer 8

succinate (4C) is oxidised into fumarate (4C) by succinate dehydrogenase. FAD is reduced to FADH2

Question 9

Describe the seventh reaction of the Krebs Cycle

Answer 9

Fumarate is converted into malate, catalysed by fumarase. Addition of a water molecule, breaking a double bond.

Question 10

Describe the final reaction of the TCA cycle

Answer 10

The last step. Dehydrogenation of malate to give oxaloacetate, the starting point of the cycle.
Catalysed by malate dehydrogenase- NAD reduced to NADH.

Question 11

What does each turn of the Krebs cycle produce

Answer 11

Each turn of the cycle produces two molecules of CO2 (waste) plus three molecules of NADH, one molecule of GTP and one molecule of FADH2.

Question 12

Describe the location of enzymes involved in the TCA cycle

Answer 12

The Krebs cycle enzymes are soluble proteins located in the mitochondrial matrix space, except for succinate dehydrogenase, which is an integral membrane protein that is firmly attached to the inner surface of the inner mitochondrial membrane. Here, it can communicate directly with components in the respiratory chain

Question 13

Describe the importance of reduced cofactors

Answer 13

The majority of the energy that derives from the metabolism of food is generated when the reduced coenzymes are re-oxidised by the respiratory chain in the mitochondrial inner membrane in a process known as oxidative phosphorylation (lecture 5).

The Krebs cycle only operates under aerobic conditions, as the NAD+ and FAD needed are only regenerated via the transfer of electrons to O2 during oxidative phosphorylation.

Question 14

What is the general strategy of amino acid degradation

Answer 14

The general strategy of amino acid degradation is to remove the amino group (which is eventually excreted as urea) whilst the carbon skeleton is either funnelled into the production of glucose or fed into the Krebs cycle. Degradation of all twenty amino acids gives rise to only seven molecules, pyruvate, acetyl CoA, acetoacetyl CoA, a-ketoglutarate, succinyl CoA, fumarate and oxaloacetate.

Question 15

What is transamination

Answer 15

Defined as a reaction in which an amine group is transferred from one amino acid to a keto acid thereby forming a new pair of amino and keto acids.
Carried out in the liver.

Question 16

Describe the metabolism of alanine

Answer 16

Alanine (C3) undergoes transamination by the action of the enzyme alanine aminotransferase.
Alanine a-ketoglutarate — pyruvate glutamate
Pyruvate can enter the TCA cycle, while glutamate is re-converted to a-ketoglutarate- used in TCA. by glutamate dehydrogenase, generating NH4+ which is ultimately converted to urea.
persistently high levels of alanine dehydrogenase can be diagnostic of hepatic orders such as Hep C.

Question 17

Describe the glycerol phosphate shuttle

Answer 17

Electrons from NADH, rather than NADH itself are carried across the mitochondrial membrane via a shuttle.
Cytosolic glycerol 3-phosphate dehydrogenase transfers electrons from NADH to DHAP to generate glycerol 3-phosphate.

2. A membrane bound form of the same enzyme transfers the electrons to FAD. These then get passed to co-enzyme Q, part of the electron transport chain (lecture 5).

Question 18

Describe the malate-aspartate shuttle

Answer 18

This system uses two membrane carriers and four enzymes.

A hydride ion (H-) is transferred from cytoplasmic NADH to oxaloacetate to give malate, a reaction catalysed by cytosolic malate dehydrogenase (MDH).

Malate can be transported into the mitochondria where it is rapidly re-oxidised by NAD+ to give oxaloacetate and NADH (catalysed by mitochondrial MDH).
A - alpha-ketoglutarate transporter, exchanges alpha-ketoglutarate for malate. malate in
B - glutamate/aspartate transporter, exchanges glutamate for aspartate aspartate out.

Question 19

Describe the role of transamination in the malate-aspartate shuttle

Answer 19

glutamate + oxaloacetate — aspartate + a-ketoglutarate

Question 20

How many ATP molecules does NADH and FADH2 produce

Answer 20

Re-oxidation of the reduced co-factors NADH and FADH2 by the process of oxidative phosphorylation (lecture 5) yields the following:

Three ATP molecules are formed by the re-oxidation of each NADH molecule.

Two ATP molecules are formed by the re-oxidation of each FADH2 molecule.

Therefore, from the Krebs Cycle:

Oxidation of 1 X acetyl CoA molecule gives 3 x NADH + 1 x FADH2 + 1x GTP = 12 ATP

Question 21

How can glycolysis and TCA cycle act as starting points for biosynthesis

Answer 21

A lot of the intermediates can be used as starting molecules for the biosynthesis of macromolecules.

Question 22

Describe how catabolic reactions are kept separate from anabolic reactions

Answer 22

NADPH takes part in anabolic reactions, whereas NADH takes place in catabolic reactions.

The use of different co-factors for sets of reactions allows electron transport in catabolism to be kept separate to that of anabolism.

Question 23

Describe the difference between NADP and NAD

Answer 23

NADP+ is a relative of NAD+, differing only by a phosphate group attached to one of the ribose rings.
Like NAD+, NADP+ can pick up two high energy electrons and in the process, a proton (H+) collectively known as a hydride ion (H-).
The phosphate group of NADP+ does not participate in electron transfer, but gives it a slightly different conformation, meaning that it will bind to different enzymes than NAD+.

The hydride ion is held in a high-energy linkage, allowing it to be easily transferred to other molecules.

Question 24

Describe some of the reactions in which NADP is a cofactor

Answer 24

Biosynthesis of cholesterol and thymidine.

Question 25

Describe the role of TCA defects in cancer

Answer 25

Mutations in the TCA genes shown in red (isocitrate dehydrogenase, succinate dehydrogenase, fumarase). have been shown to decrease TCA activity and enhance aerobic glycolysis: the preferential generation of lactate from glucose even under conditions of ample O2 (The Warburg Effect).

If we can force the cells to utilise oxidative phosphorylation instead, can we turn them into non-malignant cells?