Metabolism 4 - The Krebs Cycle

Question 1

What is acetyl coA?

Answer 1

A molecule which contains a high energy thioester bond, which is hydrolysed to donate acetate to other molecules.

Question 2

How does the krebs cycle start?

Answer 2

Oxaloacetate is converted to citrate by the addition of acetyl-coA.
Enzyme - Citrate synthase
Makes HS-coA

Question 3

What is the second reaction of the krebs cycle?

Answer 3

Citrate is converted to isocitrate by aconitase. (Isomerisation)

Question 4

What is the third reaction of the krebs cycle?

Answer 4

Isocitrate is converted to a-ketoglutarate by isocitrate dehydrogenase (using NAD+ and releasing CO2)

Question 5

What is the fourth reaction of the krebs cycle?

Answer 5

a-ketoglutarate is converted to succinyl coA. Uses NAD+, HS-CoA and catalysed by the a-ketoglutarate dehydrogenase complex.

Question 6

What is the fifth reaction of the krebs cycle?

Answer 6

Succinyl-CoA is converted to succinate using succinate CoA synthetase. This reaction makes GTP.

Question 7

What is reaction 6 of the Krebs cycle?

Answer 7

Succinate is converted to fumarate using succinate dehydrogenase. This enzyme is located at the inner mitochondrial membrane, and FAD is reduced.

Question 8

What is the seventh stage of the krebs cycle?

Answer 8

Fumarate is converted to malate, with the addition of H2O. This is catalysed by fumarase

Question 9

What is the final stage of the krebs cycle?

Answer 9

The dehydrogenation of malate, catalysed by malate dehydrogenase, to reform oxaloacetate.

Question 10

What are the net products of the krebs cycle?

Answer 10

There are two molecules of CO2 produced, three molecules of NADH, one molecule of FADH2, and one molecule of GTP.

Question 11

What is the location of the enzymes in the krebs cycle enzymes?

Answer 11

All the enzymes, bar succinate dehydrogenase, are foudn in the mitchondrial matrix. Succinate dehydrogenase is in the inner mitochondrial membrane, it communicates directly with components in the electron transport chain.

Question 12

How does degradation of amino acids occur?

Answer 12

The amino group is removed and excreted as urea. This is done by a transamination reaction. The carbon skeleton is funnelled into glucose production or enters the Kress cycle.

Question 13

Define a transamination reaction

Answer 13

A reaction where an amino group is transferred from one amino acid to a keto acid, forming a new pair of amino and keto acids

Question 14

Describe the transamination of alanine

Answer 14

Alanine reacts with a-ketaglitarate by alanine aminotransferase. This makes pyruvate and glutamate.

Question 15

What are the two pumps involved in NADH transportation?

Answer 15

The glycerol phosphate shuttle and the malate aspartate shuttle.

Question 16

Where is the glycerol phosphate shuttle used?

Answer 16

Skeletal muscle and brain

Question 17

Where in the body is the malate-aspartate shuttle used?

Answer 17

The liver, kidney and heart.

Question 18

Describe the process that occurs in the glycerol phosphate shuttle

Answer 18

• Glycerol 3-phosphate dehydrogenase transfers electrons from NADH to dihydroxyacetone phosphate to generate glycerol 3-phosphate (cytosol)
• A membrane bound form of the same enzyme transfers electrons to FAD
• Electrons are passed on to coenzyme Q

Question 19

Describe the reactions that occur in the Malate-Aspartate shuttle.

Answer 19

• The main reactions are oxaloacetate taking an electron from NAD to make malate, which is then transported across the membrane and oxidised to make NADH.
• So this can continue, transamination reactions occur. Oxaloacetate and glutamate react to form a-ketoglutarate and aspartate, and vice versa

Question 20

Why do transamination reactions occur in the malate-aspartame shuttle?

Answer 20

• Malate travels into the mitochondria through an alpha-keyoglutarate transporter.
• A glutamate aspartate transporter is also needed so the oxaloacetate can react with glutamate

Question 21

How are catabolic and anabolic pathways kept separate?

Answer 21

NADPH is used in anabolic reactions, and NADH is used in catabolic reactions. This keeps electron transport separate.

Question 22

How are NADP+ and NAD+ different?

Answer 22

They are different by one phosphate group.

Question 23

Give examples of pathways where NADP is used.

Answer 23

Thymidine synthesis and the synthesis of cholesterol.