Lecture 4: Generation of reducing equivalents

Question 1

What is the chemical difference between NAD+ and NADP+?

Answer 1

NADP+ has a phosphate group where NAD+ has a hydroxyl group in the ribose of the adenosine. This phosphate tag enables enzymes to distinguish between NAD+ and NADP+.

Question 2

Do NAD and NADP carry electrons in the same or in different ways?

Answer 2

The same way

Question 3

How many electrons are carried by a) NAD+ and b) NADP+?

Answer 3

a) 2

b) 2

Question 4

What are NAD+ and NADP+ each used for?

Answer 4

NAD is an activated carrier of electrons for fuel oxidation and is reduced in the oxidation of substrates in glycolysis and the TCA cycle. NAD’s electrons are eventually passed to oxygen.
NADP acts as an electron carrier in reductive biosynthesis. High potential electrons are needed because the precursors are more oxidised than the products (e.g. in fatty acid biosynthesis).

Question 5

How is NADP mainly produced?

Answer 5

Via the pentose phosphate pathway

Question 6

What is the advantage of using different electron carriers for oxidation and reductive biosynthesis?

Answer 6

This means oxidation and reductive biosynthesis can be regulated separately.

Question 7

When is a high NAD+ concentration needed?

Answer 7

During catabolism (fuel oxidation)

Question 8

When is a high NADP concentration needed?

Answer 8

During anabolism (biosynthesis)

Question 9

What is the [NAD+] : [NADH] ratio in the cytoplasm?

Answer 9

High, about 700

Question 10

What is the [NADP+] : [NADPH] ratio in the cytoplasm?

Answer 10

Very low, about 0.005

Question 11

Name the two phases which the pentose phosphate pathway is split into.

Answer 11

Oxidative and non-oxidative phases

Question 12

What happens in the oxidative phase of the pentose phosphate pathway?

Answer 12

Glucose-6-phosphate is oxidised to 6-phosphogluconolactone by the enzyme glucose-6-phosphate dehydrogenase, with NADP+ being converted simultaneously to NADPH + H+. As NADP+ stimulates this step, it increases the rate of the oxidative phase.
Then the 6-phosphogluconolactone is converted into 6-phosphogluconate by the enzyme 6-phosphogluconolactonase, with the simultaneous conversion of H2O to H+.
Then the 6-phosphogluconogluconate is converted to ribulose-5-phosphate by 6-phosphogluconate dehydrogenase, using NADP+ and releasing NADPH and CO2 (oxidative decarboxylation)
Ribulose-5-phosphate then enters the non-oxidative phase of the pentose phosphate pathway.

Question 13

What is another name for the pentose phosphate pathway?

Answer 13

The pentose phosphate shunt

Question 14

What happens in the non-oxidative phase of the pentose phosphate pathway?

Answer 14

Ribulose-5-phosphate generated in the oxidative phase is isomerised to an aldehyde or a ketone. The aldehyde is called ribose-5-phosphate and the enzyme generating it is called ribulose-5-phosphate isomerase. The ketone is called xylulose-6-phosphate and the enzyme generating it is called ribulose-5-phosphate epimerase.

Then the enzyme transketolase converts one molecule of the aldehyde and one molecule of the ketone into one molecule of glyceraldehyde-3-phosphate (3C) and one molecule of sedoheptulose-7-phosphate (7C).

Then the enzyme transaldolase converts these into one molecule of fructose-6-phosphate and one molecule of erythrose-4-phosphate.

Question 15

What does the rate of the pentose phosphate pathway depend on?

Answer 15

The ratio of NADP+ to NADPH

Question 16

In which tissues is reductive biosynthesis common?

Answer 16

• Tissues which carry out fatty acid synthesis (liver, adipose tissue)
• Tissues which synthesise cholesterol/steroid hormones (liver, adrenal glands, ovary/testes)

Question 17

What is used to prevent oxidative damage in all cells, especially red blood cells?

Answer 17

NADPH

Question 18

What is xylulose-6-phosphate (ketone) converted to by transketolase if there is no ribose-5-phosphate (aldehyde)?

Answer 18

fructose-6-phosphate and glyceraldehyde-3-phosphate

Question 19

What is ribulose-5-phosphate used for?

Answer 19

Generating nucleotides for DNA and RNA, so the oxidative phase (when ribulose-5-phosphate is synthesised) is very important into rapidly dividing cells, e.g. skin, bone marrow, intestinal cells

Question 20

What happens to ribulose-5-phosphate if nucleotides are not needed?

Answer 20

If nucleotides are not needed, ribulose-5-phosphate enters the non-oxidative phase of the pentose phosphate cycle and is converted back into intermediates of glycolysis and gluconeogenesis. Glucose-6-phosphate is a particularly important generated intermediate in red blood cells, which are terminal cells so don’t divide. Therefore, they don’t need nucleotides, but do need NADH. They don’t want to waste carbons in the nucleotides, so the non-oxidative phase of the pentose phosphate pathway is important in red blood cells.

Question 21

Give the overall scheme for the pentose phosphate pathway.

Answer 21

In the oxidative phase 3 x Glucose-6-phosphate releases 6 NADPH and 3 CO2 to give 3 x Ribulose-5-phosphate.

In the non-oxidative phase some ribulose-5-phosphate becomes nucleotides. The rest becomes 3 x 5C sugars, which then become 2 x 6C sugars (Fructose-6-phosphate) and 1 x 3C sugar (Glyceraldehyde-3-phosphate), which are then recycled back to the start of the pathway (3 x glucose-6-phosphate).

Question 22

Describe the different fates of glucose-6-phosphate.

Answer 22

• enters glycolysis for ATP production, especially in brain and muscle (becomes pyruvate –> Lactate and CO2 + H2O
• converted to free glucose in the liver by glucose-6-phosphatase. The free glucose then enters the blood and becomes available for use by other tissues
• enters pentose phosphate pathway to give ribose and NADPH
• if the blood glucose concentration is high, glucose-6-phosphate may be converted into glucose-1-phosphate and then added to glycogen.