Biochemistry- carbohydrate metabolism 2

Question 1

Describe the importance of NAD+

Answer 1

The [NAD+] in the cytosol is low. As it is used up for form NADH, its concentration
further decreases. Mitochondria can use the NADH with oxygen to produce more NAD,
to allow glycolysis to further continue. However, this is only possible if mitochondria
and a good supply of oxygen is available.

 If mitochondria and oxygen are not available, producing lactate is a method to continue
producing NAD to allow glycolysis to continue.

 Lactate is made at low levels by most tissues, but more is made when oxygen is scarce
of there are few mitochondria (e.g. erythrocytes).

 Can be taken up by other tissues (e.g. brain, liver, heart) and metabolised.

 Can be catabolized (back to pyruvate) or used for gluconeogenesis.

Question 2

Describe the glycerol- 3-P shuttle

Answer 2

Less efficient than the malate-aspartate shuttle as FADH2 yields less ATP than NADH. If we just relied on the malateaspartate shuttle, then we would be limited in how quickly complex 1 can function, therefore having the glycerol-3-P shuttle
aids the speed (not efficiency) of glycolysis.

 Provides an additional method of regenerating cytosolic NAD, less lactate production needed during high rates of glycolysis.
 Highly expressed in brown adipose tissue, skeletal muscle and brain (most likely to support high rates of glycolysis in these
tissues).
 The glycerol-3-P can also be used for triacylglycerol synthesis in some conditions.

Question 3

Describe the pentose phosphate pathway including the oxidative and non oxidative stage

Answer 3

There is an alternative pathway for glucose, and serves different purposes for different cells.
 NADPH:
o Used as the reducing power for biosynthetic reactions, lipid or cholesterol synthesis
(e.g. in the liver, adipose tissue, adrenal glands, gonads).
o Protects against oxidative damage, particularly in cells exposed to oxidative damage (erythrocytes, cells of the lens
and cornea).
 Ribulose 5-phosphate: can be used for the synthesis of DNA, RNA and coenzymes. This is important for rapidly dividing cells
(bone marrow, skin, intestinal mucosa).
o Cells that don’t require the pentose sugars can combine them into glycolytic intermediates.

1. Oxidative stage (for when 5 carbon sugars are useful)
    Generates NADPH, and five-carbon sugars that can be used for the synthesis of RNA, DNA and co-enzymes
2. Non-oxidative stage (for when 5 carbon sugars are not useful and just want the NADPH)
    Unneeded 5 carbon sugars can be fed back into glycolysis, or glycolytic intermediates can be used to make 5 carbon sugars.
    Transketolase: transfers 2 carbon atoms. Produces glyceraldehyde 3-phosphate.
    Transaldolase: transfers 3 carbon atoms. Produces fructose -6-phosphate.

Question 4

Describe oxidative stress (as a result of the electron transport chain)

Answer 4

The production of NADPH by the pentose phosphate pathway
is important in protection against oxidative stress.

 Catalase: has four bound NADPH that prevent the hydrogen
peroxide from damaging it.
 NADPH maintains most of the cellular glutathione in a reduced
state, which is used to deal with damaging lipid peroxides.
 Glutathione is also probably involved in maintaining exposed
protein sulphydryl groups in a reduced form.
 Glutathione peroxidase: a class of enzymes that help to reverse
the damage caused by oxidative stress.

Question 5

Describe Glucose-6-P dehydrogenase deficiency

Answer 5

The most common human enzyme defect, affecting 400 million worldwide. Most are asymptomatic

 140 mutations of the G6PD gene have been identified, mostly single-base substitutions that affect the stability of the
enzyme.
 Because erythrocytes have no mitochondria, the pentose phosphate pathway is their only source of NADPH.
 Oxidative stress caused by infections and drugs puts too much stress on the erythrocytes, which lyse, causing haemolytic
anaemia.
 This deficiency appears to offer some protection against the malaria parasite, which is sensitive to oxidative stress. Some
anti-malarial drugs take advantage of this weakness. Malaria parasite are sensitive to oxidative stress.

Question 6

Describe glycogenolysis

Answer 6

There are different types of bonds in glycogen, so different enzymes are
required to break it down. Phosphorylase is the main enzyme. It doesn’t break the bonds by hydrolysis, but through
phosphorolysis. This means that the product is phosphorylated.
 Muscle: glycogen is broken down primarily in response to exercise, for its own use.
 Liver: glycogen is broken down primarily during fasting. Regardless, the purpose of the glycogen is to supply glucose to
other tissues. Unlike muscle, liver contains G-6-Pase which produces glucose that can be released into the blood via the
glucose transporters.

Question 7

Describe 3 glycogen storage disease

Answer 7

 Lack of glucose-6-phosphatase (GSD-Ia, Van Gierke disease): most common and severe GSD. Rapid hypoglycaemia after
meals. Trapped G-6-P feeds into glycogenesis and fat synthesis and glycolysis leading to hyperlactacidaemia. Liver becomes
enlarged (may be noticeable at birth).
 Muscle phosphorylase deficiency (GSD-V, McArdle disease): one of the most common GSDs, range in severity. Often
diagnosed in adults. Cannot metabolise muscle glycogen, resulting in muscle weakness and poor tolerance of exercise.
Regular moderate exercise, with a short warm-up (10-15 minutes) improves exercise capacity.
 Liver phosphorylase deficiency (GSD-VI, Hers disease): cannot mobilise liver glycogen, liver becomes enlarged, mild
hypoglycaemia (gluconeogenesis can still provide glucose during fasting).

Question 8

How is gluconeogenesis regulated

Answer 8

Gluconeogenesis is required in a range of metabolic states, and is
strongly linked to amino acid catabolism.
 Gluconeogenesis is decreased in the fed state, but declines fairly
slowly, because the key proteins are largely regulated at the level
of transcription.
 In diabetes, the lack of insulin signalling means that gluconeogenesis continues at high levels in the fed state.
 FOXO1 inhibits the transcription of gluconeogenesis genes