Energy Storage (Glycogen and Fat)

Question 1

Which cells cannot use oxidative phosphorylation

Answer 1

RBCs

Neutrophils

Inners cells of kidney

Lens of the eye

Question 2

Where is glycogen stored

Answer 2

In liver in granule

In skeletal muscle as intra and intermyofibrillar glycogen

Question 3

What enzymes are involved in glycogen synthesis (glycogenesis)

Answer 3

Hexokinase (glucokinase in the liver) - step 1

Phosphoglucomutase - step 2

Glycogen synthase - forms a1-4 bonds

Branching enzyme - forms a1-6 bonds

Question 4

Which enzyme are involved in glycogenolysis

Answer 4

Glycogen phosphorylase

De-branching enzyme

Phosphoglucomutase

Glucose-6-phosphatase

Question 5

How is glycogenolysis in the liver different to that in the muscle

Answer 5

In the liver, there is glucose-6-phosphatase which converts glucose-6-phosphate to glucose which can be transported around the body

In the muscle there is no glucose-6-phosphatase as the glucose-6-phosphate goes straight into glycolysis

Question 6

How is glycogen metabolism regulated

Answer 6

Regulated by controlling the activites of enzymes catalysing irreversible reactions in the biosynthetic and degradative pathways - glycogen synthase and glycoen phosphorlyase

Question 7

How are glycogen synthase and glycogen phosphorylase regulated

Answer 7

Subjected to allosteric control (AMP activates muscle phosphorylase) and control by convalent modification

Glucagon and adrenaline cause phosphorylation which activates glycogen phosphorylase and inhibts glycogen synthase

Insulin causes dephosphorylation, activating glycogen synthase and inhibiting glycogen phosphorylase

Question 8

What are the features of glycogen storage disease

Answer 8

Increased or decreased amounts of glycogen causing:

• Tissue damage if excessive storage
• Fasting hypoglycaemia
• Poor exercise tolerance

Glycogen structure may be abnormal

Usually liver and/or muscle are affected

Question 9

Name some examples of glycogen storage diseases

Answer 9

von Gierke’s disease - glucose-6-phosphatase deficiency

McArdle disease - muscle glycogen phosphorylase deficiency

Question 10

What are glycogen storage diseases

Answer 10

Number of inherited disorders of glycogen metabolism resulting from an abnormality in enzymes of glycogen metabolism

Question 11

What are the possible substrates for gluconeogenesis

Answer 11

Pyruvate, lactate and glycerol - converted to glucose

Essentail and non-essential glycogenic amino acids

Question 12

Which enzymes are involved in gluconeogenesis from pyruvate

Answer 12

PEPCK - bypass step 10

Pyruvate carboxylase - bypass step 10

Glucose-6-phosphatase - bypass step 1

Fructose 1,6-bisphosphatase - bypass step 3

Question 13

How is gluconeogenesis regulated

Answer 13

Glucagon and cortisol stimulate gluconeogenesis by increased amount of PEPCK and fructose 1,6-bisphosphatase, and increasing activity of fructose 1,6-bisphosphatase

Insulin inhibits gluconeogenesis by decreasing amount of PEPCK and fructose 1,6-bisphosphatase, and decreasing activity of fructose 1,6-bisphosphatase

Question 14

When are triacylglycerols mobilised

Answer 14

During prolonged aerobic exercise and stress stituations like starvation and during pregnancy

Question 15

Which hormones stimulate storage of triacylglycerols and which inhibit storage

Answer 15

Insulin stimulates

Glucagon, adrenaline, cortisol, growth hormone and thyroxine inhibit

Question 16

Describe lipogenesis

Answer 16

Fatty acids are synthesised from acetyl CoA using ATP and NADPH

Acetyl CoA comes from mitochondria in combination with oxaloacetate as citrate, which is cleaved in the cytoplasm to release acetyl CoA

Most steps in the pathway are carried out by fatty acid synthase complex

Fatty acids are built up sequentially from acetyl CoA by cycle of reactions adding 2C per turn of cycle as malonyl CoA

Question 17

How is malonyl CoA produced

Answer 17

Malonyl CoA is produced from acetyl CoA by the enzyme acetyl CoA carboxylase with the loss of CO₂

Question 18

How is fatty acid synthesis regulated

Answer 18

Regulated by activating and inhibiting acetyl CoA carboxylase

Activators - insulin (dephosphorylate), and citrate (allosteric)

Inhibitors - glycagon and adrenaline (phosphorylate), and AMP (allosteric)

Question 19

What enzyme is involved in breaking down TAG and what hormones stimulate and inhibt this process

Answer 19

Hormone sensitive lipase

Glucagon and adrenaline stimulate by phosphorylation

Insulin inhibits by dephosphorylation