Carbohydrate metabolism

Question 1

What is hypoglycaemia and what are the symptoms

Answer 1

Low levels of glucose causing muscle weakness, loss of coordination, mental confusion, sweating and in worst cases coma and death

Question 2

What is hyperglycaemia and what are the symptoms

Answer 2

High levels of glucose causing non enzymatic modification of proteins (cataracts and lipoproteins)

Question 3

What affects blood glucose

Answer 3

Intake of glucose increases blood glucose concentration vastly and it will gradually fall back down
Exercise doesn’t affect blood glucose level

Question 4

Where and how is excess glucose stored

Answer 4

Liver is the main storage of glucose

Its stored as glycogen

Question 5

Why is this storage molecule efficient

Answer 5

Its highly branched so enzymes can easily break it down for quick access to the glucose
Glycogen doesn’t affect water potential therefore it doesn’t draw in large quantities of water

Question 6

What is the structure of glycogen

Answer 6

Glycogen includes alpha 1-4 which are straight bonds and alpha 1-6 which allows branching

Question 7

What enzymes are involved in the formation of glucose

Answer 7

glycogen synthase and a branching enzyme

Question 8

Glycogen synthesis

Answer 8

1. Glucose 6 phosphate is made into glucose 1 phosphate by phosphoglucomutase
2. This is then activated by UTP (uridine triphosphate) to create UPD-glucose and Pi as a bi product, this is done UDP-glucose-pyrophosphorylase
3. UDP-glucose attaches to glycogenin
4. Glycogen synthase increases the chain length to 11 residues
5. At 11 residues the branching enzyme forms an alpha 1-6 bond, therefore extending the molecule creating branches

Question 9

Why glycogen and not fat

Answer 9

Fat can not be mobilised as readily
Fat cannot be used as an energy source in the absence of oxygen
Fat cannot be converted in to glucose

Question 10

What enzymes are involved in glycogen breakdown

Answer 10

Phosphorylase breaks alpha 1-4 links 
Translocase 
De-branching enzyme alpha 1-6 
Phosphoglucomutase converts G1P to G6P 
Glucose 6 phosphatatse converts G6P to glucose

Question 11

What is glycogen phosphorylase

Answer 11

A key enzyme in glycogenolysis to form glucose-1-phosphate
An allosteric enzyme activated by phosphorylation but modulated by other factors
Inactive - phosphorylase b
Active - phosphorylase a

Question 12

How is glycogen phosphorylase regulated

Answer 12

1. ATP is made into cyclic AMP through adenylate cyclase
2. This activates protein kinase A
3. Which activates phosphorylase kinase
4. This phosphorylates phosphorylase b into phosphorylase a

Question 13

What hormones regulate glycogenolysis

Answer 13

Insulin inhibits
Glucagon stimulates in the liver
Adrenaline stimulates in the muscle
Cortisol is weak stimulus

Question 14

How is phosphorylase regulated in muscle

Answer 14

Can be activated by 5’-AMP without being phosphorylated
5’-AMP is formed when ATP is depleted
ATP and G-6-P can block 5’-AMP activation
ATP binds on the same site

Question 15

How is phosphorylase regulated in the liver

Answer 15

Phosphorylase a is inhibited by glucose

Question 16

How is phosphorylase b activated in muscle and liver

Answer 16

Ca2+ ions activate phosphorylase b during muscle contraction, you only get maximum activity with phosphorylation and Ca2+ activation
In liver, alpha adrenergic activation stimulates Ca2+ release

Question 17

When and how is glycogen synthase regulated

Answer 17

Activated when glucose is in large supply
Activated though ATP and G6P
Its inactivated by phosphorylation by protein kinase A
Its deactivated by dephosphorylation by protein phosphatase-1

Question 18

When and how is glycogen phosphorylase regulated

Answer 18

Activated when glucose is in short supply
Inactivated by ATP and G6P
Activated by phosphorylation by phosphorylase b kinase
Inactivated by dephosphorylation by protein phosphatase-1

Question 19

What does the pentose phosphate pathway do

Answer 19

Generates ribose 5 phosphate and NADPH

Question 20

Whats NAPDH used for

Answer 20

required for fatty acid synthesis when there is excess circulating glucose

Question 21

Why would this pathway be reversed

Answer 21

When the body needs to build up its circulating glucose levels