Carbohydrate Metabolism

Question 1

where does glycogen synthesis primarily occur?

Answer 1

in the liver and muscle

Question 2

what happens if [blood glucose] drops?

Answer 2

• hypoglyceamia

- muscle weakness, loss of coordination, sweating, hypoglycaemic coma, death

Question 3

what happens if [blood glucose] too high?

Answer 3

• hyperglycaemia

- non enzymatic modification of proteins, hyperosmolar coma

Question 4

what happens when there is excess glucose in the body?

Answer 4

• glycogenesis
• pentose phosphate pathway
• fatty acid synthesis

Question 5

what happens if there is a lack of glucose in the body?

Answer 5

• gluconeogenesis

- glycogenlysis

Question 6

what happens to glucose in the liver?

Answer 6

• glycokinase phosphorylates glucose to G6P
• could be converted back to glycogen
• can be converted to ribose-5-phosphate
• can be converted to pyruvate

Question 7

what are the steps in glycogenesis

Answer 7

• G6P is converted to G1P by phosphoglucomutase
• G1P is converted to UDP-Glucose by UDP-glucose-pyrophophorylase using UTP
• UDP-glucose is combined with glycogenic to initiate glycogenesis as it acts as a primer needed by glycogen synthase (GS) to attach to glucose molecules
• GS facilitates the addition of additional glucose monomers via 1,4-glycosidic bonds

Question 8

how are the branches in glycogen formed?

Answer 8

for every 11 glucose monomers added, some are transferred by branching enzymes via 1,6-glycosidic bonds

Question 9

why is glycogen used as a store and not glucose?

Answer 9

• glucose is osmotically active
• glycogen is more compact
• the branching structure allows for glycogen to rapidly be mobilised

Question 10

what is glycogenolysis?

Answer 10

the breakdown of glycogen the [BG] is low

produces G1P nad glucose

Question 11

what are the enzymes required to break down glycogen?

Answer 11

• phosphorylase: breaks α 1-4 bonds
• translocase: transports G6P to ER for further modification
• debranching enzyme: debranches glycogen by acting on the 1,6-glycosidic bonds
• phosphoglucomutase: converts G1P to G6P

Question 12

what are the enzymes required to form glucose?

Answer 12

• phosphorylase
• transferase
• debranching enzyme
• phosphoglucomutase
• glucose-6-phophatase: converts G6P to glucose in liver and kidney (NOT muscle)

Question 13

how is glycogen broken down?

Answer 13

step 1: break α 1,4 bonds of glycogen using phosphorylase to form G1P until there are only 4 residues attached from the α 1,6 bond

step 2: removal of α 1,6 bonds using the debranching enzyme. transferase moves 3 residues to the end of the non-reducing chain

step 3: phosphoglucomutase converts G1P to G6P

step 4: G6P is converted to glucose by glucose-6-phosphatase

Question 14

how does the enzyme glycogen phosphorylase work?

Answer 14

• forms G1P
• it is a large multi subunit enzyme
• it is regulated by allosteric interactions that signal the cell’s energy state
• also regulated by reversible phosphorylation
• there are 2 forms, active (A) and inactive (B) and is activated by phosphorylase β kinase

Question 15

how is phosphorylase β kinase activated?

Answer 15

• when [BG] high insulin is released into the bloodstream
• insulin activates cAMP, a secondary messenger
• cAMP activates kinase which is able to activate phosphorylase

Question 16

how is glycogen breakdown inhibited in liver?

Answer 16

-the presence of glucose inhibits glycogenolysis by phosphorylase

Question 17

how is phosphorylase B activated in muscle?

Answer 17

by 5’ AMP and NOT phosphorylation

-G6P blocks 5’ AMP activation

Question 18

how is phosphorylase kinase regulated?

Answer 18

• phosphorylase Kinase is under dual regulation via two different receptor types.
• the most important is through the elevation of cAMP and the activation of PKA.
• the other is calcium-mediated through the alpha adrenergic/IP3 pathway.

Question 19

how is glycogenesis and glycogenolysis reciprocally regulated?

Answer 19

• by glycogen synthase and glycogen phosphorylase
• glycogen synthase is activated when [glucose] high
• glycogen phosphorylase activated when [glucose] low

Question 20

what is the significance of the pentose-phosphate pathway?

Answer 20

• metabolic pathway parallel to glycolysis
• produces ribose-5-phosphate sugar used in making RNA and DNA and NADPH
• no ATP is used or formed

Question 21

what is the significance go gluconeogenesis?

Answer 21

• the body’s preferred fuel for the brain is glucose
• the body uses more glucose than stored in reserves so constantly needing to make more
• synthesis from non-carbohydrate source

Question 22

wha are the steps in gluconeogenesis?

Answer 22

• pyruvate carboxylase converts pyruvate to oxaloacetic acid
• oxaloacetic acid is converted phosphoenol pyruvate by phosphoenol pyruvate carboxykinase
• phosphoenol pyruvate is converted to C3 molecule GAP
• GAP converted to fructose-1,6-bisphosphate
• fructose-1,6-bisphosphate converted to G6P by fructose bisphosphatase
• glucose-6-phosphatase converts G6P to glucose

Question 23

what are gluconeogenic aa?

Answer 23

• aa that can be converted back to glucose

- can be fed in at diff parts of the process

Question 24

how does pyruvate get converted to oxaloacetate in the liver?

Answer 24

• pyruvate brought into the mito (of liver) by pyruvate carrier
• pyruvate converted to oxaloacetate by pyruvate carboxylase
• in order to leave the liver oxaloacetate is converted to malate
• malate is brought out of the mito where it is converted back to oxaloacetate
• then the process of continues

Question 25

what are the most important substrates in glyconeogenesis?

Answer 25

-alanine, lactate, glycerol

Question 26

what does glyconeogenesis do and where does it take place?

Answer 26

• covnerts pyruvate to glucose

- takes place mainly in the liver and little in kidney but hone starvation, kidney gluconeogenesis increases by 40%

Question 27

what is the fate of G6P in the liver and muscle?

Answer 27

-depends on the tissue

• for muscle: can be used for ATP synthesis for its own use (muscle use)
• muscle cannot control [bg] as it does not contain the enzymes

-in liver: has enzymes to convert G6P to glucose so controls [bg]