Gluconeogenesis

Question 1

Site of gluconeogenesis

Answer 1

In liver & kidney (cortex) to lesser extent in intestinal epithelium partly in cytoplasm & partly in mitochondria (1st step only)

Question 2

Why is glucose not synthesized from simple reversal of glycolysis?

Answer 2

1. The overall equilibrium of glycolysis strongly favours pyruvate formation
2. There are 3 irreversible reactions in glycolysis which are the ones bypassed in gluconeogenesis by alternative reactions.
   Thus glucose is synthesized by special pathway

Question 3

List organs that require continuous glucose supply

Answer 3

1. RBCs
2. Retina
3. Lens
4. Brain (can use ketone bodies)

Question 4

Identify conditions chch by active gluconeogenesis

Answer 4

Fasting: starts 4-6 hrs after last meal & reaches full capacity after depletion of liver glycogen (12-18 hrs)
DM, Cushing
Severe muscular exercise:
1. Provides lactate for Cori cycle
2. Inc sympathetic activity by CAs which stimulate gluconeogenesis
3. Dec glycogen stores & need for glucose

Question 5

Mention & compare the 2 cycles related to gluconeogenesis

Answer 5

Both start from skeletal muscle to liver & supply substartes for gluconeogenesis (pyruvate)
1. Lactic acid cycle (Cori cycle), wash excess lactate
2. Glucose-alanine cycle (Cahill cycle), wash excess ammonia

Question 6

List substrates of gluconeogenesis, their sources & corresponding intermediate

Answer 6

1. Lactate from anaerobic glycolysis in RBCs & SKM. Pyruvate through pyruvate dehydrogenase in liver
2. Glycerol from lipolysis. DHAP through phosphorylation followed by oxidation in liver cells.
3. Glucogenic & mixed AAs (alanine most important) from proteolysis. Pyruvate by transdeamination of alanine in liver.
4. Propionate from odd-number FFA valine, isoleucine, methionine. Succinyl CoA by 4 enzymes: acyl-CoAsynthetase, carboxylase, racemerase, mutase (vitamin B12)

Question 7

Describe how glucogenic amino acids enter gluconeogenesis

Answer 7

Hydrolysis of tissue proteins, glucogenic amino acids generates a-keto acids, such as, a-ketoglutarate can enter TCA and form OAA, a direct precursor of PEP.

Question 8

All amino acids can share in gluconeogenesis except……

Answer 8

Leucine & lysine

Question 9

GR: Adipose tissue cannot synthesize glucose

Answer 9

Bec they lack glycerol kinase

Question 10

Describe pathway through which propionate is converted to succinyl CoA

Answer 10

1. Activated into propionyl CoA by acyl-CoA synthetase
2. Propionyl CoA is carboxylated by propionyl CoA carboxylase to D-meythmalonyl CoA requires CO2, biotin, ATP, Mn/Mg
3. Converted to L-form by racemerase
4. Converted to succinyl CoA by methymalonyl-CoA mutase (isomerase) requires B12

Question 11

List reactions unique to gluconeogenesis

Answer 11

1. Carboxylation of pyruvate in mitochondria by pyruvate carboxylase (uses 1 ATP for each pyruvate)
2. Decarboxylation of cytosolic oxaloacetate by PEP-carboxykinase to PEP
3. Dephosphorylation of fructose 1,6 bisphosphate by fructose 1,6 bisphosphatase forming fructose 6-phosphate
4. Dephosphorylation of glucose 6-phosphate by glucose 6-phosphatae releasing free glucose

Question 12

Importance of pyruvate carboxylation & its coenzymes

Answer 12

1. Provide an important substrate for gluconeogenesis
2. Provide OAA that can replenish TCA cycle intermediates that may become depleted
3. Overcomes irreversible conversion of PEP into pyruvate by pyruvate kinase in glycolysis
   CoEs: biotin, Mn,Mg, ATP

Question 13

The enzyme which aids OAA transport across mitochondria is……

Answer 13

Malate dehydrogenase

Question 14

…..is required for PEPCK

Answer 14

GTP

Question 15

Fructose 1,6-bisphosphatase bypasses…..of glycolysis, while glucose 6-phosphatase bypasses….

Answer 15

PFK-1
Hexokinase/glucokinase

Question 16

Muscle gluconeogenesis doesn’t release blood glucose?

Answer 16

Due to absence of glucose 6-phosphatase

Question 17

Describe role of covalent modification in regulation of gluconeogenesis

Answer 17

Glucagon & epinepherine bind its G-protein coupled receptor together with an elevation in cAMP & PKA activity leading to phosphorylation of pyruvate kinase & PEP is diverted to gluconeogenesis.

Question 18

Describe role F2,6B in well-fed & fasting states

Answer 18

Well-fed state (under control of insulin) PFK-2 is activated (dephosphorylated) & stimulates glycolysis (by inhibiting fructose 1,6 bisphosphatase) sparing F1,6B allowing it to proceed with glycolysis
Fasting (under control of glucagon/epinepherine) PFK-2 is deactivated (phosphorylation) —fructose 2,6 bisphosphatase is activated —dec F2,6B —inhibition of glycolysis by activation fructose 1,6 bisphosphatae —inc gluconeogenesis.

Question 19

……. Inhibits fructose 1,6 bisphosphatase & activates PFK1

Answer 19

AMP

Question 20

……is an allosteric activator of pyruvate carboxylase

Answer 20

Acetyl CoA

Question 21

…..is repressed by insulin

Answer 21

PEPCK

Question 22

List the energy consuming steps of gluconeogensis

Answer 22

1. Pyruvate carboxylase (ATP)
2. PEPCK (GTP)
3. Phosphoglycerate kinase (ATP)
4. GAP kinase (NADH)

Question 23

With respect gluconeogenesis energy consumption, starting with pyruvate uses….ATP, with oxaloacetate uses….., with glycerol…….

Answer 23

6
4
2

Question 24

List the importance of gluconeogenesis

Answer 24

1. Gluconeogenesis maintains blood glucose level esp for nervous system & erythrocytes. After an overnight fast, gluconeogenesis & glycolysis make up equal contributions to blood glucose
2. Supplies glucose which is important in maintaining the level of intermediates of TCA cycle although fatty acids are the main source of acetyl coA in that case
3. Clears lactate produced by muscle & erythrocytes, and glycerol produced from adipose tissue
4. Glucose needed for lactose formation in mammary glands

Question 25

Mention a disease of gluconeogenesis

Answer 25

Von Gierke disease, glucose 6-phosphatase deficiency, is usually fatal as hypoglycemia causes coma & death.