Regulation of glucose and gluconeogenesis

Question 1

regulation of glycolysis

Answer 1

• Glycolysis is regulated to maintain constant ATP levels
  
  • Glycolysis if AM= high and ATP=low
  • Gluconeogenesis if AMP is low
  • Complex regulation among ATP, NADH levels along with regulation of hexokinase, PFK-1 and pyruvate kinase
    Glucagon, epinephrine and insulin regulate expression/function of glycolytic enzymes

Question 2

hexokinase izosime in liver and muscle

Answer 2

• Hexokinase I to IV
  
  • Hexokinase I and II:
    1. Predominant in muscle
    2. High affinity for glucose (activity reaches max quickly)
    3. Allosterically inhibited by glucose 6-phosphate (its product)
  • Hexokinase IV:
    1. Predominant in liver
    2. Lower affinity for glucose
    3. Not inhibited by glucose 6-phosphate so its activity continues when hexokinase I and 111 are inhibited by glucose 6-phosphate
    4. Activity increase at high glucose concentration
    5. Glucose escapes glycolysis at low concentration
    6. Inhibited by a regulatory protein (liver specific: GKRP)

Nuclear binding protein takes away the hexoikinase IV away from the cytoplasm to the nucleus (where it is away from all of the enzymes of glycolysis) when the concentration of glucose is low (high concentration of fructose-6-phosphate) to make sure that liver does not compete with other organs for the glucose. It is stimulates by the fructose-6-phosphate which is a product downstream of the hexokinase 4 and the glucose (the substrate) , when its concentration is high, bring back the hexokinase 4 inside the cytosol to stimulate the glycolysis

Question 3

what happens in the liver at high and low concentration of glucose

Answer 3

• High blood glucose: hexokinase IV (do glycolysis)

Low blood glucose: glucose-6-phosphatase (glucose is generated, not used)

Question 4

regulation of PFK-1 and FBPase-1

Answer 4

1. PFK-1 (phosphofructokinase-1):
   
   • High ATP allosterically inhibits PFK-1
   • High AMP or DP relieve ATP-inhibition
     2. FBPase-1 (fructose 1,6-biphosphatase):
   • High AMP allosterically inhibits FBPase-1
     * * encourage gluconeogenesis

Question 5

fructose- 2,6 biphosphate

Answer 5

*** it is NOT a glycolytic intermediate
Acts as allosteric activator of PFK-1 and inhibitor of FBPase-1

SO:
Fructose 1,6, biphosphate= glycolysis
Fructose 2,6 biphosphate = regulator of 2 enzymes (PFK-1 and FBPase)

Question 6

why FBPase2 and PFK-2 are unusal + how are theyr regulated

Answer 6

PFK2 and FBPase2 = same gene, same enzyme
PFK1 and FBPase1= two enzymes, two different genes
- Enzyme PFK-2/FBPase-2 is unusual: it is a bifunctional enzyme, expressed for single gene PFKFB1
- PFK-1 and FBPase-1 are encoded by 2 genes, PFKM and FBP1
- PFK-2/FBAe-2 is regulated by phosphorylation:
1. If it is phosphoryated *by glucagon, which is a cAMP-dependent protein kinase) so FBPase-2 is activate which means that glycolysis is inhibited but gluconeogenesis is stimulates and the concentration of F26BP is reduce
If its dephosporylated by insulin, PFK-2 is activated so stimulates glycolysis and inhibits gluconeogenesis + increase concentration of F26BP

Question 7

regulation of pyruvate kinase in the liver

Answer 7

In the liver only:
- Glucagon activates PKA where it will phosphorylate the pyruvate kinase (inactive form)
- When the glucagon levels drop, a protein phosphatase (PP) dephosphorylate oyruvate kinase, activating it. This mechanism prevents the liver from consuming glucose by glycolysis when blood glucose is low: instead, the liver exports glucose. And the pyruvate kinase will go in all glycoltyic tissues, including liver
- There :
- Fructose 1,6 biphospahte activates pyruvate kinase to create pyruvate from PEP
- ATP, acetyl-coa, long-chain FA inhibits it because dont want more energy than they already have
Accumulation of alanine, which can be synthesized from pyruvate in one step, inhibits pyruvate kinase, slowing the production of pyruvate by glycolysis (so the product inhibits its own formation)

** muscle isoform is not regulated by PKA phosphorylation (does not have receptors for glucagon)

Question 8

fate of pyruvate in liver (regulation by acetyl-coa)

Answer 8

Pyruvate can be convert to glucose and glucogen via gluneogenesis or oxidized to acetyl-coa for energy production.
The first enzyme in each path is regulate allosterically; acetyl-coa, produced either by FA oxidation or by the pyruvate dehydrogenase complex, stimulates pyruvate carboxylase and inhibits pyruvate dehydrogenase

Pyruvate can be converted into acetyl-CoA which go into TCA
- Low glucose concentration= pyruvate is taken up to gluconegenesis to pruduct glucose
When there is a high concentration of acetyl-CoA, it will inhibits is own production and it will also stimulate pyruvate to go through gluconeogenesis

Question 9

some of the genes regulated by insulin

Answer 9

Increased expression:
	1. Hexokinase II and IV
	2. PFK-1
	3. PFK2-FBPase-2
	4. Pyruvate kinase 
	** 4 of them are essential for glycolysis, which consume glucose for energy
	5. Glucose 6-phosphate dehydrogenase
	6. 6-phosphogluconate dehydrogenase
	7. Malic enzyme
	** 5-7 produce NADPH, which is essential for conversion of glucose to lipid
So 1-7 are involve in glycolysis

Decreased expression:

1. PEP carboxykinase
2. Glucose-6-phosphatase * * essential for glucose production by gluconeogenesis

Question 10

transcriptional regulation of enzymes (ChREBP)

Answer 10

General steps:
1. Xylulose 5-phosphate (in the pentose phosphate pathway) stimulates protein phosphatase 2 (PP2A) which will cause the dephosphorylation of serine of the ChREBP (carbohydrate response element binding protein= TF) in the cytosol
2. ChREP will go in the nucleus where it there will be dephosphorylation of threonine by PP2A (activates by xylulose 5-phosphate)
3. ChREBP can associate with Mix (partner protein) and bind to ChoRe (carbohydrate response element)
Turns on synthesis of several enzymes: pyruvate kinase, FA synthase and acetul-coa carboxylase (the first enzyme in the path to FA synthesis) which will activate glycolysis and FA synthesis

Question 11

role of the insulin

Answer 11

1. Insulin causes the activation of PKB which will phosphorylates FOXO1 (TF) so it will be tagged by the attachment of ubiquitin fro degradation by proteasomes
   
   2. FOXO1 that is dephosphorylated can go in the nucleus and act as TF (bind to DNA) and it will activate gluconeogenesis by activating PEP carboxykinase and glucose 6-phosphatase
      * * so FOXO1 suppresses: transcription of enzymes of glycolysis, pentose phosphate pathway and FA synthesis
      
      • ** SO INSULIN ACTIVATES GLYCOLYSIS BY MAKING SURE THAT FOXO1 IS DEGRADED