1. Biochemistry department
Gluconeogenesis
Dr. Sooad Al-Daihan
Biochemistry department

2. Introduction
Some tissues, such as the brain, and red blood cells require a continuous supply of glucose as a metabolic fuel.
During a prolonged fast, hepatic glycogen stores are depleted, and glucose is formed from precursors such as lactate, pyruvate, glycerol, and α-ketoacids by a special pathway, gluconeogenesis, that requires both mitochondrial and cytosolic enzymes.

4. Gluconeogenesis occurs mainly in liver.
It also occurs to a more limited extent in kidney & small intestine under some conditions. 
Synthesis of glucose from pyruvate utilizes many of the same enzymes as Glycolysis.
Almost reverse of glycolysis except for 3 reactions ,which are essentially irreversible.
Hexokinase (or Glucokinase)
Phosphofructokinase
Pyruvate Kinase.
These steps must be bypassed in Gluconeogenesis.

5. Four enzymes are needed to reverse the 3 irreversible steps of glycolysis:
Mitochondrial ‐ Pyruvate Carboxylase (liver, kidney but not in muscle)
Cytoplasmic ‐ Phosphoenolpyruvate (PEP) Carboxykinase
Cytoplasmic ‐Fructose‐1,6,‐Bisphosphatase
Cytoplasmic ‐Glucose 6‐Phosphatase

6. 1-Bypass of Pyruvate Kinase:
In gluconeogenesis PK is bypassed by 2 enzyme catalyzed reactions:
1- Pyruvate Carboxylase :
Pyruvate is carboxylated to oxaloacetate in the mitochondria .
pyruvate + HCO3- + ATP  oxaloacetate + ADP + Pi
2- PEP Carboxykinase :
Oxaloacetate is decarboxylated and phosphorylated to yield PEP in the cytosol .
oxaloacetate + GTP  PEP + GDP + CO2

7. Transport of oxaloacetate to the cytosol

8. fructose‐1,6‐bisP + H2O fructose‐6‐P + Pi
2- Bypass of Phosphofructokinase:
In gluconeogenesis PFK bypassed by Fructose 1,6 ‐ bisphosphatase reaction (Removes phosphate group)
fructose‐1,6‐bisP + H2O fructose‐6‐P + Pi
3- Bypass of Hexokinase (or Glucokinase)
In gluconeogenesis this reaction bypassed by glucose 6‐phosphatase reaction (Removes phosphate group) :
Glucose-6‐P + H2O Glucose+ Pi
Free glucose is formed by the action of glucose‐6‐ phosphatase in liver and kidney while it is absent in muscles and adipose tissues
Glucose can not be formed by these organs

9. Total Energy Cost 6 high energy bonds used per glucose synthesized
4 are needed to convert pyruvate to PEP
2ATP are utilized by pyruvate carboxylase
2GTP are utilized by PEP carboxyKinase
2 are utilized for reversal of phsphoglycerate kinase

10. Precursors for Gluconeogenesis
i- Cori Cycle
Lactate released by active skeletal muscle or red blood cells is carried to the liver where it is converted to glucose by gluconeogenic pathway (Cori cycle) and released for reuptake by skeletal muscle
ii- Glucose-Alanine Cycle
Protein broken down in skeletal muscle during exercise
Amino acids converted to alanine and released by skeletal muscle
Taken up by liver and converted to glucose and released for reuptake by skeletal muscle

11. iii- Glycerol
Glycerol released from adipocytes and skeletal muscle during lipolysis.
 Glycerol enters gluconeogenic pathway as dihydroxyacetone phosphate “active form of glycerol”.
 2 glycerol required to make one glucose in liver and kidney in fasting or low CHO diet .
Glycerol cannot be utilized in adipose tissue due to the lack of glycerol kinase in the adipose tissue.

12. iv- Propionate:
Propionyl‐CoA is converted to the TCA intermediate, succinylCoA.
This conversion is carried out by the ATP‐requiring enzyme, propionyl‐CoA carboxylase then methylmalonyl‐CoA epimerase and finally the vitamin B12 requiring enzyme, methylmalonyl‐CoA mutase
The utilization of propionate in gluconeogenesis only has quantitative significance in ruminants.

13. Regulation of Gluconeogenesis
Control of glycolysis and gluconeogenesis is reciprocal
Gluconeogenesis and Glycolysis are regulated by similar effector molecules but in the opposite direction
When one pathway is activated , the other is inhibited
Gluconeogenesis is subject to both:
- Hormonal control by Glucagon, Cortisol, Adrenaline and Insulin
- Allosteric regulation of gluconeogenic enzymes

14. Hormonal Regulation Glucagon, Cortisol, Adrenaline
Are secreted during fasting, stress and sever muscular exercise
Induce gluconeogenic enzymes
Repress glycolytic enzymes
Insulin
Secreted after CHO meal
Induce glycolytic enzymes
Repress gluconeogenic enzymes

15. Allosteric regulation

16. Biochemistry department
Uronic Acid Pathway
Dr. Sooad Al-Daihan
Biochemistry department

17. Overview
It is an alternative oxidative pathway for glucose that doesn’t lead to ATP generation.
It includes oxidation of glucose to
Glucuronic acid
Ascorbic acid
It occurs mainly in the liver cytoplasm.

18. Metabolic reactions
Glucose 6-phosphate is isomerized to glucose 1-phosphate
Glucose 1-phosphate reacts with uridine triphosphate (UTP) to form uridine diphosphate glucose (UDPGlc) in a reaction catalyzed by UDPGlc pyrophosphorylase
UDPGlc is oxidized at carbon 6 by NAD-dependent UDPGlc dehydrogenase in a two-step reaction to yield UDP-glucuronate 1 2 3

19. UDP Glucuronic acid (active form) is needed in:
Conjugation to less polar compounds as bilirubin, steroids and some drugs making them more water soluble (detoxicated) .
Synthesis of glycosaminoglycans (mucopolysaccharide) as heparin, hyaluronic acid.
In plants and some animals (not Human) glucuronic acid serves as a precursor of L-ascorbic acid.
The uronic acid pathway also provides a mechanism by which dietary D-xylulose enter the central pathway.