Biochemistry department Gluconeogenesis Dr. Sooad Al-Daihan Biochemistry department

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.

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.

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

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

Transport of oxaloacetate to the cytosol

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

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

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

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.

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.

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

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

Allosteric regulation

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

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.

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

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.