Aulani "Biokimia" Presentation4 Aulanni’am Biochemistry Laboratory Chemistry Departement Brawijaya University Carbohydrate Metabolism Cont….

Aulani "Biokimia" Presentation4 Gluconeogenesis Some tissues, such as brain, RBCs, kidney medulla, testes, embrionic tissues and exercising muscle require a continuing supply of glucose as a metabolic energy. The human brain requires over 120 gm of glucose per day. Mammalian cells make glucose from simpler precursors. Liver glycogen can meet these needs only for 10 to 18 hours without dietary carbohydrate.

Aulani "Biokimia" Presentation4 Glycolysis and Gluconeogenesis in the liver

Aulani "Biokimia" Presentation4 Gluconeogenesis

Aulani "Biokimia" Presentation4 Pyruvate precursors n The direct Glc reserves are sufficient to meet Glc needs for about a day! n Gluconeogenic pathway makes Glc from pyruvate precursors. n Triacyl glycerol > Glycerol + Fatty acids n Glycerol is a precursor of glc, glycerol enters glycolytic pathway as dihydroxyacetone phosphate.

Aulani "Biokimia" Presentation4

Gluconeogenesis is NOT a reversal of glycolysis n Several reactions MUST differ because of the irreversible steps. HK (hexokinase) PFK (phosphofructokinase) PK (pyruvate kinase)

Aulani "Biokimia" Presentation4

n The stoichiometry of gluconeogenesis is: 2pruvate + 4ATP + 2GTP + 2NADH + 6H 2 O >Glc +4ADP + 2GDP + 6Pi + 2NAD + + 2H + In contrast, the stoichiometry of reversal of glycolysis is : 2 pyruvate + 2ATP + 2NADH + 2H2O > Glc + 2ADP + 2Pi + 2NAD + The difference is 4ATP, this is needed to turn energetically unfavorable process to a favorable one !

Aulani "Biokimia" Presentation4 Gluconeogenesis and glycolysis are reciprocally regulated u Both glycolysis and gluconeogenesis are highly exorgonic under cellular conditions so there is no thermodynamic barrier. u But, amounts and activities of the distinctive enzymes of each pathway are controlled so that both pathways are not highly active at the same time.

Aulani "Biokimia" Presentation4

Substrate cycles u F-6-P > F 1,6BisP u < u A pair of reactions such as the above one is called “substrate cycle” u There is also some cycling in irreversible reactions. u “Imperfection” in metabolism? u They are sometimes referred as “futile cycles” Futile cycles amplify metabolic signals! The other potential biological role of substrate cycles is the generation of heat produced by the hydrolysis of ATP.

Aulani "Biokimia" Presentation4

Lactate and alanine formed by contracting muscle are used by other organs u Lactate is a dead end in metabolism. u Lactate should be converted to pyruvate. u The plasma membranes of most cells are highly permeable to lactate and pyruvate, therefore they easily diffuse to go to liver! u Excess lactate enters the liver and is converted pyruvate first then glucose. Thus, the liver restores the level of glucose necessary for active muscle cells, which derive ATP from the glycolytic conversion of glucose into lactate. Contracting skeletal muscle supplies lactate to the liver, which uses it to make glucose. These reactions constitute CORI CYCLE.

Aulani "Biokimia" Presentation4 LDH enzyme u Lactate > Pyruvate by LDH (lactate dehydrogenase). u The interconversion of pyruvate and lactate are done by different subunits of LDH. LDH is a tetramer.

Aulani "Biokimia" Presentation4 The Cori Cycle

Aulani "Biokimia" Presentation4 Cooperation between glycolysis and gluconeogenesis

Aulani "Biokimia" Presentation4 Two alternative fates of pyruvate

Aulani "Biokimia" Presentation4 2 nd control point

Aulani "Biokimia" Presentation4 Hormonal Control u The special role of liver to maintain constant blood glucose level requires additional control mechanisms. u When blood glucose decreases, glycogen increases and glucose is released. u This hormonal regulation in liver is mediated by fructose-2,6-bisphosphate, which is a allosteric effector for PFK-1, and F-1,6-bisphosphate

Aulani "Biokimia" Presentation4 Role of F2,6BP in regulation of Glycolysis and Gluconeogenesis

Aulani "Biokimia" Presentation4 What is F-2,6-BP? u It is structurally related to F-1,6-BP. u It is not an intermediate. u It is a “regulator” u F-2,6-BP activates PFK-1 and glycolysis. u FBPase and PFK-2 are part of the same enzyme! u An increase in glucagon (during starvation) leads to a decrease in F-2,6-BP overall which goes to a decrease in glycolysis, an increase in glucone ogenesis u A decrease in glucagon (after carbohydrate rich diet) leads to an increase in F-2,6-BP and an increase in glycolysis. u Therefore, F-2,6-BP acts as an intracellular signal indicating “glucose abundant”.

Aulani "Biokimia" Presentation4