Glycolysis Alice Skoumalová

Glucose: the universal fuel for human cells Sources:  diet (the major sugar in our diet)  internal glycogen stores  blood (glucose homeostasis) Glucose oxidation:  after a meal: almost all tissues  during fasting: brain, erythrocytes

Glycolysis:  oxidation and cleavage of glucose  ATP generation (with and without oxygen)  all cells  in the cytosol (the reducing equivalents are transferred to the electron-transport chain by the shuttle) ATP is generated: 1. via substrate-level phosphorylation 2. from NADH 3. from oxidation of pyruvate Regulation of glycolysis: 1. Hexokinase 2. Phosphofructokinase 3. Pyruvate Kinase Generation of precursors for biosynthesis:  fatty acids  amino acids  ribosis-5-P

Anaerobic glycolysis  a limited supply of O 2  no mitochondria  increased demands for ATP Lactic acidemia  in hypoxia

Phosphorylation of glucose:  irreversible Glucose 6-P:  cannot be transported back across the plasma membrane  a precursor for many pathways that uses glucose Hexokinases Glucokinase (liver, β-cell of the pancreas)  high K m

Michaelis-Menten kinetics

1. Conversion of glucose 6-P to the triose phosphates 2. Oxidation and substrate-level phosphorylation

1. Conversion of glucose 6-P to the triose phosphates irreversible regulation essential for the subsequent cleavage

Substrate-level phophorylation 2. Oxidation and substrate-level phosphorylation

Summary of the glycolytic pathway: Glucosis + 2 NAD P i + 2 ADP 2 pyruvate + 2 NADH + 4 H ATP + 2 H 2 O ∆G 0´ = - 22 kcal (it cannot be reversed without the expenditure of energy!)

Aerobic glycolysis:  involving shuttles that transfer reducing equivalents across the mitochondrial membrane

Glycerol 3-phosphate shuttle:

Malate-aspartate shuttle:

Anaerobic glycolysis: Energy yield 2 mol of ATP dissociation and formation of H +

Daily lactate production115 (g/d) Erythrocytes29 Skin20 Brain17 Sceletal muscle16 Renal medulla15 Intestinal mucosa8 Other tissues10 Major tissues of lactate production: (in a resting state)

Cori cycle: Lactate can be further metabolized by:  heart, sceletal muscle Lactate dehydrogenase: a tetramer (subunits M and H)

Lactate dehydrogenase Pyruvate + NADH + H + lactate + NAD + LD 5 isoenzymes: Heart (lactate) Liver, muscle (pyruvate)

Biosynthetic functions of glycolysis:

Regulation

Fructose 2,6-bis-phosphate:  is not an intermediate of glycolysis!  Phosphofructokinase-21. activated by Fructose 6-P 2. inhibited through phosphorylation - cAMP-dependent protein kinase (inhibition of glycolysis during fasting) tissue-specific isoenzymes (low K m, a high afinity) glucokinase (high K m ) the rate-limiting, allosteric enzyme tissue-specific isoenzymes

the liver isoenzyme - inhibition by cAMP-dependent protein kinase (inhibition of glycolysis during fasting) Lactic acidemia: increased NADH/NAD + ratioinhibition of pyruvate dehydrogenase

Summary  Glycolysis - generation of ATP (with or without oxygen)  The role of glycolysis in different tissues  Lactate production  Regulation