Glycolysis

To recognize the sole energy producing pathway (glycolysis) in RBCs To understand the different reactions of glycolysis To know what specifies glycolysis in RBCS. To retrieve the importance of 2,3,BPG To know how specific metabolism of RBCs suits its function

Glycolysis is the sequence of reactions that convert glucose into pyruvate (or lactate) with the concomitant production of small amount of ATP

Glycolysis occurs in the cytosol Glycolysis occurs in all tissues Glycolysis can proceed either in presence and in absence of O2

Glycolysis Two phases: First phase; Energy–investment phase (preparatory): converts glucose (6 C) to two Glyceraldehyde-3-P (3 C)

Glycolysis Two phases: -Second phase; Energy–generation phase (pay off): produces two pyruvates or two lactates

RX1: Hexose Kinase 1st step in glycolysis This is a priming reaction - ATP is consumed here in order to get more later

Rx 2: Phosphoglucoisomerase Reversible reaction

Rx 3: Phosphofructokinase PFK is the committed step in glycolysis! The second priming reaction of glycolysis Committed step means PFK is highly regulated

Phosphofructokinase is highly regulated ATP inhibits PFK, AMP reverses inhibition PFK increases activity when energy status is low PFK decreases activity when energy status is high

Reaction 4: Aldolase I Hexose cleaved to form two trioses, Reversible Reaction C1 thru C3 of F1,6-BP -> dihydroxyacetone phosphate C4 thru C6 -> Glyceraldehyde 3 phosphate

Reaction 4: Aldolase I

Rx 5: Triose Phosphate Isomerase (TPI) Reversible rxn Conversion of DHAP to G-3-P by TPI maintains steady state [G-3-P]

Any Questions

Glycolysis - Second Phase In this phase, 2 G-3-P are converted to 2 pyruvates (aerobic) or 2 lactates (anaerobic)

Glycolysis - Second Phase Second phase involves two very high energy phosphate intermediates 1,3 BPG Phosphoenolpyruvate

Glycolysis - Second Phase Substrate level energy (phosphorylation) production of 4 ATP (2 X 2 ATP from each 3C molecule) Net ATP yield for glycolysis is 4-2 = 2 ATP

Rx 6: Glyceraldehyde-3P-Dehydrogenase G3P is oxidized and phosphorylated to 1,3-BPG Reversible rxn Pi is used as phosphate donor Production 1 NADH + H+ (3 ATP from Respiratory Chain aerobically but not in RBCs)

Rx 6: Glyceraldehyde-3P-Dehydrogenase NADH generated in this reaction is re-oxidized by respiratory electron transport chain (generates 3 ATP) but NOT in RBCs. ETC will regenerate NAD for more glycolysis to run but NOT in RBCs. If no ETC, transfer of pyruvate to lactate is needed.

Rx 7: Phosphoglycerate Kinase (PGK) ATP synthesis from a high-energy phosphate This is referred to as "substrate-level phosphorylation“

Rx 8: Phosphoglycerate Mutase Phosphoryl group moves from C-3 to C-2 Mutases are isomerases that transfer phosphates from one hydroxyl to another

Rx 9: Enolase Reversible rxn "Energy content" of 2-PG and PEP are similar Enolase just rearranges to a form a compound from which energy can be released in hydrolysis

Rx 10: Pyruvate Kinase Substrate level phosphorylation generates second ATP Regulatory step Allosterically activated by AMP, F-1,6-bisP Allosterically inhibited by ATP and acetyl-CoA

Rx 10: Pyruvate Kinase Substrate level phosphorylation generates second ATP Hereditary deficiency of pyruvate kinase causes hemolytic anemia as it decreases the net ATP yield of glycolysis in RBCs especially if 2,3,BPG is produced.

Fates of Pyruvate

Lactate formation    Under anaerobic conditions pyruvate is converted to lactate by the enzyme lactate dehydrogenase Important for the regeneration of NAD+ under anaerobic conditions.

Lactate forming cells Cells of lens and cornea of the eye, kidney medulla, testes, leukocytes, and RBCs, because these tissues are all poorly vascularized and/or lack mitochondria. 2.In exercising skeletal muscle.

Lactate utilization: The direction of the lactate dehydrogenase reaction depends on: the relative intracellular conc.of pyruvate and lactate. the ratio of NADH/NAD+ in the cell. In the liver, heart and CNS, the ratio of NADH/NAD+ is lower than in exercising muscle. These tissues oxidize lactate (obtained from the blood) to pyruvate. In the liver, pyruvate is either converted to glucose by gluconeogenesis or oxidized in the TCA cycle. Heart muscle and CNS oxidizes lactate to CO2 and H2O via the TCA cycle.