Glycolysis Under Anaerobic Conditions “Anaerobic” Glycolysis 2 ADP + 2 Pi 2 ATP glucose 2 lactate Hydrolysis of ATP Produced: DGo´ = 2 x -7.3 = -14.6 kcal/mol Complete Oxidation of Glucose DGo´ = -686 kcal/mol

Glycolysis Under Aerobic Conditions “Aerobic” Glycolysis 2 ADP + 2 Pi 2 ATP glucose 2 pyruvate 2 NAD+ 2 NADH Catabolism of Glucose by Glycolysis, TCA, and Ox. Phos. 30 ADP + 30 Pi 30 ATP Glucose + 6 O2 6 CO2 + 6 H2O Hydrolysis of ATP Produced DGo´ = 30 x -7.3 = -219 kcal/mol Complete Oxidation of Glucose O2 DGo´ = -686 kcal/mol

Circumstances Under Which the Anaerobic Glycolytic Pathway is Used 1. Anaerobic bacteria In eukaryotic cells under conditions that are completely anaerobic (rare) In aerobic eukaryotic cells under conditions where the mitochondrial electron transport chain is saturated (w/ reducing equivalents (NADH, FADH2), pyruvate, or both; common under conditions of strenuous exercise).

Regeneration of NAD+ in the Cytosol Aerobic Conditions (Processing of Cytosolic NADH and Pyruvate by TCA/Oxidative Phosphorylation) Glycerol Phosphate Shuttle E-FAD is an enzyme in the inner mitochondrial membrane involved in oxidative phosphorylation

Thermodynamics of Glycolysis Glycolysis is not like this .. Glycolysis is more like this ..

b) DG0 depends only on the identity of the products and reactants True or False? http://biochem.uthscsa.edu/hardies-bin/survey.pl a) b) DG0 depends only on the identity of the products and reactants b) The reaction will always flow in the direction where DG is negative. c) If DG0 is near 0, then the reaction can be easily driven from backwards by pumping excess product into the reaction. These reactions are called “reversible”. d) If DG0 is << 0, (such that P/R at any reasonable cellular levels can not reverse the sign of DG), then the reaction is called “irreversible”.

Hexokinase Glycolysis Glycogen synthesis Pentose Phosphate Pathway Irreversible reaction (means that DGo´ is “large” and negative, DGo´ = -4.0 kcal/mol) (2) Two forms (isozymes) (3) Not a major regulatory enzyme of the pathway since G-6-P can also be used for glycogen synthesis or the pentose phosphate pathway

True or False? http://biochem.uthscsa.edu/hardies-bin/survey.pl The high Km of liver glucokinase relative to muscle hexokinase means that in the fed state with limiting glucose, the liver glycogen will be replenished before muscle glycogen. Km is the substrate concentration where the enzyme exhibits ½ maximal velocity. We normally expect the first irreversible reaction in that pathway that is “committed” to a particular product to be the regulated enzyme.

Phosphoglucoisomerase Reversible reaction (means that DGo´ is close to 0) (2) F-6-P is NOT irreversibly “committed” to the glycolytic pathway

Phosphofructokinase (PFK) Irreversible reaction; catalyzes the “committed” step in the pathway Important regulatory enzyme; regulated allosterically in the following manner: Stimulated by AMP and ADP Inhibited by ATP and Citrate

Idealized allosteric feedback inhibition shift caused by +/- feedback inh. velocity -I +I [S] ambient [S]

Aldolase “Pulled” toward pyruvate

Glyceraldehyde-3-phosphate dehydrogenase (1) Oxidation-reduction reaction (conversion of aldehyde to carboxylic acid phosphate ester) (2) 2 NADH are synthesized for each molecule of glucose that enters the pathway (3) Reversible (NOTE - NOT all energy yielding reactions are irreversible)

3-phosphoglycerate kinase 2 ATP are synthesized for each molecule of glucose that enters the pathway (2) Reversible a) Due to the fact that the phosphoryl transfer potentials of 1,3-bisphosphoglycerate and ATP are comparable b) Second example of an energy yielding reaction that is reversible

Pyruvate Kinase (1) Irreversible reaction (DGo´ = -7.5 kcal/mol). Thermodynamically this “pulls” the series of reactions from F-1,6-bisP to pyruvate (2) Regulates the pathway: Activated by AMP, ADP, F-1,6-bisP Inhibited by ATP

Regulation of Glycolysis Irreversible steps of the pathway are potential points of regulation: hexokinase, PFK, and pyruvate kinase Pyruvate kinase Hexokinase Product inhibition by glucose-6-phosphate “Substrate” stimulation by F-1,6-bisP

PFK is the Most Important Regulatory Enzyme of the Glycolytic Pathway “Product” Inhibition: Citrate Inhibits Regulation by Energy Charge: Stimulation by ADP Allosterically inhibition by ATP PFK is also regulated by F-2,6-bisphosphate (hormonally controlled) Surplus of citrate occurs when the TCA cycle produces a sufficient amount of ATP