glycolytic intermediates, but when this happens,
the associated hydrogen ions cause intracellular
pH to fall. Some lactate will diffuse into the extra-
cellular space and will eventually begin to accu-
mulate in the blood. The lactate that leaves the
muscle fibres is accompanied by hydrogen ions,
and this has the effect of making the buffer capac-
ity of the extracellular space available to handle
some of the hydrogen ions that would otherwise
cause the intracellular pH to fall to a point wherebiochemistry of exercise 25
GlucoseGlycolysisATP
ADPADP
ATPADP
ATPATP
ADPGlucose-6-phosphateHexokinase
Phosphoglucomutase
Glucose-1-phosphateGlycogen
Pi Glycogen
phosphorylaseGlucose-phospate isomeraseFructose-6-phosphateFructose-1,6-bisphosphate6-phosphofructokinaseAldolaseDihydroxyacetone phosphate Glyceraldehyde-3-phosphate
Triose-phosphate
isomerasePi, NAD+
NADHNADH
NAD+NAD+
NADHGlyceraldehyde-phosphate
dehydrogenase1,3-diphosphoglycerate
Phosphoglycerate kinase3-phosphoglycerate
Phosphoglyceromutase
2-phosphoglycerate
EnolasePhosphoenolpyruvatePyruvate kinasePyruvateLactate dehydrogenaseLactateCoA
Pyruvate dehydrogenaseAcetyl-CoA TCA cycleMitochondrionCytosolCleavageSUBSTRATE
LEVEL
PHOSPHORYLATION
OF ADPSugar
activationFig. 2.3The reactions of glycolysis. Glucose, a six-carbon sugar, is first phosphorylated and then cleaved to form
two molecules of the three-carbon sugar glyceraldehyde-3-phosphate, which is subsequently converted into
pyruvate, accompanied by the formation of ATP and reduction of NAD+to NADH. Glycolysis makes two
molecules of ATP available for each molecule of glucose that passes through the pathway. If muscle glycogen is the
starting substrate, three ATP molecules are generated for each glucose unit passing down the pathway. Pyruvate
may enter the mitochondria and be converted into acetyl-CoA, or be reduced to form lactate in the cytosol.
Enzymes are set in small type; Pi, inorganic phosphate; TCA, tricarboxylic acid.