Early in the investigations of glycolysis, it was realized that fermentation in yeast was similar to the breakdown of glycogen in muscle. When a muscle contracts under anaerobic conditions, glycogen disappears and lactate appears. When oxygen is made available, aerobic recovery takes place and lactate is no longer produced. If muscle contraction occurs under aerobic conditions, lactate does not accumulate as pyruvate and NADH, the products of glycolysis. Pyruvate and NADH can enter the mitochondria where they are oxidized further to CO₂, H₂O, and NAD (Figure 1). When oxygen is in short supply, mitochondrial reoxidation of NADH formed during glycolysis is impaired. As there is a limited supply of NAD to sustain glycolysis the NADH has to be reoxidized. The NADH is reoxidized by reducing pyruvate to lactate. This allows NAD to be available for the early steps in the pathway, so permitting glycolysis to continue. While glycolysis can occur under anaerobic conditions, this has a price. It limits the amount of ATP formed per mole of glucose oxidized, so that much more glucose must be metabolized under anaerobic than aerobic conditions to supply the same quantity of ATP to supply cellular work (Table1). In yeast and some other microorganisms, pyruvate formed in anaerobic glycolysis is not reduced to lactate, but is decarboxylated and reduced to form ethanol.

Fig1. Summary of glycolysis. ⊖, blocked under anaerobic conditions or by absence of mitochondria containing key respiratory enzymes, as in erythrocytes.

Table1. ATP Formation in the Catabolism of Glucose

مواضيع ذات صلة

Regulation of the Citric Acid Cycle Depends Primarily on a Supply of Oxidized Cofactors

The Citric Acid Cycle Takes Part in Fatty Acid Synthesis

The Citric Acid Cycle Takes Part in Gluconeogenesis, Transamination, & Deamination

Reactions of The Citric Acid Cycle Generate Reducing Equivalents & CO2

The Citric Acid Cycle Provides Substrates for The Respiratory Chain

Carbohydrates Occur in Cell Membranes & in Lipoproteins

Dyslipidemia and Cardiovascular Risk

Lipoprotein and Lipid Metabolism

Biomedically, Glucose is the Most Important Monosaccharide

Saccharides are Aldehyde or Ketone Derivatives of Polyhydric Alcohols

A Supply of Oxidizable Fuel is Provided in Both The Fed & Fasting States

Many Metabolic Fuels are Interconvertible