The metabolism of cells includes: (1) All the individual chemical reactions, (2) The sequences of these reactions, (3) The interrelationships that exist among the reaction sequences, and (4) The various mechanisms that regulate the reactions. Individual reactions may be energy producing (the exergonic reactions) or energy consuming (endergonic reactions). Commonly, the primary reactants or substrates are converted into final products [...]
Initially the existence of a pathway is identified by observing that the consumption of certain reactants leads to the accumulation of certain products. For example, the consumption of sugar and the production of carbon dioxide and alcohol during alcoholic fermentation is a pathway that has been known for centuries; the overall reaction is, By quantitative chemical analysis, it is possible [...]
Occasionally, several different enzyme molecules, all of which catalyze the same chemical reaction, have been isolated from a single tissue. Such families of enzymes are called isoenzymes or isozymes. Among the various isoenzymes the lactic dehydrogenases have been most extensively studied, and five different forms have been identified. All are composed of four polypeptide chains of two types called M [...]