Chemistry and Biology of Proteins - F. Haurowitz 1953

Proteins with Enzymatic Properties
Other Enzymes

The myogenic fraction of Muscle contains enolase in crystalline form; it precipitates from muscle extract at lower ammonium sulfate concentrations than phosphoglyceraldehyde dehydrogenase [165]. Enolase has also been isolated from Yeast. It catalyzes the reversible conversion of 2-phosphoglyceric acid into phosphoenolpyruvic acid;

Class="center">2-Phosphoglyceric acid ⇄ phosphoenolpyruvic acid + H₂O.

The enzyme is precipitated from yeast extracts by nucleic acid and mercury salts, allowing the ISOLATION OF A crystalline mercury derivative of the enzyme [166]. The Molecular Weight of the purified enzyme is 66,000 [167]. Each enzyme molecule contains 1 magnesium atom.

Zymohexase is an enzyme complex consisting of aldolase and isomerase; aldolase catalyzes the reversible Cleavage of a fructose-1,6-diphosphate molecule into two triosephosphate molecules:

Fructose-1,6-diphosphate ⇄ triosephosphate + triosephosphate.

Aldolase from yeast apparently requires the presence of metals for its activity, whereas muscle aldolase remains active even in the presence of poisons that affect metal-containing Enzymes [169]. Processing the muscle of 20–30 rats yields 1 g of crystalline aldolase [169]. In recent years, data regarding the Amino Acid Composition of aldolase and phosphoglyceraldehyde dehydrogenase have been obtained [165]. These data are presented in Table 1.

Finally, lipoxidase should be mentioned as the last oxidative enzyme obtained in crystalline form. It was extracted from soybeans and purified using ammonium sulfate [168]. This enzyme catalyzes the Oxidation of Unsaturated Fatty acids by atmospheric oxygen; its molecular weight is 90,000–100,000 [168]. It has not yet been established whether lipoxidase possesses a non-protein active group.

Yeast and animal Tissues contain several enzymes that catalyze the phosphorylation or dephosphorylation of Organic compounds. Phosphorylation is known to be coupled with oxidation-reduction reactions; therefore, these enzymes have already been discussed generally along with oxidoreductases. Hexokinase, which catalyzes The conversion of glucose into glucose-6-phosphate, has been obtained in crystalline form by ammonium sulfate precipitation [170, 171]. The molecular weight of hexokinase is 96,000. Phosphotransferase, one of the most active enzymes in yeast Fermentation, has also been obtained in crystalline form [172]. The enzyme that catalyzes the attainment of equilibrium in the reaction

1,3-Diphosphoglyceric acid + ADP ⇄ 3-phosphoglyceric acid + ATP

belongs to the Nucleoproteins. It has not yet been established whether hexokinase and phosphotransferase possess non-protein prosthetic groups. The MECHANISM OF ACTION of these enzymes was discussed above.

The protein component of carboxylase (the second enzyme playing a crucial role in fermentation) is obtained by treating the enzyme with dilute alkali solutions. The prosthetic group of yeast carboxylase is thiamine pyrophosphate [173], the phosphate groups of which are apparently linked to the protein component via magnesium ions. It is also possible that the apoenzyme and coenzyme are linked through the quaternary nitrogen of thiamine [174]. Bacterial Decarboxylases, which cleave carbon dioxide from Tyrosine and Other Amino Acids, contain Pyridoxal phosphate as a prosthetic group [175]. The protein component of these decarboxylases has not yet been studied.

Proteins also take part in the photochemical reaction that leads to oxygen production in green plants. Biological oxygen evolution can be reproduced in vitro by irradiating a chloroplast suspension in solutions of iron oxalate and potassium ferricyanide [176] or quinone [177]. Oxygen is evolved in both cases. In these experiments, METABOLISM/14.html">Chloroplasts containing proteins and Lipids cannot be replaced by protein-free chlorophyll.