Coenzyme Activity
The action of biotin as a required component in some specific carboxylations and ecarboxylations is well known. Oxalacetate stimulates the growth of L. arabinosus in a medium deficient in both aspartic acid and biotin, This probably results from the failure of the deficient organism to condense pyruvic acid with CO2 to form oxaiacetate, which could then be transaminated, forming aspartic acid. It has been demonstrated that C1402 from ~,aHC1403 was incorporated into aspartic acid to a much smaller extent by livers from biotin- eficient chicks than by livers from normal chicks. Biotin is known to constitute the coenzyme of a variety of carboxylating (C02 fixation) enzymes.
Propionyl CoA carboxylase reacts with CO2 and ATP; in this form the enzyme incorporates CO2 into propionic acid (as propionyl CoA) to form methyl malonyl CoA. The CO2 reacts with biotin to form a biotin-C02 combination (an active CO2 molecule). The point of attachment of the CO2 is somewhat unsettled, but according to Lynen and co-workers the CO2 combines with the nitrogen atom as indicated in the structure shown. Another carboxylation reaction results in malonic acid from acetic. In these carboxylation reactions the process is visualized as a two-step mechanism :
CO2 + biotin-enzyme + ATP ---> CO2-biotin-enzyme + ADP + Pi CO2-biotin-enzyme + acetyl CoA ---> malonyl CoA + biotin-enzyme
The point of attachment of CO2 in oxalacetic transcarboxylase is also on the nitrogen of biotin indicated, according to Wood and others. In the production of urea in the body, one step involves the reaction of carbamyl phosphate and ornithine to yield citrulline. MacCleod and others showed that livers from biotin-deficient rats were capable of carrying out this conversion at a rate of only 50 per cent that of normal livers. In microorganisms the role of biotin in purine synthesis is well established. One step in the synthesis involves CO2 fixation (5-aminoimidazole ribotide (AI)- + -C02 ---> 5-amino-4-imidazolecarboxylic acid ribotide (AICA)).
It is at this point that biotin is involved, since AI accumulates in Saccharomyces cerevisiae under conditions of biotin deficiency and is utilized in the formation of AlCA upon supplying the vitamin.
Interestingly enough, biotin also appears to be involved in a subsequent step, the formation of the carboxamide derivative. The details of these and other reactions concerned with purine synthesis can be found. Some interesting relations of biotin to enzyme and other protein synthesis in animals has been reviewed. Specific roles of biotin enzymes in lipid synthesis in animals are indicated in a review by Vagelos.
Fatty acid synthesis is decreased in animals during biotin deficiency. As might be expected, CO2 transfer is the significant reaction here involving biotin enzymes. Oxybiotin can be utilized by various microorganisms, and it has been demonstrated that higher animals (chicken) can also utilize this molecule without conversion into biotin. Biocytin and desthiooiotin are all used in place of biotin by various microorganisms. Pimelic acid (HOOC(CH2)5COOH) stimulates biotin synthesis in some microorganisms and is growth-promoting, in others.
