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PubMed1973

Histidine uptake in mutant strains of Neurospora crassa via the general transport system for amino acids.

A transport double mutant of Neurospora crassa has been isolated that has only one of the three transport systems capable of l-histidine uptake. The substrate specificity of the remaining transport system, termed the general transport system, has been fully characterized with regard to the contributions to binding of the side chain, the alpha-amino group, and the carboxylate group. The positively charged alpha-amino group is necessary for binding; the negatively charged carboxylate group is of less importance, since its replacement by a neutral carbonyl functional group does not completely abolish binding. The greatest structural latitude for binding was found in the side chain; affinity for alpha-amino acids was uniformly high except for l-aspartic and l-glutamic acids, l-asparagine, and l-proline. Thus, this transport system is "general" with these restrictions.

Amino Acids↗

PubMed1973

Specificity of the tyrosine-phenylalanine transport system in Bacillus subtilis.

l-Tyrosine and l-phenylalanine enter cells of Bacillus subtilis via a system of active transport that exhibits complex kinetic behavior. The specificity of the transport system was characterized both at low concentrations of transport substrate (where affinity for l-tyrosine or l-phenylalanine is high but capacity is low) and at high concentrations (where affinity is low but capacity is high). Specificity was not found to differ significantly as a function of either l-tyrosine or l-phenylalanine concentration. Kinetic analysis showed that the relationship between the uptake of l-phenylalanine and l-tyrosine is strictly competitive. Neither l-tyrosine nor l-phenylalanine uptake was competitively inhibited by other naturally occurring l-amino acids, indicating the importance of the phenyl side chain to uptake specificity. Hence, it is concluded that l-tyrosine and l-phenylalanine are transported by a common system that is specific for these two amino acids. The abilities of analogue derivatives of l-tyrosine and l-phenylalanine to inhibit the uptake of l-[(14)C]tyrosine and l-[(14)C]phenylalanine competitively were determined throughout a wide range of substrate and inhibitor concentrations. In this manner, the contributions of the side chain, the alpha-amino group and the carboxyl group to uptake specificity were established. It is concluded that the positively charged alpha-amino group contributes more significantly to uptake specificity than does the negatively charged carboxyl group. The recognition of a phenyl ring is an essential feature of specificity; other amino acids with aromatic side chains, such as the indole and imidazole rings of l-tryptophan and l-histidine, do not compete with l-tyrosine and l-phenylalanine for uptake. The presence of the p-hydroxy substitutent in the side chain (as in l-tyrosine) enhances the uptake of the aryl amino acid analogues investigated.

Amino Acids↗