The alpha-helix [Pauling, L., Corey, R. B., & Branson, H. R. (1951) Proc. Natl. Acad. Sci. U.S.A. 37, 205-211] is a common motif in both proteins and peptides. Despite intense investigation, predictive understanding of helices is still lacking. A recent hypothesis [Presta, L. G., & Rose, G. D. (1988) Science 240, 1632-1641] proposed that the structural specificity of helices resides, in part, in those residues that flank helix termini. If so, then signals that arrest helix propagation--i.e., helix stop signals--should be found among these flanking residues. Evidence is presented for the existence of one such signal, a reciprocal backbone-side-chain hydrogen-bonding interaction, dubbed the capping box. In proteins, the capping box is found uniquely at helix N-termini. In peptides, the capping box can function as a helix stop signal, as shown in the work of Kallenbach and co-workers.

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Rat 3-hydroxyisobutyrate dehydrogenase exhibits significant amino acid sequence homology with 6-phosphoglucohate dehydrogenase, D-phenylserine dehydrogenase from Pseudomonas syringae, and a number of hypothetical proteins encoded by genes of microbial origin. Key residues previously proposed to have roles in substrate binding and catalysis in sheep 6-phosphogluconate dehydrogenase are highly conserved in this entire family of enzymes. Site-directed mutagenesis, chemical modification, and substrate specificity studies were used to compare possible mechanistic similarities of 3-hydroxyisobutyrate dehydrogenase with 6-phosphoghiconate dehydrogenase. The data suggest that 3-hydroxyisobutyrate and 6-phosphogluconate dehydrogenases may comprise, in part, a previously unrecognized family of 3-hydroxyacid dehydrogenases.

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