Differential geometry and protein conformation. V. Medium-range conformational influence of the individual amino acids

A previous differential geometric analysis of the conformational properties of the various amino acids has been extended to study their influence on folding over a larger backbone interval. In addition, statistical effects associated with variation in the number of the individual amino acids in the database have been treated in greater detail, using a simulation method. It is found that the amino acids can be divided into three groups on the basis of their conformational influence over four-C^α units in the interval i ? 6 ? j ? i + 6. Group Ia is composed of seven amino acids (His, Leu, Ala, Met, Lys, Gln, Ile) that encourage the formation of A_R-helical structure. Group Ib (Glu, Phe, Trp, Val, Asp) is composed of amino acids with some helix-forming tendency but that also show positive extended-strand formation tendency. They therefore act as a bridge between group Ia and group II (Cys, Gly, Asn, Pro, Arg, Ser, Thr, Tyr) that contains amino acids that encourage the formation of extended structure and bends. The detailed four-C^α conformational properties of each of the amino acids are shown, and the ability of amino acids to exert conformational influence in both directions along the backbone is examined. It is shown that, in general, such influence extends farther in the N-terminal direction than in the C-terminal direction. A framework is briefly sketched for using the present data to investigate actual folding mechanisms.