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Amino acid compositions of 356 proteins are expressed as points in an 18 dimensional space of 18 axes representing the contents of amino acids. The proteins are classified into four groups of intra- and extracellular enzymes and nonenzymes according to analysis of the distribution of the points. The groups have a significant correlation to four folding types of secondary structures, and extra- and intracellular proteins to those with and without disulfide bond(s), respectively. The location and function of a protein seem to determine its amino acid composition and folding type.
Nishikawa et al. (Thu,) studied this question.