The high resolution refined structures of 23 enzymes were analyzed to determine the properties of amino acids involved in active site regions. These regions were found to be rich in G-X-Y or Y-X-G oligopeptides, where X and Y are polar and non-polar residues, respectively, that are small and with low polarity. Other regions of the enzyme molecules have significantly fewer of these sequences. These features suggest that glycine residues may provide flexibility necessary for enzyme active sites to change conformation, and the G-X-Y or Y-X-G oligopeptides may be a motif for the formation of enzyme active sites.