Reptile lysozyme: the complete amino acid sequence of soft-shelled turtle lysozyme and its activity.

Soft-shelled turtle egg-white lysozyme was purified and sequenced. Lysozyme was reduced and carboxymethylated to fragment it with trypsin, V8 protease and CNBr. The peptides yielded were purified by RP-HPLC and sequenced. Every trypsin peptide was overlapped by V8 protease peptides and CNBr fragment. The amino acid sequence was compared with other lysozymes. This lysozyme has an extra Gly residue at N-terminus, which was found in pheasant lysozyme. Further, this lysozyme has an insertion of a Gly residue between 47 and 48 residues when compared with chicken lysozyme, as found in human lysozyme, therefore it proved that this lysozyme has the largest number of amino acids (131 aa) in chicken type lysozymes. The amino acid substitutions were found at subsites E and F. Namely Phe34, Arg45, Thr47, and Arg114 were replaced by His, Tyr, Arg, and Tyr, respectively. The time course using N-acetylglucosamine pentamer as a substrate showed a reduction of the rate constant for glycosidic cleavage and increase of binding free energy for subsites E and F, which proved the contribution of amino acids mentioned above for substrate binding at subsites E and F.