Carboxypeptidases from Germinating Soybeans. II. : Stability and Some Chemical and Kinetic Properties of Carboxypeptidases S_a and S_b

Abstract

Carboxypeptidase S_a was composed of 716 amino acid residues, 90 hexose residues (as galactose), and 31 hexosamine residues (as glucosamine) ; these were Trp₁₄, Lys₄₂, His₁₅, Arg₂₁, Asp₇₇, Thr₄₂, Ser₆₀, Glu₅₀, Pro₃₄, Gly₄₈, Ala₅₇, Val₅₀, Met₆, Ile₅₆, Leu₉₃, Tyr₂₂, and Phe₃₄. Carboxypeptidase S_b was composed of 969 amino acid residues but was free from hexose and hexosamines ; these were Trp₂₁, Lys₆₂, His₁₉, Arg₃₅, Asp₁₀₉, Thr₄₉, Ser₇₃, Glu₈₄, Pro₄₉, Gly₈₈, Ala₇₄, Gys (half)₄, Val₆₂, Met₁₅, Ile₄₉, Leu₇₅, Tyr₃₄, and Phe₄₇. Carboxypeptidase S_a was stable in the pH range from 4.5 to 6.5 and in the temperature range from 0 to 50°, but carboxypeptidase S_b was quickly denatured at pH values below 5.0 or over 6.0. They were completely inhibited by the detergents, sodium dodecyl sulfate and cetyltrimethylammonium bromide, but not by Tween 80 or Triton X-100. The Km and V_max values of carboxypeptidase S_a for the hydrolysis of Z-Glu-Phe were 0.16 M and 0.29 μmol/min/·mg, respectively, and those of carboxypeptidase S_b were 0.053 M and 11.36 μmol/min·mg, respectively.