Covalent structure of turnip peroxidase 7. Cyanogen bromide fragments, complete structure and comparison to horseradish peroxidase C.

The complete amino acid sequence of turnip peroxidase TP 7, the principal isoperoxidase during winter in turnip, Brassica napus L., variety Blanc dur d'hiver, has been determined by sequence analysis of cyanogen bromide fragments and of tryptic peptides. The turnip peroxidase TP 7 enzyme is composed of 296 amino acids, one hemin group and one neutral carbohydrate side chain attached through asparagine. The molecular weight of the polypeptide part is 31,060, and including hemin and carbohydrate the molecular weight of the native enzyme is close to 33,400. The isoelectric point of turnip peroxidase TP 7 is 11.6. Comparison of turnip peroxidase TP 7 and horseradish peroxidase HRP C shows that they contain four similarly located disulfide bridges and have pyrrolidone carboxylyl N termini. Their common evolutionary origin is distant as their amino acid sequences are only 49% identical. Furthermore, turnip peroxidase TP 7 differs significantly from three other isoperoxidases of turnip root, turnip peroxidases TP 1, TP 2 and TP 3, and from horseradish peroxidase HRP C in its physico-chemical and enzymatic properties, and its pronounced season-dependent appearance. All these differences of turnip peroxidase TP 7 and of the others suggest they serve separate biological functions.