Amino-acid Sequence of Fragment 90-148 of Human Erythrocyte Carbonic Anhydrase B

Abstract

TWO types of carbonic anhydrase, which differ strikingly in catalytic activity, have been recognized in human erythrocytes and in the red cells of several mammalian species^1–3. The “low” and “high” activity forms have been respectively designated B and C. Their synthesis is under the dependence of two different autosomal genes⁴ but the physiological significance of the presence of isoenzymes in the red cells is still unknown. The two types of carbonic anhydrase have a molecular weight of about 30,000. They consist of a single polypeptide chain of approximately 260 amino-acid residues acetylated on its amino end^5,6 and they contain one zinc atom mol⁻¹ which is essential for catalytic activity. As forms B and C exhibit many differences in their amino-acid composition, carbonic anhydrase has proved to be a very suitable material for studies of structure-activity relationships. The first investigation in this direction was undertaken on the human isoenzymes B and C by X-ray diffraction in conjunction with amino-acid sequence determination. Data on the three dimensional structure of form C at 2.0 Å resolution have been recently published^7,8. X-ray analysis of form B is in progress⁹. The complete primary structure of any carbonic anhydrase has not yet been elucidated. The amino-acid sequence of form C is less well known than that of form B, which has been determined^10–11 except for 1 the segment between residues 90 and 148. The sequence of this part of the molecule is the object of this paper. It completes the knowledge of the primary structure of human form B which is thus the first carbonic anhydrase fully sequenced.