Carp Muscle Calcium-binding Protein: II. STRUCTURE DETERMINATION AND GENERAL DESCRIPTION

Abstract

The structure of crystalline carp muscle calcium-binding protein (parvalbumin) has been determined by x-ray diffraction techniques to nominal 1.85-A resolution. Isomorphous and anomalous scattering data were measured for three heavy atom derivatives, 3-chloromercuri-2-methoxypropyl urea, mercury bromide, and ethyl mercury chloride, to 2.0-A resolution using precession photography. As described in Paper III in this series the 2.0-A phases were refined and the 2.0- to 1.85-A phases were determined by use of the tangent formula. The electron density map is interpreted in terms of the 108 amino acid sequence described in Paper I in this series. A calcium ion is bound in the loop between helix C and helix D and a second calcium is bound in the EF loop. The entire CD region is related to helix E, the EF loop, and the terminal helix F by an approximate intramolecular 2-fold axis. Although it does not bind calcium the AB region has a structure similar to the CD and EF regions and appears to have resulted from a gene triplication. The molecule is generally spherical with a well defined hydrophobic core, one-seventh of its total volume, composed of side chains of phenylalanine, isoleucine, leucine, and valine. All of the polar side chains are at the surface except those associated with calcium binding and with an invariant internal salt bridge between arginine-75 and glutamic acid-81.