Calcium binding proteins. Elucidating the contributions to calcium affinity from an analysis of species variants and peptide fragments

Abstract

This paper describes the sequence homology of calcium-binding proteins belonging to the troponin C superfamily. Specifically, this similarity has been examined for 276 twelve-residue calcium-binding loops. It has been found that, in the calcium-binding loop, several residues appear invariant, regardless of the species of origin or the affinity of the protein. These residues are Asp at position 1 (+ X of the coordinating position of the calcium), Asp or Asn at position 3 (+ Y), Gly at position 6, Ile at position 8, and Glu at position 12 (− Z). It has also been found that conservation of certain residues can vary in similar sites in similar proteins. For example, position 3 (+ Y) in site 3 of troponin C is always an Asn, whereas in calmodulin the residue is always Asp. This study also examined the calcium-binding affinities of peptide fragments comprising the loop, helix–loop, loop–helix, and helix–loop–helix. These were compared with larger enzymatic or chemically generated protein fragments in an effort to understand the various contributions to the calcium-binding affinity of a single-site versus a two-site domain as found in troponin C and calmodulin. Based on free energy differences, it was found that a 34-residue helix–loop–helix peptide represents about 60% of the binding affinity found in the intact protein. Cooperativity with a second calcium binding site accounted for the remaining 40% of the affinity.Key words: calcium-binding proteins, sequence homology, peptide fragments, species variants, calcium affinity.