Calcium-binding proteins

Abstract

Calcium binding, which appears to be either specific or physiologically significant, has been reported for 70 norminally "different" proteins. First, I catalog these proteins. Only a few recent or general references can be cited. Those proteins that serve critical physiological functions or that may be regarded as chemical prototypes are discussed in more detail. Second, I present several generalizations. Inevitably the logic here is cyclic in that the generalizations dictate which data from all those available are actually presented. In order to focus our attention in the catalog, I outline these generalizations: 1. An examination of the five calcium-binding proteins of known structure does not reveal a correlation between ligand type and/or geometry and Ca2+ affinity or selectivity. 2. For many of the enzymes supposedly activated or stabilized by Ca2+ the available data dot not allow one to judge the physiological significance of the calcium binding or its contribution of the enzymic mechanism. 3. Of the enzymes requiring Ca2 and concanavalin A, only the nuclease of Staphylococcus and possibly phospholipase appear to bind Ca2 at the active site. 4. The calcium affinities of most of the extracellular enzymes are low, pKd = 3 to 4. This is consistent with the fact that the Ca2+ concentration of the extracellular environment is about 10(-3) M. 5. The cytosol concentration of free Ca2+ in most if not all eukaryotic cells is from 10(-6) to 10(-8) M. Following a stimulus to the cell after which Ca2+ functions as a second messenger, the free Ca2+ may rise to 10(-6) to 10(-5) M. The reported affinities of most enzymes of the cytosol are too low to be physiologically significant. 6. Several intracellular enzymes or enzyme activators have pKd values between 5 and 8. These enzymes therefore may be turned on and off or "modulated" in response to extracellular stimuli. 7. There is a distinct conceptual difference between extracellular enzymes that are "activated" by Ca2+ and those intracellular enzymes that are modulated by Ca2+. The activated proteins bind Ca2+ upon secretion or upon incorporation into a secretory vesicle and retain it throughout their functional lifetimes. The modulated proteins may bind and release Ca2+ many times in response to varying concentrations of this second messenger. 8. Several of the calcium-modulated proteins contain a characteristic conformation, consisting of a helix, calcium-binding loop, and second helix, referred to as the "EF hand". These proteins are homologous, that is, evolutionarily related.