Calcium

Abstract

The focus of this chapter is on the biological chemistry of calcium, building from a basic understanding of the unique physicochemical and structural properties that both differentiate it from other competing metal cations, as well as lending it the required functional characteristics to promote biological activity. Following a brief introduction to key kinetic, thermodynamic and coordination chemistry properties of calcium, pathways of uptake will be described for intestinal cells and ultimately serum for general cellular distribution. This will include a review of various categories of membrane transporters (both active and passive) and the role of vitamin D and its derivatives and other hormones in regulating calcium transport, as well as the important role for bone as a central store of calcium and mechanisms of release into the blood. Subsequently, the speciation of calcium in blood as a hydrated cation, as albumin-bound forms, and complexed to other specialized proteins will be described, as well as the important role for calciumactivated proteins in blood clotting and cell signalling mechanisms. The latter topic will bring us back to the intracellular role of calcium as a second messenger and a review of intracellular depositories of calcium in the endoplasmic and sarcoplasmic reticula and in mitochondria. Also discussed are the mechanisms of membrane transporters that facilitate the rapid increase or decrease in cytosolic calcium levels that underlie its role as a second messenger. These include voltage- and receptor-gated channels, as well as symports and antiports involving co-transport of sodium and hydrogen ions. This provides an opportunity to understand further the role of proteins in buffering intracellular calcium levels, as well as in promoting signalling pathways. Finally, the similarities in eukaryotic and prokaryotic calcium biology will be detailed, from the viewpoint of their roles in transport, buffering and signalling. A comment will cover the chemistry of ionophores secreted/excreted by prokaryotae, which can equilibrate calcium concentrations across membranes, killing target organisms and serving as a defence mechanism for select bacteria. A final section will detail some important questions and topics that remain to be addressed. The reader will find additional general details and insights on several of the topics and proteins discussed in this chapter in several texts that are worthy of review. 1-6