Low-molecular-weight compounds

A number of coordination compounds found in organisms have relatively low molecular weights. Ionophores, molecules that are able to carry ions across lipid barriers, are polydentate ligands designed to bind alkali and alkaline-earth metal ions; they span membranes and serve to transport such ions across these biological barriers. Molecular receptors known as siderophores are also polydentate ligands; they have a very high affinity for iron. See also Ionophore.

Other low-molecular-weight compounds are metal-containing cofactors that interact with macromolecules to promote important biological processes. Perhaps the most widely studied of the metal ligands found in biochemistry are the porphyrins; iron protoporphyrin IX (see illustration) is an example of the all-important complex in biology known as heme. Chlorophyll and vitamin B₁₂ are chemically related to the porphyrins. Magnesium is the central metal ion in chlorophyll, which is the green pigment in plants used to convert light energy into chemical energy. Cobalt is the central metal ion in vitamin B₁₂; it is converted into coenzyme B₁₂ in cells, where it participates in a variety of enzymatic reactions.