ABSTRACT
Proteins are at the interface between chemistry and biology: on the one hand they are among the largest of chemically well-defined molecules, while on the other hand they are perhaps the smallest systems which display the complexity, quirkiness, and downright intractability of living systems. One reason for the fascination of the protein folding problem is that it represents an unusually concrete and limited case of the whole problem of reductionism. Most of the small metal-rich proteins form approximately cylindrical structures with either a simple up-and-down or a Greek key topology, but where the elements forming the cylinder are a mixture of helices and extended strands. As a general rule, disulfides appear to be incompatible with either metals or prosthetic groups, a dichotomy which allows the separation of the small irregular proteins into disulfide-rich and metal-rich. Presumably all proteins rearrange to some extent during the final stages of folding in order to settle into an optimal balance of all energetic factors.
