Protein folding unique fitness

Another approach is to map the arrangement of secondary structural elements onto the known tertiary structures of other proteins. Currently, approximately one hundred unique protein folds have been identified. There is some question as to if this is an upper limit. If this is indeed the case, then the protein of unknown structure must adopt a known topological fold. The secondary structural elements are mapped onto the template of the different known protein structures. The best fits, as judged by the environmental factors (solvent accessibility) of the individual amino acids, are then further analyzed as probable folds. This procedure is referred to as threading the secondary elements into three-dimensional structures [28],... 

Considerating the number and relative stringency of the functional criteria satisfied by the protein folds - - self-organizing robustness in conjunction with marginal stabihty, diverse architectures, a hydrophobic core fit for organic synthesis, diverse bulk properties, etc. - it seems likely that few other types of polymer will be equally fit. At present, I think that current knowledge is consistent with the possibility that the protein folds represent an ensemble of natural forms uniquely fit for the mechanism we call life. If correct, this not only would support Henderson s contention that the cosmos is fine-tuned for carbon-based life, but would further restrict this statement to the protein-based variety of life that exists currently on earth. 

These studies fit the general patterns of this story. When proteins started to get picky about metals, the first protein folds bound iron, then manganese (soon), and molybdenum (also soon) appeared. Zinc, calcium, and copper came later, or in the case of porphyrins, never showed up at all. Zinc s line looks different from the others, appearing earlier but growing more slowly, showing a unique pattern over time. 

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