Proteins are molecules evolutionarilly selected

All biological species have developed continuosly starting out from single or from a very limited number of ancestral species. As chance may have played a major role in priming events of the past, we have to accept the fact that evolutionary directions may never be fully explained in terms of cause and effect.

Protein evolution involves several changes of few residue, insertions and deletions of many residue, gene doubling, and genes fusion.

In protein structure, the basical step of the historical process is the change of an amino acide residue in the polypeptide chain. In the course of time such changes accumulate, so that eventually all similarities between initial an resultant amico acid sequence are eliminated. But as a rule even after all sequence similarities between two homologous proteins have vanished, the clain folds still resemble each other.

The tendency for replacements at a given residue position varies appreciably. Single amino acid substitutions are not the only differences between homologous proteins. Larger steps are insertions and deletions of single residues or stretches of residues, which affect not only side chains but also the main chain. Insertions and deletions cause great difficulties in sequence comparisons of distantly related proteins.

In such cases knowledge of the three-dimensional protein structures helps considerably. It reveals in general that internal residues vary slowly, whereas the differences between homologous proteins (amino acid changes or deletions and insertions of chain loops) accumulate on the surface. Consequently, the sequences of distantly related proteins can be aligned on the basis of residues that are geometrically at corresponding positions in the tertiary structures..

Several proteins were selected to be stable because it is biologycally advantageous:

1) Flexibility - core action Proteins need to change their conformation to bind to substratum, interact with others proteins and effect the catalysis. Alosteric interactions are done throught structurals transitions which allow non-contiguous regions communicate each other. Similarlly, proteins change quickly between different conformational states, serving as signs translators and energy converter

2) Short life time of rising proteins Intermediary structures must have short life time in proteins folding, the activation barrier for the next produtive state shouldn't be large.

3) Partial unfolding The transport of one protein from one cellular compartment to another, only can occur wheter if it can unfold itself sufficiently to cross the membrane cell chanel.

4) Easy dismounting One example are the colagen molecules which can be degraded over controled way to remodel tissues.

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