Amino acid sequence calcium-binding motif

Table 2.2 Amino acid sequences of calcium-binding EF motifs in three different proteins Pamalbumin VKKAFAI I DQDKSGFIEEDELKLFLQNF Calmodulin FKEAFSLFDKDGDGT I TTKELGTVMRSL Troponin-C LADCFR I FDKNADGF I D lEELGE I LRAT...

When proteins fold into their tertiary structures, there are often subdivisions within the protein, designated as domains, which are characterised by similar features or motifs. A protein domain is a part of the protein sequence and structure that can evolve, function and exist independently of the rest of the protein chain. Many proteins consist of several structural domains. One domain may appear in a variety of evolutionarily related proteins. Domains vary in length from about 25 up to 500 amino acids. The shortest domains, such as zinc fingers , are stabilised by metal ions or disulfide bridges. Domains often form functional units, such as the calcium-binding EF hand domain of calmodulin. As they are self-stable, domains can be swapped by genetic engineering between one protein and another, to make chimera proteins. 

Protein families are proteins related by structure or function. A protein family may be structurally diverse but have a particular cluster of amino acids at the active site that defines the class according to some catalytic function (e.g., dehydrogenases and kinases). Alternatively, proteins may have a structural motif that defines the class (e.g., helix-loop-helix motif of the EF-hand calcium-binding proteins). Proteins with identical function in different organisms often have slightly different primary structures (see below). The presence of certain amino acids relative to others in primary sequences allows putative protein sequences from the Human Genome Project, for example, to be classified into general protein families. Whether this initial classification is valid remains to be seen. 

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