Proteins are macromolecules composed of polymeric chains of amino acids linked together in a sequence that is unique for each protein. They provide much of the structure of the cell and comprise the largest percentage of the cell mass.1 The amino acid building blocks that make up proteins are drawn from a standard repertoire of 20 amino acids that are the common for all living cells.1 Millions of proteins of diverse structure and function are found in all living organisms. The amino acid sequences of more than 2.3 million proteins have been determined, or predicted based on DNA sequence, and have been catalogued in searchable protein databases.2 Approximately 74% of the catalogued proteins are organized into 7677 different families according to their relatedness in structure and function.2 The same families of proteins whose structure and function are related can be found across different orders in plant and animal kingdoms. "For distantly related species, nature doesn't reinvent the wheel. Similar proteins involved in essential cellular functions are often similar across species."3 For example, a recent comparison of the protein-protein interactions for three distantly related species (yeast, worm, fly) found some conservation in the proteins and patterns of interactions, although differences were also noted.4 Humans share proteins with similar amino acid sequence and function with other organisms, as observed for the hemoglobin a chain where the percentage of identical amino acids (human/animal) ranges from 35% for lamprey to 56% for frog and 70% for chicken.5 Genome sequences reveal that vertebrates have inherited nearly all of their protein domains from invertebrates; only 7% of identified human protein domains are vertebrate-specific.1
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