Sunday, February 8, 2009

Understanding the Broad Compositions of Proteins

In spite of these diverse biological functions, proteins have relatively homogeneous compositions. All proteins are linear polymers of the same 20 types of amino acids in different combinations. The major difference between proteins is in the sequence in which the amino acids are assembled into polymeric chains. The secret to their functional diversity lies partly in the chemical diversity of the 20 amino acids, but primarily in the diversity of the three-dimensional structures that these amino acid building blocks can form by linking in different sequences. The amazing functional properties of proteins can be understood only in terms of their relationship to the three-dimensional structures of proteins.

Now we know that the amino acid sequence of a protein and thereby its three-dimensional structure is specified by a gene. But, this is not completely true. Even though the gene sequence specifies the amino acid sequence of the protein, the three-dimensional structure is also influenced by a number of other factors. The number of proteins produced in a system always exceeds the number of genes. The Human Genome Project has announced the presence of about 35,000 genes. But the actual number of proteins encoded by these genes exceeds the number of genes.

This is mainly because of the various types of molecular modifications such as deletion of amino acids, chemical modifications of certain amino acids, addition of other macromolecules and groups such as phosphate groups (phosphorylation), acetyl groups (acetylations), sugar and other types of carbohydrates (glycosylations), lipids, etc. All these chemical modifications of proteins just after their formation are collectively called post-translational modifications. Actually, these post-translational modifications are responsible for the diversity in the three-dimensional structures and functions along with the amino acid sequence prescribed by the respective genes. A number of proteins are expressed in all cells irrespective of their functional specialization. Such proteins are called housekeeping proteins, required for the basic life activities of all cells_ But there are certain proteins, which are unique to certain cells. Hemoglobin in erythrocytes (RBC), collagen, myosin, etc., in muscle cells are some examples. This is called cell-specific or organ-specific or tissue-specific gene expression. The expressions of these genes are under the control of very specific regulatory proteins or other types of small molecules called transcription factors.

Tags: Bio Technology, Bio Genetics, Proteins

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