Sunday, February 8, 2009

An Introduction to Structure of Proteins

A single cell develops into a multicellular embryo through a large number of complicated biochemical reactions mediated and controlled by various types of proteins expressed during the course of its development. Every function in the living cell depends on proteins. They make us who we are and make our cells operate properly. A cell cannot function without proteins. The shape of a protein determines its biological activity. A single protein may have a varying structure and more than one function. Proteins have many different biological functions. Proteins are even classified according to their biological roles. The key to appreciating how different proteins function in these different ways lies in an understanding of protein structure and their three-dimensional shape. Proteins interact with other molecules such as small molecules, other proteins, nucleic acids, lipids, etc., and these interactions form the basis of their biological roles. Structural complementarity is the means of molecular recognition that allows molecules to interact. The structure of one molecule is complementary to that of its partner(s) in the interaction, like pieces in a puzzle, or a lock and its key. Proteins, by virtue of their architectural diversity, are ideal for such complementary interactions. In short, the structure or the molecular shape of the protein determines its function. Therefore, to understand the function and biological role of a protein it is essential to understand the structure and three-dimensional shape of the protein in detail.

The detailed study of the structure of proteins requires protein extraction and purification to its homogeneity. The purified protein has to be analyzed by various biochemical and instrumental methods to get the details about its chemical composition. The pure protein obtained has to be crystallized to study its three-dimensional shape by x-ray crystallography (an x-ray diffraction technique). Another equally powerful technique to elucidate the three-dimensional structure of protein is NMR spectroscopy. The structural study of protein, thus has two parts-the first part is determination of amino acid sequence and the second part is the elucidation of the three-dimensional shape of the protein formed by the specific folding of the polypeptide chain controlled by a number of molecular forces. The extraction and purification and its crystallization are the preconditions for the detailed structural and functional studies.

The ability to sequence polypeptides was a major step forward in the understanding of the relationship between protein structure and function. It was Dr. Frederick Sanger who developed the basic chemical method for sequencing proteins during the 1940s. He showed for the first time, that proteins are a linear polymer of amino acids, linked in a continuous sequence by peptide bonds. He received the Nobel Prize in 1958 for determining the sequence of the peptide hormone insulin. The peptide bond is formed between the alpha-amino and alpha-carboxyl groups of two adjacent amino acids. Pehr Edman modified this process of amino acid sequencing by introducing a new reagent-phenyl-isothiocyanate for the sequential removal of amino acids and their identification in a protein. This method of sequencing is now automated and called the Edman Degradation Reaction and the instrument is called the Sequenator.

Tags: Bio Technology, Bio Genetics, Proteins

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