The Covalent Structure of Proteins: -The Function of a Protein Depends on Its Amino Acid Sequence

The bacterium Escherichia coli produces more than 3,000 different proteins; a human produces 25,000 to 35,000. In both cases, each type of protein has a unique three-dimensional structure and this structure confers a unique function. Each type of protein also has a unique amino acid sequence. Intuition suggests that the amino acid sequence must play a fundamental role in deter mining the three-dimensional structure of the protein, and ultimately its function, but is this supposition corect? A quick survey of proteins and how they vary in amino acid sequence provides a number of empirical clues that help substantiate the important relationship between amino acid sequence and biological function. First, as we have already noted, proteins with different functions always have different amino acid sequences. Second, thousands of human genetic diseases have been traced to the production of defective proteins. Perhaps one-third of these proteins are defective because of a single change in their amino acid sequence; hence, if the primary structure is altered, the function of the protein may also be changed. Finally, on com paring functionally similar proteins from different species, we find that these proteins often have similar amino acid sequences. An extreme case is ubiquitin, a 76-residue protein involved in regulating the degradation of other proteins. The amino acid sequence of ubiquitin is identical in species as disparate as fruit flies and humans. Is the amino acid sequence absolutely fixed, or in variant, for a particular protein? No; some flexibility is possible. An estimated 20% to 30% of the proteins in humans are polymorphic, having amino acid sequence variants in the human population. Many of these varia tions in sequence have little or no effect on the function of the protein. Furthermore, proteins that carry out a broadly similar function in distantly related species can differ greatly in overall size and amino acid sequence. Although the amino acid sequence in some regions of the primary structure might vary considerably with out affecting biological function, most proteins contain crucial regions that are essential to their function and whose sequence is therefore conserved. The fraction of the overall sequence that is critical varies from protein to protein, complicating the task of relating sequence to three-dimensional structure, and structure to function. Before we can consider this problem further, however, we must examine how sequence information is obtained.

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مواضيع ذات صلة

The Covalent Structure of Proteins:-Large Proteins Must Be Sequenced in Smaller Segments

The Covalent Structure of Proteins:- Short Polypeptides Are Sequenced Using Automated Procedures

The Covalent Structure of Proteins:-The Amino Acid Sequences of Millions of Proteins Have Been Determined

Working with Proteins:- Unseparated Proteins Can Be Quantified

Working with Proteins:- Proteins Can Be Separated and Characterized by Electrophoresis

Working with Proteins:- Proteins Can Be Separated and Purified

Peptides and Proteins:- Some Proteins Contain Chemical Groups Other Than Amino Acids

Peptides and Proteins: - Polypeptides Have Characteristic Amino Acid Compositions

Peptides and Proteins: -Biologically Active Peptides and Polypeptides Occur in a Vast Range of Sizes

Peptides and Proteins: -Peptides Can Be Distinguished by Their Ionization Behavior

Peptides and Proteins: - Peptides Are Chains of Amino Acids

Amino Acids: -Amino Acids Differ in Their Acid-Base Properties