The Covalent Structure of Proteins:- Small Peptides and Proteins Can Be Chemically Synthesized

Many peptides are potentially useful as pharmacologic agents, and their production is of considerable commercial importance. There are three ways to obtain a peptide: (1) purification from tissue, a task often made difficult by the vanishingly low concentrations of some peptides; (2) genetic engineering (Chapter 9); or (3) direct chemical synthesis. Powerful techniques now make direct chemical synthesis an attractive option in many cases. In addition to commercial applications, the syn thesis of specific peptide portions of larger proteins is an increasingly important tool for the study of protein structure and function. The complexity of proteins makes the traditional synthetic approaches of organic chemistry impractical for peptides with more than four or five amino acid residues. One problem is the difficulty of purifying the product after each step. The major breakthrough in this technology was provided by R. Bruce Merrifield in 1962. His innovation involved synthesizing a peptide while keeping it at tached at one end to a solid support. The support is an insoluble polymer (resin) contained within a column, similar to that used for chromatographic procedures. The peptide is built up on this support one amino acid at a time using a standard set of reactions in a repeat ing cycle (Fig. 3–29). At each successive step in the cycle, protective chemical groups block unwanted reactions. The technology for chemical peptide synthesis is now automated. As in the sequencing reactions already considered, the most important limitation of the process is the efficiency of each chemical cycle, as can be seen by calculating the overall yields of peptides of various lengths when the yield for addition of each new amino acid is 96.0% versus 99.8% (Table 3–8). Incomplete re action at one stage can lead to formation of an impurity (in the form of a shorter peptide) in the next. The chemistry has been optimized to permit the synthesis of proteins of 100 amino acid residues in a few days in reasonable yield. A very similar approach is used to synthesize nucleic acids (see Fig. 8–38). It is worth noting that this technology, impressive as it is, still pales when compared with biological processes. The same 100-amino-acid protein would be synthesized with exquisite fidelity in about 5 seconds in a bacterial cell. A variety of new methods for the efficient ligation (joining together) of peptides has made possible the assembly of synthetic peptides into larger proteins. With these methods, novel forms of proteins can be created with precisely positioned chemical groups, including those that might not normally be found in a cellular protein. These novel forms provide new ways to test theories of enzyme catalysis, to create proteins with new chemical properties, and to design protein sequences that will fold into particular structures. This last application provides the ultimate test of our increasing ability to relate the primary structure of a peptide to the three-dimensional structure that it takes up in solution.

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مواضيع ذات صلة

Overview of Protein Structure:- The Peptide Bond Is Rigid and Planar

Overview of Protein Structure:- A Protein’s Conformation Is Stabilized Largely by Weak Interactions

Protein Sequences and Evolution:- Protein Sequences Can Elucidate the History of Life on Earth

The Covalent Structure of Proteins:- Amino Acid Sequences Provide Important Biochemical Information

The Covalent Structure of Proteins:- Amino Acid Sequences Can Also Be Deduced by Other Methods

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

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

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