Peptides are formed in four ways as described next.

1. Peptides are released by enzymatic digestion of proteins that have been synthesized on ribosomes according to the information coded in mRNA. This is the major route of peptide formation in vivo. Both the hydrolysis of food proteins in the digestive tract and also release of biologically active peptides in a variety of organs are classified in this category. The production of peptides by recombinant DNA technology essentially follows this route.

2. In living organisms there are peptide-synthesizing systems that are independent of mRNA and ribosomes. A few peptides (glutathione, kyotorphin, and cyclic peptide antibiotics) are synthesized in this way.

3. Under special in vitro conditions peptide bonds are also formed by protease-catalyzed reactions (1) (condensation and aminolyses of amides or esters).

4. Peptides are chemically synthesized by a condensation of amino acids in the presence of an activating reagent for carboxyl groups such as carbodiimides. If condensation between the carboxyl group of amino acid A and the amino group of amino acid B is to occur, the amino group of A and carboxyl group of B must be protected. Protection of other functional groups in the side chains of amino acids is also necessary. Stepwise syntheses of peptides are facilitated by a solid-phase synthesis (2). In the solid-phase synthesis developed by Merrifield, i-butyloxy-carbonyl (i-Boc) amino acid is coupled to chloromethyl polystyrene resin. After the acid-catalyzed removal of the i-Boc group from the resin, the next i-Boc amino acid is coupled to the resin in the presence of carbodi-imide. Amino acids are thus coupled successively to the resin from the carboxyl terminus toward the amino terminus. Peptides are released from the resin in a strong acid such as anhydrous hydrogen-fluoride. Protecting groups for amino acid side chains are also removed by this procedure. In a new procedure, the alkaline labile 9-fluorenyl-methoxycar-bonyl (Fmoc) group is used as a blocking group for the «-amino group. In this case, recovery of the peptide from the resin is facilitated by trifluoroacetic acid.

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