Current Organic Chemistry - Volume 5, Issue 1, 2001

Peptides are essentially small version of proteins and the structure of peptides is best described as a chain of amino acids linked to each other through amide bonds. In the chemical synthesis of peptides, two procedures are primarily used, although they are both based on fundamentally same principles one is a solution method and the other being a solid-phase method carried out on a resin. The chemistry of peptide synthesis was developed based on the following basic chemical principles 1) selection of protecting groups for amino acids and deprotection and 2) peptide bond formation. Therefore, studies on peptide synthesis in solution can be directly applied to solid phase methodology. This review deals with the fundamental chemistry of peptide synthesis in solution and considers the following points (1) principle of peptide synthesis, (2) protection procedures, (3) chain elongation procedures by either stepwise or segment condensation reactions and (4) final deprotection of protected peptides.

This review describes current methods for peptide and protein syntheses, largely from a strategic point of view. The solid phase method is useful for the rapid preparation of peptides. Two major synthetic strategies have been adopted by this method, namely, the Boc and Fmoc strategies. At the final stage of the Boc solid phase method, a protected peptide resin is treated with a strong acid to obtain a free peptide. On the other hand, in the Fmoc solid phase method, a free peptide is obtained by treating a protected peptide resin with a weak acid. Both solid phase methods are quite useful for the preparation of peptides with molecular weights in the vicinity of five thousand. Ligation methods were developed to overcome the molecular weight barrier existing in a solid phase method. Building blocks used for ligation are prepared by the solid phase method, or more recently by biological methods. All the current ligation methods that produce a native peptide bond use peptide C terminal thiocarboxylic acids or thioesters as building blocks. Blake et al. developed a selective activation method of the C terminal carbonyl group by the combination of thiocarboxylic acid and silver ions. Based on this approach, a thioester method was developed, in which partially protected peptide thioesters are used as building blocks. Subsequently, a new ligation method was developed using peptide thioesters, in which protecting group is no longer necessary. The discovery of protein splicing phenomenon added a biological route to the preparation of peptide thioesters. A partially protected peptides segment can be also derived from an expressed peptide segment. Polypeptides with a molecular weight of more than 10 thousand can be routinely synthesized.

Short chain dehydrogenases/reductases (SDR) constitute a large protein family. The SDR family now includes more than 1,000 enzymes from humans, mammals, insects and bacteria, and exhibits a wide variety of substrate specificity for steroids, retinoids, prostaglandins, sugars, alcohols and other small molecules. These enzymes have a residue identity level of 15-30 perent. Much has been done in the last decade to understand the structure function relationships in the SDR enzymes. This review summarizes recent progress of structural and functional studies of the enzymes belonging to the SDR family (X ray crystal structure analyses and site directed mutagenesis studies). Based on these studies, the three dimensional structure, catalytic mechanism, coenzyme specificity, and substrate specificity of the SDR enzymes are discussed.