Proteins chemical total synthesis

Corresponding improvements in the semisynthesis of proteins have been developed (see Section 5.1.11). They, too, have involved intermolecular and intramolecular reactions to form normal peptide bonds between small synthetic peptides and large protein components derived from natural sources or produced by recombinant techniques. In this case, there is the potential to produce derivatives of much larger proteins than can currently be prepared by total chemical synthesis. The total synthesis and semisynthesis approaches are useful for somewhat different purposes, but can be considered complementary together, they greatly broaden the field of protein chemistry and biology. 

Native chemical ligation, which is described in Section 5.3, has proven to be a generally useful route to the total synthesis of proteins of up to medium size. A recent adaptation which utilizes a protein splicing element, an intein, to generate a reactive thioester at the C-terminus of a recombinant protein provides an interesting variant of this technique. 4,5 ... 

Carbodiimides are the diimides derived from carbon dioxide, and they are extensively used in the formation of peptide amide bonds from carboxylic acids and amines. This reaction was utilized by the Nobel laureate Sheehan in the total synthesis of penicillin. He also was the first to use water soluble carbodiimides to crosslink gelatin. Khorana, another Nobel laureate, demonstrated that carbodiimides can also be used in the synthesis of nucleotides. Today, carbodiimides are used extensively in the synthesis and modification of proteins. Proteomics is the new frontier of chemical research. 

Although the total synthesis of a protein allows complete control over the structure, including posttranslational modifications and introduction of labels at desired sites in the sequence, it is still a major undertaking for which most laboratories whose main interest is in the biology of their target proteins are not equipped. In certain cases, for example when the site of introduction of a specific chemical modification is near the C-terminus, a combination of molecular biological and chemical methods has proved to be very powerful. 

The first total synthesis of a protein by the chemical ligation approach utilized nonnative bonds at the site of ligation. In the early 1990s the Kent laboratory pioneered a thioester forming ligation that utilized a highly chemoselective reaction between a peptide with a C-terminal carbothioate group 1 and a N-terminal bromoacetyl peptide 2 at pH <4 (Scheme 2). 

Scheme 6.260 shows an example of the application of photooxygenation in the total synthesis of dysidiolide (539), a cdc25A protein phosphatase inhibitor, involving regioselective oxidation of the furan moiety of 540 in nearly quantitative chemical yield in the last step of the synthetic procedure.1447... 

Callystatin A (4) is a higly potent cytotoxic polyketide isolated from Callyspongia truncata Its absolute structure was determined by chemical method and confirmed by total synthesis. Its potent cytotoxicity (IC50 = 0.022 nmol against human pharyngeal carcinoma KB cells) was presumed to be due to the inhibition of nuclear export signal (NES)-dependent export of nuclear proteins. ... 

The term enzyme engineering [11] (see Fig. 1) describes a range of techniques from deliberate chemical modification to remodeling a wild-type enzyme by gene technology. Subti-ligase , a mutant of subtilisin BNP , was prepared by Jackson et al. [12] by protein design and used in a further total synthesis of RNase... 

---