Disulfide linkage

ENZYME KINETIC DATA IN PROTEINS STRUCTURE-FUNCTION STUDIES 

Critical to the elucidation of stmcture-function relationships of enzymes is the determination and analysis of kinetic data used in conjunction with stmctural information. The more supportive these two data sets (i.e., kinetic and structural information) become, the better will be our ability not only to understand enzyme catal5ftic mechanisms at a molecular level but also to design enzymes knowledgeably for specific end uses. 

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Sanger also determined the sequence of the A chain and identified the cysteine residues involved m disulfide bonds befween fhe A and B chains as well as m fhe disulfide linkage wifhin fhe A chain The complefe insulin sfruefure is shown m Figure 27 11 The sfruefure shown is fhaf of bovine insulin (from cattle) The A chains of human insulin and bovine insulin differ m only fwo ammo acid residues fheir B chains are identical except for the ammo acid at the C terminus... 

The immunoglobulin structure in Figure 6.45 represents the confluence of all the details of protein structure that have been thus far discussed. As for all proteins, the primary structure determines other aspects of structure. There are numerous elements of secondary structure, including /3-sheets and tight turns. The tertiary structure consists of 12 distinct domains, and the protein adopts a heterotetrameric quaternary structure. To make matters more interesting, both intrasubunit and intersubunit disulfide linkages act to stabilize the discrete domains and to stabilize the tetramer itself. 

A second kind of covalent bonding in peptides occurs when a disulfide linkage, RS-SR, is formed between two cysteine residues. As we saiv in Section 18.8, a disulfide is formed by mild oxidation of a thiol, RSH, and is cleaved by mild reduction. 

Conserved sequences that fold into large loops stabilized by three disulfide linkages. The name Kringle comes from the Scandinavian pastry that these structures resemble. They can mediate certain protein-protein interactions. 

Biocatalysis refers to catalysis by enzymes. The enzyme may be introduced into the reaction in a purified isolated form or as a whole-cell micro-organism. Enzymes are highly complex proteins, typically made up of 100 to 400 amino acid units. The catalytic properties of an enzyme depend on the actual sequence of amino acids, which also determines its three-dimensional structure. In this respect the location of cysteine groups is particularly important since these form stable disulfide linkages, which hold the structure in place. This three-dimensional structure, whilst not directly involved in the catalysis, plays an important role by holding the active site or sites on the enzyme in the correct orientation to act as a catalyst. Some important aspects of enzyme catalysis, relevant to green chemistry, are summarized in Table 4.3. 

Difluorodiphenyl disulfide, when tested on woolen swatches, is an outstanding knockdown agent and toxicant for body lice. Several compounds with p-fluorinated phenyl radicals have exhibited the same order of activity against lice, but too few compounds with the disulfide linkage have been tested to warrant an appraisal of its effect here. 

Fungal cutinases show no free SH groups but have 4 Cys residues, indicating that they are in disulfide linkage [119]. The reaction of the native enzyme with DTE was extremely slow but in the presence of SDS at its CMC rapid reduction could be observed [102]. Reduction of the disulfide bridge resulted in irreversible inactivation of the enzyme and the protein tended to become insoluble. CD spectra of cutinase in the 205-230 nm region, before and after DTE reduc-... 

Figure 1.10 Sulfhydryl groups may undergo a number of additional reactions, including acylation and alkylation. Thiols also may participate in redox reactions, which generate reversible disulfide linkages.

Figure 1.15 Polypeptide chains may be bound together through disulfide linkages occurring between cysteine residues within each subunit.

Figure 1.20 Cysteine and methionine are highly susceptible to oxidation reactions. Cysteine thiols can form disulfide linkages with other cysteine groups or be oxidized to cysteic acid. Methionine is oxidized very easily to the sulfoxide or sulfone products.

Figure 1.121 Sodium tetrathionate reacts with thiols to form reactive sulfenylthiosulfate intermediates. Another sulfhydryl-containing molecule may couple to this active group to create a disulfide linkage.

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