Principles of Protein Structure - H. Schulz 1982

Covalent Protein Structure
Disulfide Bonds
Disulfide bridges as structural components of proteins

Disulfide bridges, which lead to loop formation within the polypeptide chain, have been found in several Proteins (Pepsin, thioredoxin, the Insulin A-chain, Silk Fibroin [145], lipoamide dehydrogenase, and other pyridine nucleotide disulfide oxidoreductases [111]). Between the bridged Cysteine residues, the polypeptide chain contains 2–4 residues. Examination of models and X-Ray Diffraction Analysis show that such loops possess a flattened, rigid Structure. In Glutathione reductase and related Enzymes, the isoalloxazine ring of FAD is involved in the loop [123, 124].

Another frequently encountered structural element is the —Cys—Cys— sequence, where both residues form disulfide bridges with other cysteine residues [82]; the geometry of the peptide unit precludes The formation of a disulfide bond between adjacent residues. Thus, the —Cys—Cys— sequence provides a means for binding and bringing together three chain segments. It is highly unlikely that the presence of —Cys—Cys— fragments linked by S—S bonds in 15 proteins is a coincidence or the result of their common ancestry. Therefore, it has been suggested that the —Cys—Cys— sequence represents a particularly advantageous feature of Cell/13.html">Protein Structure and that many proteins have independently evolved this sequence during the course of evolution (Fig. 7.2).

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Fig. 4.4. Collagen biogenesis [131].

The following stages have been identified: Stage 1. Synthesis of pre-a₁ chains and pre-a₂ chains in a 2 : 1 ratio. Each chain contains 1300 residues. Stage 2. Hydroxylation of certain Pro and Lys residues. Stage 3. Attachment of sugars (Glc—Gal) to the hydroxylated residues. Stage 4. Trimer formation accompanied by the creation of interchain disulfide bridges, which appear to be located at the terminal Regions of the chains. Stage 5. Formation of a triple helix in the center of the procollagen molecule. Stage 6. Secretion of procollagen into the extracellular space. Stage 7. Removal of the globular terminal fragments of collagen. Stage 8. (Spontaneous) self-aggregation of collagen molecules (fiber formation). Stage 9. Deamination of Lys and hydroxylated residues to yield aldehydes. Stage 10. Cross-linking of polypeptide chains within the fiber via reactions involving these aldehydes as well as Lys and His side chains (for details, see [131]).