Principles of Protein Structural Organization - G. Schultz 1982

Covalent Structure of Proteins
Enzyme-Controlled Modifications of Side Chains
Conclusion

To gain a clearer picture, simplification is usually sought, and this approach has been adopted in this chapter. However, one must remain aware of the true complexity of the problem. The fact that a polypeptide chain composed of 20 Amino Acids spontaneously folds and subsequently Functions as a globular protein by no means exhausts The complexity of the matter. Typically, for example, chains associate and function as aggregates. The studied properties of such aggregates suggest that the fundamental structural units of Proteins are not individual chains, but individual domains. In proteins with unknown structures, only functional rather than Structural domains can be identified. At the same time, data from known protein structures lead to the Conclusion that the structural domain is a more fundamental concept. Known Examples of protein aggregates are so complex that even isolating monomers and oligomers presents a challenge. We apply physiological criteria according to which monomers may contain more than one polypeptide chain if the chains are covalently linked.

The rather detailed description of proteins in the preceding chapters does not include cross-links and epigenetic modifications. The most common cross-link is the disulfide bridge, which serves both mechanical and chemical purposes. Mechanically important cross-links are frequently formed utilizing the ε-amino group of Lys. During epigenetic modifications, the main chain is often cleaved. This is a vital physiological tool that delivers the required protein to the right place at the right time. Side-chain modifications are also widespread, endowing Enzymes with novel properties. All these phenomena should be kept in mind when attempting to draw specific Conclusions from the rather General Principles outlined at the beginning of this chapter.