Principles of Protein Structure - G. Schultz 1982

Covalent Protein Structure
Enzyme-Controlled Modifications of Side Chains
Other Types of Modification Reactions

Side chain modification expands THE SPECTRUM OF protein properties. Since side-chain modification endows Proteins with unique characteristics, only a few of the most General Remarks can be made in this regard. In most cases, only a single side chain or a limited number of them are modified. This Specificity is determined by the protein and modifying enzyme structures, as well as The Nature of the side chain. Examples of side-chain modifications and certain functional aspects are given in Table 4.3. Stryer [85] has provided figures illustrating the chemistry of these reactions.

Regarding the modifying group, the following observations can be made:

a) It endows the side chain with a function that cannot be performed by any of the canonical Amino Acids.

b) It does not alter the Fundamental properties of the protein; the attached group has the molecular mass of a cellular metabolite, i.e., less than 1000.

c) The modifying group can be detached from the protein by peptide bond Cleavage. This property distinguishes side-chain modification from a "prosthetic group," which may be covalently attached to a side chain but is also tightly bound to the protein via non-covalent forces. Examples of prosthetic groups covalently attached to certain proteins include FAD [160] and the heme group. For these proteins, they are, of course, in some respects side-chain modifications. It should be noted that any biological Classification suffers from a certain degree of ambiguity.