Proteins and nucleic acids are copolymers

Enzymes and nucleic acids are not simply polymers, they are copolymers. This seems to be a trivial observation, and partly is however it bears significance to an often forgotten point, that the study of the prebiotic synthesis of polypeptides and nucleic acids - and their properties - cannot ignore the general rules of copolymerization. 

These results do not depend on the absolute values of aa and ab, or ba and bb. but on their ratio, and this means that the outcome of the copolymerization cannot be predicted on the basis of the kinetic parameters of the homopolymerization of A and B alone. This also means that the copolymer product can have a composition of A and B units drastically different from the composition of the starting monomer mixture. For example, starting from a 50 50 mixture of A and B, it may happen that the resulting chains contain only a very little percentage of A. This is so, even when A has a great tendency to polymerize on its own. 

In the case of vinyl monomers (styrene, acrylonitrile, acrlylamide, isobutylene, etc.) copolymerization is generally spontaneous however, the reaction products are determined by the kinetic constants - a case of interplay between thermodynamic and kinetic factors. 

In the case of proteins or nucleic acids we do not have two, but several comonomers furthermore we are not dealing with the simple case of radical polymerization, but with the more complex polycondensation. Very little is known about the kinetics of the copolymerization of polycondensates - for example analysis of ta and re has not been done systematically for amino acids. However, a few general points can still be made on the basis of the general principles of copolymerization. One has been already mentioned that the initial composition of amino acids in the prebiotic soup may not correspond to the amino-acid composition in the chain. Thus, the fact that one given amino acid has a very small frequency of occurrence in protein chains may not necessarily mean that this amino acid was not present under prebiotic conditions the low frequency in the chains can simply be the result of the kinetics of polycondensation. Conversely, the presence of preferred residues or short sequences in protein chains might be due to the interplay of kinetic parameters, and have little to do with the initial biological constraints. 

Kinetic constants and kinetic parameters measure probabilities, and of course in a real copolymerization process one eventually should consider velocities - namely take into account the actual concentration of A and B. Then prediction of composition and preferential sequences becomes even more complicated (Billmeyer, 1984 Elias, 1997). 

---

Most naturally occurring polymers are largely homopolymers, but proteins and nucleic acids are copolymers composed of a number of different mers. While many synthetic polymers are homopolymers, the most widely used synthetic rubber, SBR, is a copolymer of styrene (S) and butadiene (B), with the R representing rubber. There are many other important copolymers. Here we will restrict ourselves to vinyl-derived copolymers. 

---