X.3.2.1 Ribozymes and coenzymes can be the relics of a time of “life without proteins”, a time when nucleic acids performed all biological functions, including catalytic functions
Some oligonucleotides, alone or in association with other molecules, can exhibit enzymatic activity, which could have placed them at an advantage in competition for the most effective self-replication.Information gained by studying ribozymes, i.e. RNA molecules exhibiting enzymatic activity (Orgel 1986) is certainly very interesting in this respect.Most of the originally studied ribozymes are active in some area of processing RNA (Orgel 1986); nonetheless, at the present time, a great many ribozymes with a broad range of enzymatic activity are known (Connell & Christian 1993).
For example, the intron contained in the precursor of ribosomal RNA of the protozoa Tetrahymena thermophilais a typical ribozyme.This ribozyme is capable both of hydrolyzing various RNA-substrates, including its own pre-rRNA, and also of catalyzing the transfer of nucleotides from one nucleotide chain to another, i.e., reactions of the type
CpU +pGpN »» CpUpN + pG
where pU, pG and pN denote the 5’-monophosphate of the relevant nucleotide (N = U, C, A, G).In these reactions, the length of one nucleotide chain increases at the expense of another chain, so that they could be very useful in an environment in which competition occurs between various oligonucleotides.
The RNA-component of the enzyme of ribonuclease P of the bacteria Escherichia coliis another ribozyme.This enzyme also contains a protein part, but this is not active in the actual catalytic reaction – splitting of the tRNA precursors (Pace & Smith 1990).Basically, ribosome is also a ribozyme, as the most important reaction that occurs on it, i.e. the formation of new peptide bonds, is catalyzed by part of the rRNA molecule, and not by one of its protein components (Dahlberg 1989; Nissen et al. 2000; Cech 2000).
Ribozymesare considered by some biologists to be molecular relics of the time when there was, as yet, no division of functions between nucleic acids and proteins and when nucleic acids also performed all the functions that have been taken over by proteins in modern organisms.Contemporary coenzymes, nonprotein components of modern enzymes, on which the actual catalytic reaction mostly occurs (White 1976), could be similar relics and simultaneously probably indirect evidence for the existence of such a phase in the evolution of lifeA remarkably large number of known coenzymes are derived from nucleotides.The protein component of an enzyme naturally greatly increases the effectiveness of catalysis primarily in that it provides for greater substrate specificity of the reaction.However, in the initial stages of evolution, at the time when primitive organisms did not yet contain such a wide range of various biomolecules as contemporary organisms, substrate specificity was not such a critical parameter and the coenzymes themselves could perform catalytic functions.