Genetic code enzymes

If enzymes responsible for DNA repair are unable to remove the DNA adduct, or if an error takes place in the repair, then the error in the genetic code remains when the cell divides. Thus, cellular proliferation is also required, in addition to a mutation, for there to be a permanent effect of a chemical compound. Accumulation of genetic errors, i.e., mutations, has been suspected to be an important factor in chemical carcinogenesis. ... 

The close connection of this enzyme family with the transfer of genetic information has made it a popular object of study when dealing with questions regarding the formation and evolution of the genetic code (see Sect. 8.1). It is now agreed that the aminoacyl-tRNA synthetases are a very ancient enzyme species which do not, however, arise from one single primeval enzyme, but from at least two, corresponding to the synthetase classes. 

An important factor in the evolution of the genetic code is certainly provided by the aminoacyl-tRNA synthetases (see Sect. 5.3.2). It is clear that the two synthetase classes are not randomly distributed across the matrix of the amino acid assignment of the genetic code. For example, with one exception, all XUX codons code for class 1 synthetases, while all XCX codons code for class 2 aminoacyl-tRNA synthetases. A possible explanation could be that the synthetases and the genetic code evolved simultaneously. However, it is more likely that these enzymes evolved when the genetic code had already been established (Wetzel, 1995). 

One important question is that of the order in which the basic mechanisms of evolution processes, leading eventually to the emergence of life, occurred. As far as the development of the genetic code is concerned, it is not clear whether the code evolved prior to the aminoacylation process, i.e., whether aminoacyl-tRNA synthetases evolved before or after the code. A tRNA species which is aminoacy-lated by two different synthetases was studied if this tRNA had important identity elements such as the discriminator base and the three anticodon bases for the two synthetases, this would be evidence that the aminoacyl-tRNA synthetases had developed after the genetic code. Dieter Soil s group, which is experienced in working with this family of enzymes, came to the conclusion that the universal genetic code must have developed before the evolution of the aminoacylation system (Hohn et al, 2006). 

The chemical functioning of a cell is the main objective of Molecular Biology (MB). The isolation, identification, and purification of biomacromolecules could be seen as the first step, in fulfilling this objective. From the catalytic point of view, enzymes are the center of attention. However, the most important biomolecule in the cell is the DNA, in which the genetic code of each enzyme is enclosed, thus representing ultimate target for MB. The biochemical and molecular processes of replication, transcription, and translation become part of the objective. 

The physiological functions of carboxylesterases are still partly obscure but these enzymes are probably essential, since their genetic codes have been preserved throughout evolution [84] [96], There is some evidence that microsomal carboxylesterases play an important role in lipid metabolism in the endoplasmic reticulum. Indeed, they are able to hydrolyze acylcamitines, pal-mitoyl-CoA, and mono- and diacylglycerols [74a] [77] [97]. It has been speculated that these hydrolytic activities may facilitate the transfer of fatty acids across the endoplasmic reticulum and/or prevent the accumulation of mem-branolytic natural detergents such as carnitine esters and lysophospholipids. Plasma esterases are possibly also involved in fat absorption. In the rat, an increase in dietary fats was associated with a pronounced increase in the activity of ESI. In the mouse, the infusion of lipids into the duodenum decreased ESI levels in both lymph and serum, whereas an increase in ES2 levels was observed. In the lymph, the levels of ES2 paralleled triglyceride concentrations [92] [98],... 

The sequences of the amino acids in the chains from which proteins are constructed are encoded in the nucleotide sequences of DNA (deoxyribonucleic acid). The coding sequence for a protein in the DNA is found in the structural gene for that protein. The RNA enzymes are also encoded by DNA genes. A fourth major theme of the book deals with the nature of the genetic code used in DNA and with the processes by which cells read and interpret the code. It also includes study of the methods by which thousands of genes have been mapped to specific positions in chromosomes, isolated, cloned, and sequenced. 

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