Ribonucleic amino acid compounds

The aminoacyl transfer reaction, one of the latter stages in protein synthesis, involves incorporation of amino acids from soluble ribonucleic acid-amino acid into ribosomal protein. This reaction requires guanosine triphosphate and a soluble portion of the cell. Evidence has been obtained with rat liver preparations that aminoacyl transfer is catalyzed by two protein factors, aminoacyl transferases (or polymerases) I and n, which have been resolved and partially purified from the soluble fraction. Transferase n activity has also been obtained from deoxycholate-soluble extracts of microsomes. With purified transferases I and n, incorporation is observed with relatively low levels of GTP its sulfhy-dryl requirement is met by a variety of compounds. The characteristics of this purified amino acid incorporating system, in terms of dependency on the concentration of its components, are described. 

Thiol groups enter some biologically important thiol compounds by the direct incorporation of cysteine itself. Most frequently this involves peptide bond formation. The incorporation of cysteine into proteins does not differ from any other amino acid involving activation as an amino acid adenylate, transfer to a specific transfer ribonucleic acid (t-RNA), and assembly by ribosomal enzymes as coded by messenger ribonucleic acid (m-RNA). It should be pointed out that cystine, the two-headed ... 

Indole contains a pyrrole ring fused with a benzene ring. Compounds derived from indole include the essential amino acid L-tryptophan (Section 27.1C) and the neurotransmitter serotonin. Purine contains a six-membered pyrimidine ring fused with a five-membered imidazole ring. Caffeine is a trimethyl derivative of an oxidized purine. Compounds derived from purine and pyrimidine are building blocks of deoxyribonucleic acids (DNA) and ribonucleic acids (RNA, Chapter 28). 

The nucleotides which consist of three parts, namely a nitrogenous base, a pentose sugar and a phosphate radical, are a very important group of compounds since one or more of them is involved in virtually every biochemical process. The adenosine di- and tri-phosphates which play an essential part in cellular energy exchanges have a nucleotide-type structure as do many of the coenzymes. Furthermore, nucleotides constitute the monomeric units of which the nucleic acids are composed that is to say nucleic acids are polynucleotides. The nucleic acids which are of two types, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are responsible for directing the synthesis of proteins. They specify the unique sequence of amino acids in any particular protein and consequently should be regarded as primordial molecules on whose existence that of the proteins depends. However, since the synthesis of the nucleic acids depends on enzymes which are themselves proteins, this poses the fundamental evolutionary question as to which came first - the biochemical version of the problem of the chicken and the egg ... 

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