Amino acid tRNA ligases

Amino add activation. Before binding to the ribosomes, tRNAs are loaded with the correct amino acids by specific ligases (7 see p. 248). It is the amino acid tRNA ligases that carry out the transfer (translation) of the genetic information from the nucleic acid level to the protein level. 

Some 20 different amino acid tRNA ligases in the cytoplasm each bind one type of tRNA (see p. 82) with the corresponding amino acid. This reaction, known as amino acid activation, is endergonic and is therefore coupled to ATP cleavage in two steps. 

The aminoacyl-tRNA S5mthetases (amino acid tRNA ligases) join amino acids to their appropriate transfer RNA molecules for protein S5mthesis. They have the very important task of selecting both a specific amino acid and a specific tRNA and joining them according to Eq. These reactions repre-... 

Eigneg E.A. and Loftfield R.B. (1974) Kinetic techniques for the investigation of amino acid tRNA ligases (aminoacyl-tRNA synthetases, amino acid activating enzymes). Methods EnzymoL, 29, 601-619. 

Translation the role of amino-acid-tRNA ligases 213... 

Translation involves two compartments the cytosol in which individual amino acids are enzymi-cally attached to their specific tRNAs by amino-acid-tRNA ligases (also called aminoacyl-tRNA synthetases) and the ribosomes in which the amino acids are correctly positioned according to the base sequence of a mRNA template and polymerized into polypeptide chains. 

In addition to their ability to load the tRNA with an amino acid, amino-acid-tRNA ligases are capable of recognizing inappropriate attachments, i.e. they have a proofreading function. If an incorrect amino acid has been attached, it may be removed by the hydrolysis of the enzyme-bound aminoacyl-adenylate intermediate. Once the anhydride linkage is cleaved, the amino acid and AMP dissociate from the active site so that the tRNA may be reloaded, hopefully with the correct molecule. The selectivity of these ligases is largely responsible for the maintenance of the fidelity of protein synthesis. 

Both molecules are folded in such a way that the 3 end and the 5 end are close together. As in DNA, most of the bases are located in the inside of the structures, while the much more polar backbone is turned outwards. An exception to this is seen in the three bases of the anticodon of the tRNA (pink), which have to interact with mRNA and therefore lie on the surface of the molecule. The bases of the conserved CCA triplet at the 3 end (red) also jut outward. During amino acid activation (see p.248), they are recognized and bound by the ligases. 

First, the amino acid is bound by the enzyme and reacts there with ATP to form diphosphate and an energy-rich mixed acid anhydride (aminoacyl adenylate). In the second step, the 3 -OH group (in other ligases it is the 2 -OH group) of the terminal ribose residue of the tRNA takes over the amino acid residue from the aminoacyl adenylate. In aminoacyl tRNAs, the carboxyl group of the amino acid is therefore esterified with the ribose residue of the terminal adenosine of the sequence. ..CCA-3. ... 

The Lipid I and II building blocks may be further elaborated by many other enzymes that modify the sugars or amino acid chains. Branched peptides are added to the Lipid I and II peptide chains either by enzymes that act in an ATP-dependent fashion similar to the MurC-F ligases [39], or by enzymes that add amino acid residues from aminoacyl tRNA intermediates, such as the S. aureus enzymes FemA, FemB and FemX, which form the pentaglycine bridge (O Fig. 3) [36], and the S. pneumoniae enzymes FemM and FemN, which form an L-Ser-L-Ala or L-Ala-L-Ala dipeptide bridge [34,35]. Lipid II is also the substrate for the sortase enzymes that catalyze the attachment of surface proteins for incorporation into peptidoglycan [40]. 

Scheme 14.24. A representation of a pathway for the formation of 5-aminolevulinic acid ( -aminolevulinic acid) from glutamate in plants using a tRNA ligase and, subsequently, amino to carbonyl transposition aided by pyridoxamine. EC numbers and some graphic materials in this scheme have been taken from links in http //www.chem.qmul.ac.uk/iubmb/ enzyme.

The aminoacyl-tRNA ligases, of course, provide the mechanism by which an amino add is activated in the presence of ATP and an activating em me specie for that amino add. The reaction is a reversible one, and can be diagramm as follows [reaction (I)], bearing in mind that our former colleague Robert Loftfield may be correct in regarding the path from free amino acid to aminoacyl-tRNA in some instances to represent a concerted reaction mechanism, without a definitive intermediate. 

---