At Least One Species Of Transfer Rna tRna Exists For Each Of The 20 Amino Acids

tRNA molecules have extraordinarily similar functions and three-dimensional structures. The adapter function of the tRNA molecules requires the charging of each specific tRNA with its specific amino acid. Since there is no affinity of nucleic acids for specific functional groups of amino acids, this recognition must be carried out by a protein molecule capable of recognizing both a specific tRNA molecule and a specific amino acid. At least 20 specific enzymes are required for these specific recognition functions and for the proper attachment of the 20 amino acids to specific tRNA molecules. The process of recognition and attachment (charging) proceeds in two steps by one enzyme for each of the 20 amino acids. These enzymes are termed aminoacyl-

tRNA synthetases. They form an activated intermediate of aminoacyl-AMP-enzyme complex (Figure 38-1). The specific aminoacyl-AMP-enzyme complex then recognizes a specific tRNA to which it attaches the aminoacyl moiety at the 3'-hydroxyl adenosyl terminal. The charging reactions have an error rate of less than 10-4 and so are extremely accurate. The amino acid remains attached to its specific tRNA in an ester linkage until it is polymerized at a specific position in the fabrication of a polypeptide precursor of a protein molecule.

The regions of the tRNA molecule referred to in Chapter 35 (and illustrated in Figure 35-11) now become important. The thymidine-pseudouridine-cyti-dine (T¥C) arm is involved in binding of the amino-acyl-tRNA to the ribosomal surface at the site of protein synthesis. The D arm is one of the sites important for the proper recognition of a given tRNA species by its proper aminoacyl-tRNA synthetase. The acceptor arm, located at the 3'-hydroxyl adenosyl terminal, is the site of attachment of the specific amino acid.

The anticodon region consists of seven nucleotides, and it recognizes the three-letter codon in mRNA (Figure 38-2). The sequence read from the 3' to 5' direction in that anticodon loop consists of a variable base-modified purine-XYZ-pyrimidine-pyrimidine-5'. Note that this direction of reading the anticodon is 3' to 5', whereas the genetic code in Table 38-1 is read 5' to 3', since the codon and the anticodon loop of the mRNA and tRNA molecules, respectively, are antiparallel in their complementarity just like all other intermolecular interactions between nucleic acid strands.

The degeneracy of the genetic code resides mostly in the last nucleotide of the codon triplet, suggesting that the base pairing between this last nucleotide and the corresponding nucleotide of the anticodon is not strictly

Amino acid (aa)

Enzyme (Enz)

AMINOACYL-tRNA SYNTHETASE

Enz •Adenosine —O — P — O — C —CH —R I I

OH NH2

Enz •Adenosine —O — P — O — C —CH —R I I

OH NH2

Enzyme (Enz)

AMINOACYL-tRNA SYNTHETASE

Enz^AMP-aa (Activated amino acid)

Aminoacyl-AMP-enzyme complex tRNA

tRNA-aa

Aminoacyl-tRNA

Figure 38-1. Formation of aminoacyl-tRNA. A two-step reaction, involving the enzyme aminoacyl-tRNA synthetase, results in the formation of aminoacyl-tRNA. The first reaction involves the formation of an AMP-amino acid-enzyme complex. This activated amino acid is next transferred to the corresponding tRNA molecule. The AMP and enzyme are released, and the latter can be reutilized. The charging reactions have an error rate of less than 10-4and so are extremely accurate.

mRNA 5'

U

• U

• U

A

• A

Anticodon

Phenylalanyl-tRNA

Acceptor arm

Phenylalanyl-tRNA

Acceptor arm

Figure 38-2. Recognition of the codon by the anticodon. One of the codons for phenylalanine is UUU. tRNA charged with phenylalanine (Phe) has the complementary sequence AAA; hence, it forms a base-pair complex with the codon. The anticodon region typically consists of a sequence of seven nucleotides: variable (N), modified purine ((Pu*), X, Y, Z, and two pyrim-idines (Py) in the 3' to 5' direction.

by the Watson-Crick rule. This is called wobble; the pairing of the codon and anticodon can "wobble" at this specific nucleotide-to-nucleotide pairing site. For example, the two codons for arginine, AGA and AGG, can bind to the same anticodon having a uracil at its 5' end (UCU). Similarly, three codons for glycine—GGU, GGC, and GGA—can form a base pair from one anticodon, CCI. I is an inosine nucleotide, another of the peculiar bases appearing in tRNA molecules.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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