Protein elongation

The principal drawbacks to the technique are its labor-intensive nature and low protein yields, which in the best cases reach only a milligram or less. Competition with release factors at the amber stop codon often results in truncated protein as the primary product [16, 17]. Suppression efficiency rates are also affected by the character of the amino acid, which determines whether it is a good substrate for the ribosome and protein elongation factors. In addition, context effects variously ascribed to the influence of neighboring mRNA... 

See p. 398 in Biochemistry (2nd ed.) for a discussion of inhibitors of protein elongation. 

As with initiation, elongation is a multistep process. In the first step, another GTP-binding protein, elongation factor Tu (EF-Tu), delivers the next aminoacyl tRNA to the A site on the ribosome. As this occurs, EF-Tu hydrolyzes GTP and leaves the ribosome (Fig. 23-5). 

Emetine and dehydroemetine exert a direct aniebicidal action on various forms nf E. Iihwlyiica. They arc protoplasmic poisons that inhibit protein synthesis in pnilo/iKil and mammalian cells by preventing protein elongation. Because their effect in intestinal amebiasis is solely. symptom-... 

The immediate segregation of proteins in the ER suggested that translocation across the membrane may be coupled to translation. The experiments of Redman and Sabatini (1966) supported this concept on a molecular level. After addition of puromycin, an inhibitor of protein elongation, to growing cells, N-terminal fragments of secretory proteins were found in the lumen of the ER, and not in the cytoplasm. In later work, Sabatini and Blobel (1970) showed that inclusion of microsomes of RER in a cell-free system translating mRNA coding for secretory pro-... 

A novel derivative of histidine present only in the eukaryotic protein elongation factor 2 (EF-2), which participates in the elongation step of protein biosynthesis. Diphtheria toxin inhibits eukaryotic protein synthesis by catalyzing a covalent modification of diphthamide (see Chapter 25). 

A charged tRNA is escorted to the A site in a complex with the protein elongation factor EF-Tu, which also carries a molecule of GTP. 

Translocation in eukaryotes involves another G protein, elongation factor EF2 (EF-G in prokaryotes) that complexes with GTP and binds to the ribosome, causing a conformational change that moves the mRNA and its base-paired tRNAs with respect to the ribosome. The uncharged tRNA moves from the P site and is released from the ribosome. The peptidyl-tRNA moves into the P site, and the next codon of the mRNA occupies the A site. During translocation, GTP is hydrolyzed to GDP, which is released from the ribosome along with the elongation factor (see Fig. 15.10). 

As an example, the transfer NOE spectrum of Ant-Ado (anthranilic acid adenine) a mimetic of an amino-acid-loaded tRNA, bound to the protein Elongation Eactor Thermo-unstable (EE-Tu) is shown in Fig. 49,a. The NOEs can be used to determine the conformation of the bound form. 

Chain elongation. Elongation includes the synthesis of all peptide bonds of a polypeptide chain. This is accomplished, with the assistance of a set of protein elongation factors, by a repetitive cycle of events in which successive aa-tRNA adds to the A site and the growing peptidyl-tRNA occupying the P site of the mRNA ribosome complex. 

It may be that we should be more concerned with why the bacterial machinery is relatively so simple. In rapidly growing bacterial cells a very substantial fraction of the total cellular protein is involved directly in protein biosynthesis (activating enzymes, ribosomal structural proteins, elongation factors etc). Evolutionary pressure to attain high growth rates may therefore have resulted in a streamlining of the bacterial protein synthesis mechanism to the bare minimum of components, in which case we should not look for some subtlety of regulation in every additional feature of complexity in the eukaryotic system. 

In the protein elongation process the EF-Tu and EF-G cycles themseves interact with the mRNA-pro-gramed ribosome cyclically and since (i) EF-TU has to be released from the ribosome before EF-G can bind to it and vice versa and (ii) GTP hydrolysis is required for the release of both factors, there is a stoichiometric relationship between the number of GTPs hydrolysed and the number of amino acids incorporated into the protein synthesized. This contrasts with the other GTPases, e.g. heterotrimeric G-proteins, Ras proteins, which continue to transmit a signal as long as they remain in the E - GTP form, a key feature of the signal amplification process. 

Muscle proteins in typical mammalian muscle tissue constitute around 20% of the muscle weight The major proportion of muscle is made up of muscle fibre proteins (elongated, threadlike cells) called myofibrillar proteins. Smaller amounts of soluble sarcoplasmatic proteins and insoluble structural proteins from connective tissue are also present (Table 2.16). Myofibrillar and sarcoplasmatic proteins are almost complete (whole) proteins, while the nutritional value of structural proteins is very low as they are almost indigestible. Table 2.17 gives the amino acid composition of some pure animal proteins Table 2.9 presents the amino acid compositions for the main types of meat proteins. 

Jaworski, J.G., Goldschmidt, E.E. and Stumpf P.K., 1974. Fat metabolism in higher plants. Properties of the palmityl acyl carrier protein Stearyl acyl carrier protein elongation in maturing Safflower seed extracts. Arch. Biochem. Biophys. 163, 769-776. 

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