Leucine, characteristics

Transfer RNA (tRNA) serves as a carrier of amino acid residues for protein synthesis. Transfer RNA molecules also fold into a characteristic secondary structure (marginal figure). The amino acid is attached as an aminoacyl ester to the 3 -terminus of the tRNA. Aminoacyl-tRNAs are the substrates for protein biosynthesis. The tRNAs are the smallest RNAs (size range—23 to 30 kD) and contain 73 to 94 residues, a substantial number of which are methylated or otherwise unusually modified. Transfer RNA derives its name from its role as the carrier of amino acids during the process of protein synthesis (see Chapters 32 and 33). Each of the 20 amino acids of proteins has at least one unique tRNA species dedicated to chauffeuring its delivery to ribosomes for insertion into growing polypeptide chains, and some amino acids are served by several tRNAs. For example, five different tRNAs act in the transfer of leucine into... 

Figure 8.11 Substrate inhibition. The enzyme L-amino acid oxidase (EC 1. 4. 3. 2) suffers substrate inhibition at concentrations of L-leucine above 3.0 mmol 1 1. A Lineweaver-Burk plot shows the characteristic bend to the usual straight line.

Three Trks have been identified, Trk A, B and C. Ligand-binding induces Trk receptor homodimerization. This activates the intrinsic cytoplasmic tyrosine kinase activity, resulting in receptor autophosphorylation and initiation of an intracellular response. A characteristic feature of the Trk family of receptors is the presence of a leucine-rich region near the N-terminal (extracellular) end of the molecule. 

T There is a relatively rare genetic disease in which the three branched-chain a-lceto acids (as well as their precursor amino acids, especially leucine) accumulate in the blood and spill over into the urine. This condition, called maple syrup urine disease because of the characteristic odor imparted to the urine by the a-lceto acids, results from a defective branched-chain a-lceto acid dehydrogenase complex. Untreated, the disease results in abnormal development of the brain, mental retardation, and death in early infancy. Treatment entails rigid control of the diet, limiting the intake of valine, isoleucine, and leucine to the minimum required to permit normal growth. ... 

Maple syrup urine disease (MSUD) is a recessive disorder in which there is a partial or complete deficiency in branched-chain o-ketoacid dehydrogenase, an enzyme that decarboxylates leucine, isoleucine, and valine (see Figure 20.10). These amino acids and their corre sponding a-keto acids accumulate in the blood, causing a toxic effect that interferes with brain functions. The disease is characterized by feeding problems, vomiting, dehydration, severe metabolic acidosis, and a characteristic maple syrup odor to the urine. If untreated, the disease leads to mental retardation, physical disabilities, and death. 

The properties of polypeptides and proteins are determined to a large extent by the chemistry of the side chain groups, which may be summarized briefly as follows. Glycine in a peptide permits a maximum of conformational mobility. The nine relatively nonpolar amino acids-alanine, valine, leucine, isoleucine, proline, methionine, phenylalanine, tyrosine, and tryptophan-serve as building blocks of characteristic shape. Tyrosine and tryptophan also participate in hydrogen bonding and in aromatic aromatic interactions within proteins. 

The R proteins, which act as receptors for Avr, and other elicitor proteins, are usually leucine-rich-repeat proteins with a characteristic nucleotide binding site attached (NB-LRR proteins).534,537 Like other cell surface receptors they participate in signaling and utilize both ion channels and Ser/Thr protein kinases.538 The Arabidopsis genome contains 150 sequences that may represent NB-LRR receptors.530... 

The structure of the isoleucyl-tRNA synthetase (IleRS) from Thermus ther-mophilus (1045 residues, Mr 120 000) has been solved, as well as its complexes with lie and Val.17 The protein contains a nucleotide binding fold (Chapter 1) that binds ATR The fold has two characteristic ATP binding motifs His-54-Val-55-Gly-56-His-57 and Lys-591-Met-592-Ser-593-Lys-594. In the L-Ile-IleRS complex, a single He is bound at the bottom of the ATP cleft, with the hydrophobic side chain in a hydrophobic pocket, surrounded by Pro-46, Trp-518, and Trp-558. L-Leucine cannot fit into this pocket because of the steric hindrance of one of its terminal methyl groups. Larger amino acids are similarly excluded from this site. In the l-Val-IleRS complex, Val is bound to the same site, but the... 

Synthetic peptides such as (Leu-Ser-Leu-Glu)12 act as channels in artificial membranes, and reproduce the hydrophobic exterior (from leucine side chains) and hydrophilic interior (from serine hydroxyl groups) characteristic of natural channel formers. 

Efforts may now have been successful Whereas normal tobacco cells require auxin for division, sequence tagged (TDNA) lines encoding an adenylyl cyclase were obtained which were auxin-independent but cAMP-dependent. From one line (axi 141), a complementary DNA encoding adenylyl cyclase has been isolated with characteristic leucine repeats and similarity to yeast adenylyl cyclase (Ichikawa et al., 1997). The result seems not to be the expression of an alternative division pathway from the normal auxin-driven division since it is blocked by auxin inhibitors and is activated by cAMP and the cyclase activator forskolin. Perhaps a link to G-protein at the membrane will now bring plant growth regulation even closer to that of animals. 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

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