Isoleucine, properties

Further complications may arise with the larger amino acids such as isoleucine, where the R side-chain itself contains a chiral carbon atom [R = CH3CH2C H(CH)3, where the asterisk denotes the second chiral centre]. This molecule is an example of a diastereomer - a molecule with more than one chiral centre. Diastereomers have different physical and chemical properties, and their interconversion is more complicated, and is termed epimerization. 

The occurrence of alanine in proteins was first shown by Schutzen-berger, who did not actually identify his product with the synthetical one Weyl in l88i obtained it as a decomposition product of silk and showed that his preparation was similar in properties to Strecker s synthetical alanine. He thus established it as a constituent of a protein molecule. The researches of Emil P ischer have shown that alanine is a constant constituent of all proteins. It is worthy of note that of the eighteen definitely determined units of a protein molecule, six of them, namely, isoleucine, phenylalanine, tyrosine, serine, histidine and tryptophane, are derivatives of a-aminopropionic acid. 

Based on the properties of the side chains, the 20 amino acids can be put into six general classes. The first class contains amino acids whose side chains are aliphatic, and is usually considered to include glycine, alanine, valine, leucine, and isoleucine. The second class is composed of the amino acids with polar, nonionic side chains, and includes serine, threonine, cysteine, and methionine. The cyclic amino acid proline (actually, an imino acid) constitutes a third class by itself. The fourth class contains amino acids with aromatic side chains tyrosine, phenylalanine, and tryptophan. The fifth class has basic groups on the side chains and is made up of the three amino acids lysine, arginine, and histidine. The sixth class is composed of the acidic amino acids and their amides aspartate and asparagine, and glutamate and glutamine. 

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. 

This editing mechanism for isoleucyl-tRNA synthetase was demonstrated directly in 1998 by X-ray crystallography on complexes of the enzyme with L-isoleucine and L-valine. Both substrates fit into the ATP-requir-ing synthetic site but neither isoleucine nor isoleucyl-tRNA will fit into the editing site which is located in an adjacent (3-barrel domain.104 105 Proofreading steps based on differing chemical properties as well as size can also be visualized.103 106... 

Broca, C., Cross, R., Petit, P., Sanvaire, Y., Manteghetti, M.,Tournies, M., Masiello, P., Gonnis, R. and Riber, C. (1 999) 4-Hydroxy isoleucine - experimental evidence of its insulinotropic and antidiabetic properties. American Journal of Physiology 277(4), E61 7-E623. 

Cutler, A.J., Sternberg, M. and Corm, E.E. (1985) Properties of a microsomal enzyme system from Linum usitatissimum (linen flax), which oxidizes valine to acetone cyanohydrin and isoleucine to 2-methylbutanone cyanohydrin. Arch. Biochem. Biophys., 238,272-9. 

Thermosensitive polyphosphazenes with (o-methoxy poly(ethylene glycol) in combination with glycine ethyl ester and depsipeptide ethyl ester or al-kylamines as substituents have been synthesized and characterized. All polymers exhibit LCST properties. Thermosensitive polymers have also been prepared by the introduction of a-amino-co-methoxy- poly(ethylene glycol) and L-isoleucine ethyl ester substituents to the phosphazene backbone. Polymer (230) shows reversible sol-gel transitions in aqueous solution depending on the temperature of the solution. ... 

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