Isoleucine interconversion

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. 

Succinyl CoA is a point of entry for some of the carbon atoms of methio-nine, isoleucine, and valine. Propionyl CoA and then methylmalonyl CoA are intermediates in the breakdown of these three nonpolar amino acids (Figure 23.26). The mechanism for the interconversion of propionyl CoA and methylmalonyl CoA was presented in Section 22.3.3. This pathway from propionyl CoA to succinyl CoA is also used in the oxidation of fatty acids that have an odd number of carbon atoms (Section 22.3.2). 

Aspartate is involved in the control point of pyrimidine biosynthesis (Reaction 1 below), in transamination reactions (Reaction 2 below), interconversions with asparagine (reactions 3 and 4), in the metabolic pathway leading to AMP (reaction 5 below), in the urea cycle (reactions 2 and 8 below), IMP de novo biosynthesis, and is a precursor to homoserine, threonine, isoleucine, and methionine (reaction 7 below). It is also involved in the malate aspartate shuttle. 

L-Amino acids are the naturally occurring form both in proteins and as free amino acids, and the ability of different experimental animals to use different optical isomers of the amino acids has been studied. The chick is only able to use the L-form of threonine, isoleucine and valine but can use D- and L-leucine equally. It can use both D- and L-tryptophan, although the D-form is less effective (see Fisher, 1954). This probably relates to the presence of specific racemases that enable certain D- and L-interconversions to take place. Factors that affect the requirements for specific amino acids which are particularly pronounced in avian species are summarised in Table 2.3. 

Racemization, the interconversion of amino acid enantiomers, occurs slowly in biological and geological systems. The rate increases with extreme pH values, high temperature, and high-ionic strength. Rates also vary between amino acids At 25°C, the racemization half-life of serine is about 400 yr, while that of isoleucine is 40,000 yr. Enantiomer analysis is used to confirm bioactivity of synthetic peptides and for geological dating. ... 

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