Isoleucine isomerization

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

Figure 5.7 Examples of (a) elimination, (b) isomerization (aldose/ketose) and (c) a complex rearrangement of the pinacol-pinacolone type found in the biosynthesis of valine and isoleucine.

FIGURE 8.19 Data showing the relative sensitivities to isomerization of /V-methylamino-and amino-acid residues under conditions of saponification, acidolysis by HBr (5.7 M = saturated) in anhydrous acetic acid and aminolysis of the mixed anhydrides by H-Gly-OBzl TosOH/EtgN.94 lie = isoleucine. 

Bouveault and Locquin have also synthesised leucine by the reduction of a-oximinoisobutylacetic acid, which was prepared in a similar way to the isomeric compound from which they obtained isoleucine. 

Of the various isomeric amino-caprok acids only leucine and isoleucine occur in the protein molecule both of them, combined with tyrosine and valine in the form of polypeptides, from which they are easily split off by enzymes, seem to form a very important part of most proteins. 

Methylmalonyl-CoA mutase is a cobalamin-linked enzyme of mitochondria that catalyzes the isomerization of methylmalonyl-CoA to succinyl-CoA. A reduction of this enzyme due to vitamin B12 deficiency will result in a metabolic block with the urinary excretion of methylmalonic acid, and the measurement of this metabolite has been used to confirm a deficiency of vitamin B12. The test has also been useful in investigating rare abnormalities of this enzyme that result in the excretion of methylmalonic acid in the presence of adequate vitamin B12. Given an oral loading dose of valine or isoleucine will increase the urinary excretion of methylmalonic acid in patients with a vitamin B12 deficiency (G4). However, Chanarin and his colleagues (CIO) found that one-quarter of their patients with pernicious anemia excreted a normal concentration of methylmalonic acid even after a loading dose of valine. Normal subjects excrete up to 15 mg of methylmalonic acid in their urine over a 24-hour period (Cll). 

Hulst, A.G. and Kientz, C.E. (1996) Differentiation between the isomeric amino acids leucine and isoleucine using low-energy collision-induced dissociation tandem mass spectrometry. J. Mass Spectrom., 31 (10), 1188-90. 

Amino acid racemization (AAR), 323 Amino acids alanine, 11 aspartic acid, 11 dental calculus, 11 enantiomeric forms, 11 ESR spectroscopy, 366-367 fossils, 10-11 D,L-isoleucine, 11 isomeric forms, 11 laboratory-produced, 11... 

Propionyl CoA Carboxylase Propionyl CoA carboxylase catalyzes the carboxylation of propionyl CoA to methyhnalonyl CoA, which undergoes a vitamin Bi2-dependent isomerization to succinyl CoA (see Figure 10.13). This reaction provides a pathway for the oxidation, through the tricarboxylic acid cycle, of propionyl CoA arising from the catabolism of isoleucine, valine, odd-carbon fatty acids, and the side chain of cholesterol. 

This enzyme s role in humans is to assist the detoxification of propionate derived from the degradation of the amino acids methionine, threonine, valine, and isoleucine. Propionyl-CoA is carboxylated to (5 )-methylmalonyl-CoA, which is epimerized to the (i )-isomer. Coenzyme Bi2-dependent methylmalonyl-CoA mutase isomerizes the latter to succinyl-CoA (Fig. 2), which enters the Krebs cycle. Methylmalonyl-CoA mutase was the first coenzyme B -dependent enzyme to be characterized crystallographically (by Philip Evans and Peter Leadlay). A mechanism for the catalytic reaction based on ab initio molecular orbital calculations invoked a partial protonation of the oxygen atom of the substrate thioester carbonyl group that facilitated formation of an oxycyclopropyl intermediate, which connects the substrate-derived and product-related radicals (14). The partial protonation was supposed to be provided by the hydrogen bonding of this carbonyl to His 244, which was inferred from the crystal structure of the protein. The ability of the substrate and product radicals to interconvert even in the absence of the enzyme was demonstrated by model studies (15). 

Amyl alcohols occur in eight isomeric forms and have the empirical formula CjHnOH. All are liquids at ambient conditions except 2,2-dimethylpropanol (neopentyl alcohol), which is a solid. Almost all amyl alcohols are manufactured in the United States by the hydroformylation of butylenes. Yeast fermentation processes for ethanol yield small amounts of 4-methyl-l-butanol (isoamyl alcohol) and 2-methyl-1-butanol (active amyl alcohol, scc-butyl-carbinol) as fusel oil. However, when the amino acids leucine and isoleucine are added to sugar fermentations by yeast, 87% and 80% yields of 4-methyl-l-butanol and 2-methyl-l-butanol, respectively, are obtained (Fieser and Fieser, 1950). These reactions are not suitable for commercial applications because of cost, but they do indicate the close structural relationship between these C5 amino acids and the C5 alcohols. The reactions occur under nitrogen-deficient conditions. If a nitrogen source is readily available, the production of the alcohols is lowered considerably. 

Figure 55-10 Electron impact positive ion mass spectra of isomeric acylglycines detected by GC/MS analysis of organic acid trimethyisily (IMS) derivatives. A, 3-liethylcrotonylglycine monO TMS ester (leucine metabolism). B, Tiglylglycine mono-TMS ester (isoleucine metabolism). As their retention times are relatively close in most chromatographic systems, proper differentiation betv een the two compounds is best achieved by evaluation of the fragment ion at m/z 82 (arrow) which is prominent in the spectrum of 3-methylcrotonylglydne but not tigiylgiycine.

---