Isoleucine epimerization

Bowen, D.Q., Hughes, S., Sykes, G.A. and Miller, G.H. (1989). Land-sea correlations in the Pleistocene based on isoleucine epimerization in nonmarine molluscs. Nature 340 49-51. 

The observed relative rates of isoleucine epimerization in heating experiments are N-terminal and di-ketopiperazine groups s> C-terminal and interior groups free isoleucine (in aqueous solution). However, the relative extents of epimerization observed in real samples is diketopiperazine > N-terminal > C-terminal > free s> interior (Mitterer Kriausakul 1984).The apparent dilemma of why C-terminal and free isoleucine can exhibit extensive epimerization but their rates of epimerization are slow is explained by the conversion of some of the more highly epimerized isoleucine from quickly (N-terminal and diketopiperazine) to slowly (C-terminal and free isoleucine) epimerizing units via diketopiperazine and hydrolysis (Fig. 5.39 Kriausakul Mitterer 1983). 

Fig. 5.40 Isoleucine epimerization in fossil foraminiferans (Glohorotalia spp.) from deep-sea sediments interpreted as sequential first order reactions with different rates (A = D-alloisoleucine, I = L-isoleucine after Muller 1984).

Muller PJ. (1984) Isoleucine epimerization in Quaternary planktonic foraminifera effects of diagenetic hydrolysis and leaching, and Atlantic-Pacific intercore correlations. Meteor Forschungsergeb. Reihe C 38, 25—47. 

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. 

D-Allo-isoleucine and L-isoleucine derivatives have been prepared from the corresponding mixture of stereoisomers via a diastereoselective hydrolysis reaction catalyzed by the enzyme alcalase (Scheme 2.6). Initially, enantiomerically and diastereomerically pure derivatives of L-isoleucine 10 were submitted to chemical epimerization to yield a 1 1 mixture of stereoisomers at the a-position. Thereafter,... 

The two epimeric 7-turn mimetics 162 and 163 were synthesized from the respective isomer of leucine 154 and 155 (Scheme 34). The two chiral reagents 156 and 159 were synthesized from (.S )-HOC isoleucine and (/5-lcucinc. respectively <1997TL6961>. 

Zur Benennung der Epimere der physiologisch vorkommenden Aminosauren mit zwei Chiralitatszentren gibt es eine besondere Vor-silbe, namlich die Vorsilbe Alio-. Demnach heifien die Epimere von Threonin und Isoleucin Allothreonin bzw. Alloisoleucin. 

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). 

Fig. 5.38 Epimerization (E) and racemization (R) of isoleucine (major pathway is epimerization of L-isoleucine and D-alloisoleucine large arrow indicates biogenic input).

The rate of hydrolysis is important, because if it were rapid it would yield the rate curve predicted by the simple first order kinetics in Eqn 5.12 for the epimerization of free aqueous isoleucine. If hydrolysis were slow, the initial part of the rate curve would also be similar to that... 

Fig. 5.39 Possible variations in molecular environment for isoleucine (ile after Mitterer Kriausakul 1984).With increasing diagenesis, hydrolytic cleavage will move interior amino acids to terminal positions in peptide chains and ultimately to free amino acids.The N-terminal unit has a free amino group, whereas the C-terminal analogue has a free carboxyl group.These two varieties of terminal groups are interconverted via the formation of diketopiperazines. HR = high epimerization rate LR = low epimerization rate HE = high epimerization extent LE = low epimerization extent.

Kriausakul N., Mitterer R.M. (1983) Epimerization of COOH-terminal isoleucine in fossil dipeptides. Geochim. Cosmochim.Acta 47, 963—6. 

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