Alanine reaction mechanism

Aspartame is relatively unstable in solution, undergoing cyclisation by intramolecular self-aminolysis at pH values in excess of 2.0 [91]. This follows nucleophilic attack of the free base N-terminal amino group on the phenylalanine carboxyl group resulting in the formation of 3-methylenecarboxyl-6-benzyl-2, 5-diketopiperazine (DKP). The DKP further hydrolyses to L-aspartyl-L-phenyl-alanine and to L-phenylalanine-L-aspartate [92]. Grant and co-workers [93] have extensively investigated the solid-state stability of aspartame. At elevated temperatures, dehydration followed by loss of methanol and the resultant cyclisation to DKP were observed. The solid-state reaction mechanism was described as Prout-Tompkins kinetics (via nucleation control mechanism). 

Under these conditions Val-Ala was almost quantitatively converted into c(Val-Ala) according to the postulated reaction mechanism (4), Figure 2 whereas an equimolar mixture of valine and alanine failed to react. The intramolecular nature of DKP formation vas further established by reactions of tripeptides. Each tripeptide studied produced a single DKP that had to result from attack of the N-terminal amino group according to Figure 2. DKP formation by... 

Fig. 8.5 Reaction mechanism of alanine racemase. (Reproduced with permission from Faraci and Walsh, Biochemistry, 27, 3268 (1988)).

Trifluoroalanine contains three fluorine atoms which are very similar in size to the hydrogen atoms in alanine. The molecule is therefore able to fit into the active site of the enzyme and take alanine s place. The reaction mechanism proceeds as before to give the dihydropyridine intermediate. However, at this stage, an alternative mechan-... 

Another measure of the asymmetric kinetic properties of the two bases in the alanine racemase mechanism is the qualitative behavior of the equilibrium overshoots observed. Overshoots are often observed in reaction progress curves run in deuterium oxide that are initiated with a single stereoisomer that is protiated at the Ca position (Fig. 7.3). The optical activity is monitored by polarimetry or circular dichroism (CD). At equilibrium, the signal is zero, since the product is a racemic mixture of d- and L-isomers. However, when there is a significant substrate-derived KIE on the reverse direction (product being fully deuterated in a two-base mecha-... 

In this context, the 3D structures of the above mentioned proteinsshare a common reaction mechanism. All of these proteins contain a conserved HXXXD motif in the active site and a catalytically active histidine residue, located at the center of the active site, with the latter considered as a general base for deprotonation of the hydroxyl group of the substrate prior to acyl group transfer (esterification). However, an apparent exception was noted with VihH - although it contained the conserved HXXXD motif, its histidine residue (Hisl26) did not serve in an equivalent role in acyl transfer catalysis, as its mutation to alanine or glycine had little effect on catalysis. ... 

Its formation from rhamnose heated with piperidine acetate in ethanol, under the same conditions that produced amino-hexose-reductones from glucose and other hexoses, was described as early as 1963 by Hodge et al., who confirmed the structure by IR and NMR data and proposed a formation pathway. The formation from Amadori intermediates was been reviewed by Vernin (1981). Numerous model systems have confirmed that it is one of the main Maillard-reaction products. For instance we will mention the formation from L-rhamnose and ethylamine (Kato et al., 1972) and from pentose/glycine or alanine, whose mechanism was proposed by Blank and Fay (1996) and Blank et al. (1998), from the intermediate Amadori compound, /V-(l-deoxy-D-pentos-l-yl)glycine. Furaneol is also formed by recombination of... 

Its formation during roasting could result from a trimolecular reaction between ethylglyoxal, ammonia and acetaldehyde resulting from the Strecker degradation of alanine. The mechanism of chain elongation reactions of glyoxal has been elucidated by Yaylayan and Keyhani (1998). 

A pantothenic acid hydrolase (pantothenase) activity has been isolated from Pseudomonas fluorescens and other Pseudomonas strains. This enzyme hydrolyzes the amide bond of pantothenic acid 2 to form pantoic acid 5 (or pantoyl lactone) and /i-alanine 7 (EC 3.5.1.22) (Equation (10)). A detailed kinetic study of the reaction mechanism has shown that the reaction is partially reversible because of the formation of an acyl—enzyme (pantoyl-enzyme) intermediate during the course of catalysis, which may react with either water or / -alanine to form pantoic acid (the product hydrolysis) or pantothenic acid (the original substrate) Such a mechanism suggests that this enzyme could act as a pantothenate synthase, as reaction of the active site serine with pantoyl lactone would result in the formation of the pantoyl—enzyme intermediate. However, no biochemical or genetic evidence is currently available to support such a hypothesis. 

The decarboxylation of L-aspartic acid to L-alanine is catalysed by a pyridoxal-P-dependent j8-decarboxylase whose reaction mechanism is clearly different from that of the a-decarboxylases since the initial step probably involves C -H bond cleavage. The steric course at during the normal decarboxylation reaction has recently been shown [23b] to be inversion. In addition to this, however, the enzyme will also catalyse the decarboxylation of amino-malonic acid to glycine and Meister and coworkers [24,25] have shown that this process involves loss of the Si carboxyl group with overall retention at C . 

From the chemical equations above it is clear that the laboratory synthesis is not a clean reaction, because it yields a mixture of the both enantiomers L-alanine and D-alanin i.e. a racemate. The appearance of the racemate can be explained from the knowledge of the reaction mechanism of nucleophilic addition to the carbonyl group. In the carbonyl group the carbon atom is bound by a double bond and lies with all three of its substituents in the same molecular plane. The CN" group as a nucleophile can attack the carbonyl carbon with the same probability from either side of the molecular plane. Consequently above-the-plane attack yields (R) enantiomer and below-the-plane attack gives the (S) enantiomer. 

There are many examples of nitrilase-catalyzed reactions in which amides form a considerable amount of the reaction products, such as the transformations of acrylonitrile analogs and a-fluoroarylacetonitriles by nitrilase 1 from Arabidopsis thaliana [17], the conversion of p-cyano-L-alanine into a mixture of L-asparagine and L-aspartic acid by nitrilase 4 from the same organism [18] or the transformations of mandelonitrile by nitrilase from Pseudomonas jhiorescens [19] or some fungi [8], Moreover, formamide is the only product of the cyanide transformation by cyanide hydratase. Therefore, this enzyme was classified as a lyase (EC 4.2.1.66), although it is closely related to nitrilases, as far as its aa sequence and reaction mechanism are concerned [3]. 

Using curved arrows, propose a mechanism for the following reaction, one of the steps in the metabolism of the amino acid alanine. 

Amino acids can be prepared by reaction of alkyl halides with diethyl acelamidomalonate, followed by heating the initial alkylation product with aqueous 1-ICl. Show how you would prepare alanine, CH3CH(NH2)C02H, one of the twenty amino acids found in proteins, and propose a mechanism for acid-catalyzed conversion of the initial alkylation product to the amino acid. 

Influence of ionic strength on the reaction rate constant. The influence of the ionic strength on the reaction rate constant was studied using KCl as electrolyte. The results obtained in this study are listed in Table 4, where we can see that the reaction rate constant for N-Br-alanine decomposition undergoes an increment of 40 % upon changing the ionic strength from 0.27M to IM, while in the case of N-Bromoaminoisobutyric acid the increment of the reaction rate constant is of about 12 %. This is an evidence of a non ionic mechanism in the case of the decomposition of N-Br-aminoisobutyric acid, as it is expected for a concerted decarboxylation mechanism. For the decomposition of N-Br-proline the increase on the reaction rate constant is about 23 % approximately, an intermediate value. This is due to the fact both paths (concerted decarboxylation and elimination) have an important contribution to the total decomposition process. 

The third reason for favoring a non-radical pathway is based on studies of a mutant version of the CFeSP. This mutant was generated by changing a cysteine residue to an alanine, which converts the 4Fe-4S cluster of the CFeSP into a 3Fe-4S cluster (14). This mutation causes the redox potential of the 3Fe-4S cluster to increase by about 500 mV. The mutant is incapable of coupling the reduction of the cobalt center to the oxidation of CO by CODH. Correspondingly, it is unable to participate in acetate synthesis from CH3-H4 folate, CO, and CoA unless chemical reductants are present. If mechanism 3 (discussed earlier) is correct, then the methyl transfer from the methylated corrinoid protein to CODH should be crippled. However, this reaction occurred at equal rates with the wild-type protein and the CFeSP variant. We feel that this result rules out the possibility of a radical methyl transfer mechanics and offers strong support for mechanism 1. 

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