Maleate Isomerase | |||||||||
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Identifiers | |||||||||
EC no. | 5.2.1.1 | ||||||||
CAS no. | 9023-74-9 | ||||||||
Databases | |||||||||
IntEnz | IntEnz view | ||||||||
BRENDA | BRENDA entry | ||||||||
ExPASy | NiceZyme view | ||||||||
KEGG | KEGG entry | ||||||||
MetaCyc | metabolic pathway | ||||||||
PRIAM | profile | ||||||||
PDB structures | RCSB PDB PDBe PDBsum | ||||||||
Gene Ontology | AmiGO / QuickGO | ||||||||
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In enzymology, a maleate isomerase (EC 5.2.1.1), or maleate cis-tran isomerase, is a member of the Asp/Glu racemase superfamily discovered in bacteria. It is responsible for catalyzing cis-trans isomerization of the C2-C3 double bond in maleate to produce fumarate, [1] which is a critical intermediate in citric acid cycle. [2] The presence of an exogenous mercaptan is required for catalysis to happen. [3]
Maleate isomerase participates in butanoate metabolism and nicotinate and nicotinamide metabolism. [4] It is an essential enzyme for the last step of metabolic degradation pathway of nicotinic acid. Recently, maleate isomerase has been an industrial target for degradation of tobacco waste. [5] [6] It is also got attention for its involvement in aspartic acid and maleic acid production. [7] [8] [9]
Maleate isomerase has been utilized by multiple bacteria species, including Pseudomonas fluorescens, [3] Alcaligenes faecalis , [10] Bacillus stearothermophilus , [11] Serratia marcescens [8] , Pseudomonas putida [12] and Nocardia farcinica . [1] [5] The enzyme has a molecular weight of 74,000 and a turnover number of 1,800 moles per mole of protein per min. [3]
Analogous to other Asp/Glu racemase members, maleate isomerase is formed by two identical protomers, with a flat dimerization surface. [13] [14] Each protomer of maleate isomerase has two domains connected by a pseudo-twofold symmetry, with each domain contributes one catalytic cysteine, which is crucial to the isomerase activity at the active site. [5] Experiment shows that substitution of either cysteine by serine significantly reduces the rate of reaction of the enzyme. [1]
In addition to catalytic cysteines, a few other residues at the active site are important for the recognition of the substrate and help stabilize reaction intermediates. [5] [1] For example, maleate isomerase from Pseudomonas putida S16 uses Asn17 and Asn169 form hydrogen bonds with the carboxylate group of the maleate distal to Cys82. [5] Tyr139 hydrogen bonds with the carboxylate group of the maleate proximal to Cys82. [5] Pro14 and Val84 make van der Waals interactions with the C2 and C3 carbon atoms of the maleate. [5]
The mechanism of maleate isomerase is considered to be similar to other Asp/Glu racemase members, though have not been fully understood. One proposed reaction mechanism of Nocardia farcinia maleate isomerase is as follows. [1] [9] At the active site of maleate isomerase, Cys76 is first deprotonated to be more readily act as a nucleophile. [1] The sulfur atom of the deprotonated Cys76 then carries a direct nucleophilic attack to the C2 atom of the maleate, covalently bonding to the C2 atom. [9] [1] Concomitantly, thiol proton of Cys194 is transferred onto the C3 atom of the maleate to form a succinyl-cysteine intermediate. [9] [1] The newly formed C2–C3 single bond is then rotated, with Cys76S–C2 bond dissociated, and C3 atom of the maleate deprotonated by Cys194, thus forming fumarate with regeneration of a neutral Cys194. [9] [1] In certain type of bacteria, maleate seems completely buried inside the cavity of maleate isomerase and cannot be seen on the surface of the enzyme. [5]
Maleate isomerase can be used to produce fumaric acid, an important building block material for polymerization and esterification reactions, from the isomerization of maleic acid. [7] Maleic acid is produced from maleic anhydride. [7]
Maleic acid can also be converted into fumaric acid by thermal or catalytic cis–trans isomerization. [15] [16] However, these conversion methods are occurring at high temperatures that causes formation of by-products from maleic and fumaric acids, as a result, yields are below the equilibrium yields. [17] This problem was the main motivation for the alternative enzymatic strategy with maleate isomerase that would facilitate isomerization without by-products. [7]
It is known that, even at moderate temperatures, natural maleate isomerase is unstable. [18] For that reason, heat-stable maleate isomerases are engineered and applied. [7] For example, thermo-stable maleate isomerases derived from Bacillus stearothermophilus, Bacillus brevis, and Bacillus sporothermodurans were used to improve the process. [7] [17] In a study using Pseudomonas alcaligenes XD-1, conversion rate from maleic acid into fumaric acid could be achieved as high as 95%. [19] [20] [7]
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