Enantioselectivity in the enzymatic dehydration of malate and tartrate: Mirror image specificities of structurally similar dehydratases

by Selleckchem | Feb 12 2024

Malate (2-hydroxysuccinic acid) and tartrate (2,3-dihydroxysuccinic acid) are chiral substrates; the former existing in two enantiomeric forms (R and S) while the latter exists as three stereoisomers (R,R; S,S; and R,S). Dehydration by stereospecific hydrogen abstraction and antielimination of the hydroxyl group yield the achiral products fumarate and oxaloacetate, respectively. Class-I fumarate hydratase (FH) and L-tartrate dehydratase (L-TTD) are two highly conserved enzymes belonging to the iron-sulfur cluster hydrolyase family of enzymes that catalyze reactions on specific stereoisomers of malate and tartrate. FH from Methanocaldococcus jannaschii accepts only (S)-malate and (S,S)-tartrate as substrates while the structurally similar L-TTD from Escherichia coli accepts only (R)-malate and (R,R)-tartrate as substrates. Phylogenetic analysis reveals a common evolutionary origin of L-TTDs and two-subunit archaeal FHs suggesting a divergence during evolution that may have led to the switch in substrate stereospecificity preference. Due to the high conservation of their sequences, a molecular basis for switch in stereospecificity is not evident from analysis of crystal structures of FH and predicted structure of L-TTD. The switch in enantiomer preference may be rationalized by invoking conformational plasticity of the amino acids interacting with the substrate, together with substrate reorientation and conformer selection about the C2C3 bond of the dicarboxylic acid substrates. Although classical models of enzyme-substrate binding are insufficient to explain such a phenomenon, the enantiomer superposition model suggests that a minor reorientation in the active site residues could lead to the switch in substrate stereospecificity.

Comments:

It sounds like you're delving into the fascinating world of enzyme stereospecificity and the molecular mechanisms underlying substrate preference in enzymes like fumarate hydratase (FH) and L-tartrate dehydratase (L-TTD). The ability of these enzymes to selectively act on specific stereoisomers of malate and tartrate is intriguing, especially considering their evolutionary divergence and the lack of clear structural insights into their switch in substrate stereospecificity.

Enzyme stereospecificity, particularly regarding chiral substrates like malate and tartrate, often involves a complex interplay of enzyme-substrate interactions. While traditional models of enzyme-substrate binding might not fully explain the switch in substrate preference observed in FH and L-TTD, the enantiomer superposition model could shed light on this phenomenon.

The enantiomer superposition model suggests that a minor reorientation or adjustment in the active site residues of an enzyme could lead to a switch in substrate stereospecificity. This implies that subtle changes in the orientation or flexibility of amino acids within the active site could influence how the enzyme accommodates and interacts with different stereoisomers of substrates like malate and tartrate.

Conformational plasticity of the amino acids involved in substrate binding, along with potential substrate reorientation and conformer selection around the C2-C3 bond of the dicarboxylic acid substrates, might collectively contribute to the switch in substrate specificity observed between these enzymes.

The lack of evident differences in crystal structures between FH and L-TTD suggests that understanding their substrate specificity switch may require exploring dynamic aspects, such as protein flexibility, subtle conformational changes, and the interplay between the enzyme and its substrate in the active site.

Further investigations, perhaps through computational modeling, molecular dynamics simulations, or mutagenesis studies focusing on active site residues, could provide valuable insights into the molecular basis of the switch in substrate stereospecificity observed in these enzymes.