Cinnamate sensitivity of MCT4 in mammals is also low, suggesting that the characteristic is conserved from fish to mammals. An unexpected finding was the presence of another high-affinity monocarboxylic acid transporter, MCT1b, in the arterial capillaries of the rete mirabile. Since the rete mirabile is a typical countercurrent system in which transport between inflow and the outflow is generally passive, the presence of the transporter MCT1b only on the inflow side was at first a puzzle. Considering that gas gland cells demand a large amount of glucose to generate lactic acid by glycolysis, we later realized that this localization of MCT1b is ideally suited for delivering glucose to the gas gland cells. To prevent glucose loss from the inflow vessels to the outflow vessels of the countercurrent system, the intercellular space among the endothelial cells of the arterial capillaries must be tightly sealed by tight junctions, which in turn become a barrier for recycling lactic acid through the paracellular pathway from the outflow to inflow vessels. However, this barrier could be circumvented by the transendothelial cell pathway mediated by MCT1b. Sequence comparisons show that Rv0045c shares a low sequence identity, to other members of the a/b hydrolase fold family, however, the consensus sequence G-X-S-X-G of the nucleophile elbow and the catalytic residues are Yohimbine-Hydrochloride highly conserved. Similar to other a/b hydrolases, it has been previously shown that Rv0045c can hydrolyze ester bonds within a series of p-nitrophenyl derivatives. The purified enzyme can effectively hydrolyze p-nitrophenyl derivatives with short hydrocarbon chains. We identified p-nitrophenyl caproate as the most suitable substrate of Rv0045c at the assay conditions of 39uC and pH 8.0. To understand the active site and enzymatic Ascomycin mechanism of esterase Rv0045c, we determined the crystal structure of the enzyme and performed docking experiments. The a/b hydrolase fold family has been structurally well characterized and comprises a variety of enzymes including esterases, lipases, epoxide hydrolases, dehalogenases, proteases, and peroxidases, catalyzing myriad reactions.