Reaction Mechanism for Lactate Dehydrogenase

When the loop, which contains Arg 101, covers the coenzyme and forces the end of the end of the nicotinamide ring associated with the hydrogen transfer deep into the active site, it will be close enough to the substrate that transfer of a hydrogen can occur rapidly and easily.

Once the coenzyme is bound to the enzyme, a site for the substrate will be generated. The substrate will bind forming a ion pair with Arg 171 and a hydrogen bond with His 195. At this point you have the substrate(lactate or pyruvate) hydrogen bonded with His 195 and ion paired with Arg 171, and the nicotinamide ring of the coenzyme all in close proximity.

In the oxidation of latate, lactate will then accept a proton from His 195 and ultimately give a proton to NAD+ to form pyruvate and NADH. In the oxidation of lactate to pyruvate, His 195 acts as a general base in removing a proton from lactate.

In the reverse reaction, thing will begin with NADH donating a proton to pyruvate. Then through a series of transfers (illustrated below in the schematic) pyruvate will donate a proton to His 195, thus forming lactate and NAD+. In this reverse reaction the imidazolium group of His 195 acts as a general acid.