These molfiles show how the pyruvate is strapped into place in one orientation by R171, N140, R199 and H195, and how the NAD is positioned by M163, I32, S99 and D53. This arrangement is what causes the LDH to synthesize L-lactate and not D-lactate. In the Reaction SPL file, all the reaction participants (interactors) are defined at the top level overall LDH reaction, where pyruvate and NADH (Fig. ) is on the left side of the arrow (CSM), L-lactate and NAD + (Fig. ) on the right side (PRD) and the LDH as the catalyst (CAT). In Reaction SPL we can then drill down into the reaction and establish at least 3 sub-steps: