Two things that I don't understand. First is the diagram shows a H from NADH joining pyruvate then a H+ leaving pyruvate. There is a change in charge here. Yet after LDH there is 2 extra hydrogens on lactate. It would only make sense if the H+ arrow going off was an arrow going on. I also am not sure why the OH group on the top left of lactate cannot dissociate. Perhaps it would disrupt the shape of the molecule too much or maybe be too easily pulled back on?