Lactate dehydrogenase has glutamine at its active site, allowing it to act on pyruvic acid, and complete the reaction, and form lactate. Any mutation to this vital piece of amino acid can lead to wrongful production of any product or no production at all.
In some cases, when a mutation occurs at this glutamine position, and it is replaced by arginine, the lactate dehydrogenase becomes more reactive to a different substrate, which is malate. This is caused due to higher affinity of malate to the enzyme, when arginine is replaced with glutamine.
Pyruvic acid has more affinity towards the enzyme with glutamine in place. It is because of the change in active site due to the mutation, allowing different substrate to interact with it more efficiently.