Isozymes are different enzymes involved in catalysing the same reaction. These can be used to regulate metabolic pathways (including amino acid synthesis), through inhibition. Isozymes tend to be splice variants of the same gene, allowing for different tissues to have different regulatory processes - this allows the different amino acid requirements that different tissues have to be included in regulating metabolic pathways. For example, one regulatory domain may be inhibited by threonine, but a different regulatory domain (incorporated by alternative splicing) may inhibit the catalytic activity on the binding of lysine.
The enzyme lactate dehydrogenase catalyses the conversion of pyruvate to lactate. This enzyme is a tetramer, with two different monomers: M (typically found in higher concentrations in muscle tissue) or H (generally found in higher concentrations in heart tissue). M subunits are inhibited by lactate, and catalyse the conversion of pyruvate → lactate. H subunits are inhibited by pyruvate, and catalyse the conversion of lactate → pyruvate. The ratio of H to M subunits varies based on the metabolic activity of the tissue, with a higher proportion of H subunits being obtained with endurance training (such as for a marathon, …). H subunits allow for oxidative phosphorylation to be favoured through preventing the loss of lactate. More M subunits allows for anaerobic respiration to be favoured, synthesising more lactate.