The function of FAHD1 (FAH domain containing oxaloacetate decarboxylase 1, Ensembl gene identifier ENSG00000180185) is as follows. Tautomerase that converts enol-oxaloacetate, a strong inhibitor of succinate dehydrogenase, to the physiological keto form of oxaloacetate (PubMed:38287013). It is thereby required to maximize aerobic respiration efficiency by preventing succinate dehydrogenase inhibition (PubMed:38287013). Also acts as a weak oxaloacetate decarboxylase (ODx), catalyzing the decarboxylation of oxaloacetate (OAA) to pyruvate and CO(2), and as such is likely a regulatory enzyme in the TCA cycle (PubMed:25575590, PubMed:30348641). Also displays acylpyruvase activity, being able to hydrolyze acetylpyruvate and fumarylpyruvate in vitro (PubMed:21878618, PubMed:30348641). Exhibits only a weak hydrolase activity on methylacetopyruvate and acetylacetone, and no activity toward acetoacetyl-CoA (PubMed:21878618). {ECO:0000269|PubMed:21878618, ECO:0000269|PubMed:25575590, ECO:0000269|PubMed:30348641, ECO:0000269|PubMed:38287013}.