The function of Ensembl gene identifier ENSG00000138029 (HADHB, hydroxyacyl-CoA dehydrogenase trifunctional multienzyme complex subunit beta) is as follows. Mitochondrial trifunctional enzyme catalyzes the last three of the four reactions of the mitochondrial beta-oxidation pathway (PubMed:1550553, PubMed:16423905, PubMed:26474213, PubMed:29915090, PubMed:30850536, PubMed:8135828). The mitochondrial beta-oxidation pathway is the major energy-producing process in tissues and is performed through cycles of four consecutive reactions (PubMed:26474213, PubMed:29915090). Each beta-oxidation cycle shortens the fatty acyl-CoA by two carbons, yielding one acetyl-CoA (for the citric acid cycle), one FADH(2), and one NADH (which donate electrons to the respiratory chain for ATP production) (PubMed:26474213, PubMed:29915090). These cycles repeat until the chain is fully degraded to acetyl-CoA units (PubMed:26474213, PubMed:29915090). Among the enzymes involved in this pathway, the trifunctional protein, responsible for the hydration, dehydrogenation, and thiolysis steps, shows specificity for long-chain fatty acids, such as those from dietary and stored fats (PubMed:30850536). Mitochondrial trifunctional enzyme is a heterotetrameric complex composed of two proteins, the trifunctional enzyme subunit alpha/HADHA carries the 2,3-enoyl-CoA hydratase and the 3-hydroxyacyl-CoA dehydrogenase activities while the trifunctional enzyme subunit beta/HADHB described here bears the 3- ketoacyl-CoA thiolase activity (PubMed:16423905, PubMed:26474213, PubMed:29915090, PubMed:30850536, PubMed:38372965, PubMed:8135828). These activities have been experimentally confirmed on a few substrates derived from beta-oxidation of long-chain saturated fatty acids such as palmitate (hexadecanoate) and laurate (dodecanoate) (PubMed:1550553, PubMed:8135828, PubMed:8163672, PubMed:8651282). In addition, based on its established catalytic mechanism, and combined genetic interaction or mutant phenotype evidence, it is predicted to act also on other substrates, including long-chain unsaturated fatty acids such as oleate (9Z-octadecenoate), linoleate (9Z,12Z-octadecadienoate), linolenate (9Z,12Z,15Z-octadecatrienoate), and others (Probable) (PubMed:26474213). {ECO:0000269|PubMed:1550553, ECO:0000269|PubMed:16423905, ECO:0000269|PubMed:29915090, ECO:0000269|PubMed:30850536, ECO:0000269|PubMed:38372965, ECO:0000269|PubMed:8135828, ECO:0000269|PubMed:8163672, ECO:0000269|PubMed:8651282, ECO:0000303|PubMed:26474213, ECO:0000303|PubMed:29915090, ECO:0000303|PubMed:30850536, ECO:0000305|PubMed:38372965}.