HACL1 (2-hydroxyacyl-CoA lyase 1) is a peroxisomal thiamine pyrophosphate (TPP)-dependent enzyme that catalyzes the cleavage of 2-hydroxyacyl-CoA intermediates, removing the first carbon as a carboxyl group during fatty acid α-oxidation 1. The enzyme plays two primary metabolic roles: degradation of 3-methyl-branched fatty acids such as phytanic acid, and shortening of 2-hydroxy long-chain fatty acids to facilitate subsequent β-oxidation 2. HACL1 catalyzes the cleavage between the first and second carbons of 2-hydroxyacyl-CoA substrates, producing (n-1) fatty aldehydes and formyl-CoA 1, and is particularly important for endogenous biosynthesis of heptadecanoic acid (C17:0) in liver 3. Clinically, HACL1 dysfunction is relevant to metabolic disease pathophysiology. Deficiency in HACL1 activity leads to accumulation of 2-hydroxy C18:0 and reduced odd-chain fatty acid synthesis, shifting articular chondrocytes from anabolic to catabolic metabolism—a mechanism implicated in osteoarthritis pathogenesis 4. Additionally, HACL1 genetic variants have been identified as novel loci associated with glycemic phenotypes and type 2 diabetes risk in population studies 5. TPP binding is essential for enzyme catalytic activity but not for peroxisomal targeting 1, suggesting that thiamine deficiency could impair HACL1 function without affecting protein localization.