COQ5 (Coenzyme Q5 Homolog, Methyltransferase): COQ5 (Coenzyme Q5 Homolog, Methyltransferase) is an enzyme involved in the biosynthesis of coenzyme Q (CoQ), a vital molecule for mitochondrial energy production. It catalyzes a key methylation step in the CoQ pathway. Mutations in COQ5 can lead to CoQ deficiency, affecting mitochondrial function and contributing to disorders of energy metabolism.