The function of COQ5 (coenzyme Q5, methyltransferase, Ensembl gene identifier ENSG00000110871) is as follows. Methyltransferase required for the conversion of 2- decaprenyl-6-methoxy-1,4-benzoquinol (DDMQH2) to 2-decaprenyl-3-methyl- 6-methoxy-1,4-benzoquinol (DMQH2). {ECO:0000255|HAMAP-Rule:MF_03191, ECO:0000269|PubMed:38425362}.