Elucidating biosynthetic pathways for vitamins and cofactors
3-Methylcrotonyl-Co A carboxylase (3MCC), like PCC, is also a heterododecameric enzyme (subunit composition: α).Similar to the activities of PCC, the α-subunit possesses the BC and BCCP activities and the β-subunit possesses the CT activity. The α-subunit of 3MCC is encoded by the MCCC1 gene and the β-subunit is encoded by the MCCC2 gene.The enzyme ACC is a cytosolic enzyme that is the rate-limiting enzyme of fatty acid synthesis.
As discussed in detail in the Fatty Acid and Triglyceride Metabolism page, humans express two forms of ACC (ACC1 and ACC2) both of which are multifunctional enzymes possessing all three biotin-dependent carboxylating activities.Indeed, the frequency is high enough, and the resultant symptoms severe enough, that current neonatal disease testing includes analysis for defects in the activity of this enzyme.The most severe symptoms associated with biotin deficiency and profound biotinidase gene defects are the result of the accumulation of toxic metabolic intermediates.The biotinidase gene (gene symbol: BTD) is located on chromosome 3p25.1 and is composed of 4 exons that encode a 543 amino acid protein.The clinical significance of defects in the BTD gene are described below.
An important autosomal recessive inherited disorder that leads to biotin deficiency is biotinidase (BTD) deficiency.