Methylcrotonyl CoA carboxylase (MCC) (3-Methylcrotonyl CoA carboxylase, methylcrotonoyl-CoA carboxylase) is a biotin

Biotin

Biotin, also known as Vitamin H or Coenzyme R, is a water-soluble B-complex vitamin discovered by Bateman in 1916. It is composed of a ureido ring fused with a tetrahydrothiophene ring. A valeric acid substituent is attached to one of the carbon atoms of the tetrahydrothiophene ring...

-requiring enzyme located in the mitochondria. MCC uses bicarbonate

Bicarbonate

In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 as a carboxyl group source to catalyze the carboxylation of a carbon adjacent to a carbonyl

Carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 group performing the 4th step in processing leucine

Leucine

Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...

, an essential amino acid.

Function

During branched-chain amino acid degradation, MCC performs a single step in the breakdown of leucine to eventually yield acetyl CoA and acetoacetate. MCC catalyzes the carboxylation of 3-methylcrotonyl CoA to 3-methylglutaconyl CoA. 3-Methylglutaconyl CoA is then hydrated to produce 3-hydroxy-3-methylglutaryl CoA. 3-Hydroxy-3-methylglutaryl CoA is cleaved into two molecules, acetoacetate and acetyl CoA.

Point mutations and deletion events in the genes coding for MCC can lead to MCC deficiency, an inborn error of metabolism

Inborn error of metabolism

Inborn errors of metabolism comprise a large class of genetic diseases involving disorders of metabolism. The majority are due to defects of single genes that code for enzymes that facilitate conversion of various substances into others...

 which usually presents with vomiting, metabolic acidosis

Metabolic acidosis

In medicine, metabolic acidosis is a condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body. If unchecked, metabolic acidosis leads to acidemia, i.e., blood pH is low due to increased production of hydrogen by the body or the...

, very low plasma glucose concentration, and very low levels of carnitine

Carnitine

Carnitine is a quaternary ammonium compound biosynthesized from the amino acids lysine and methionine. In living cells, it is required for the transport of fatty acids from the cytosol into the mitochondria during the breakdown of lipids for the generation of metabolic energy. It is widely...

 in plasma.

Mechanism

Bicarbonate is activated by the addition of ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

, increasing the reactivity of bicarbonate. Once bicarbonate is activated, the biotin portion of MCC performs nucleophilic attack on the activated bicarbonate to form enzyme-bound carboxybiotin. The carboxybiotin portion of MCC can then undergo nucleophilic attack transferring the carboxyl group to the substrate, 3-methylcrotonyl CoA, to form 3-methylglutaconyl CoA.