MCD is a protein tetramer, an oligomer formed by a dimer of heterodimers related by an axis of binary symmetry with a rotation angle of about 180 degrees.

As a result, the conformational changes synchronised in the pair of subunits facilitates the catalysis despite the reduction of the number of available active sites.

Each monomer of that structure exhibits a large hydrophobic interface with the possibility to form an inter subunit disulfide bridge.

[6] Another important PTM is the formation of an interchain disulfide bond in the amino acid cysteine in position 206, which may take place in peroxisomes, as the cytosolic and mitochondrial environments are too reducing for this process.

[3] The enzyme malonyl-CoA decarboxylase (MCD) functions as an indirect via of conversion from malonic semi aldehyde to acetyl-CoA in peroxisomes.

This is due to the fact that the beta oxidation of long chain fatty acids with an odd number of carbons produces propionyl-CoA.

Malonyl-CoA also plays an important role inside the mitochondria, where it is an intermediary between fatty acids and acetyl-CoA, which will be a reserve for the Krebs cycle.

Malonyl-CoA concentrations are crucial in the intracellular energetic regulation and the production or degradation of this metabolite delimits the use of glucose or lipids to produce ATP.

The diseases related with MCD can be caused by its mislocalization, mutations affecting the gene MLYCD, its accumulation in peroxisomes and, mainly, its deficiency.

MCS deficiency is a rare autosomal disorder that is widely diagnosed by neonatal screening and it is caused by mutations in MLYCD.

It causes many symptoms: brain abnormalities, mild mental retardation, seizures, hypotonia, metabolic acidosis, vomiting, excretion of malonic and methylmalonic acids in urine, cardiomyopathies, and hypoglycemia.

In peroxisomes, the accumulation of MCD substance also causes pathological symptoms, which are similar to MCS deficiency: malonic aciduria, a lethal disease in which patients (normally children) have delayed development and can suffer from seizures, diarrhoea, hypoglycaemia and cardiomyopathy, as well.