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Things worth knowing about "Carboxylase"
Carboxylases are enzymes that catalyze the addition of CO2 to organic molecules. They play a central role in metabolism and depend on vitamin B7 (biotin) as a key cofactor.
What Is a Carboxylase?
A carboxylase is an enzyme that catalyzes a carboxylation reaction – the biochemical addition of a carboxyl group (–COOH) to an organic substrate using carbon dioxide (CO2). These enzymes are found in virtually all living organisms and perform essential roles in both energy metabolism and biosynthetic pathways.
Carboxylases belong to the broader enzyme class of ligases and typically require cofactors for their activity, with biotin (vitamin B7) being the most common and important cofactor in human biochemistry.
Biological Function and Importance
Carboxylases are involved in central metabolic pathways, including:
- Fatty acid synthesis: Acetyl-CoA carboxylase catalyzes the first and rate-limiting step of fatty acid biosynthesis, converting acetyl-CoA to malonyl-CoA.
- Gluconeogenesis: Pyruvate carboxylase converts pyruvate to oxaloacetate – a key step in synthesizing glucose from non-carbohydrate sources.
- Citric acid cycle (Krebs cycle): Carboxylation reactions play an important role in replenishing intermediates of the citric acid cycle (anaplerotic reactions).
- Amino acid metabolism: Propionyl-CoA carboxylase is involved in the breakdown of certain amino acids and odd-chain fatty acids.
- Photosynthesis: RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) is perhaps the most well-known carboxylase in nature and fixes atmospheric CO2 during carbon assimilation in plants.
Biotin as a Key Cofactor
Most carboxylases in the human body are biotin-dependent enzymes. Biotin is covalently attached to a lysine residue of the enzyme and acts as a carrier of the carboxyl group during the reaction. The four main biotin-dependent carboxylases in humans are:
- Acetyl-CoA carboxylase (ACC1 and ACC2)
- Pyruvate carboxylase (PC)
- Propionyl-CoA carboxylase (PCC)
- 3-Methylcrotonyl-CoA carboxylase (MCC)
A biotin deficiency impairs the activity of all these enzymes simultaneously and can lead to serious metabolic disorders.
Carboxylase Deficiencies and Associated Diseases
Genetic defects in individual carboxylases or in biotin metabolism can cause rare but serious diseases:
- Multiple carboxylase deficiency: Caused by a defect in holocarboxylase synthetase or biotinidase, simultaneously impairing several biotin-dependent carboxylases. Symptoms include metabolic acidosis, neurological impairment, skin abnormalities, and immune dysfunction.
- Isolated pyruvate carboxylase deficiency: Results in severe lactic acidosis, neurological deficits, and hypoglycemia, primarily affecting newborns and infants.
- Propionyl-CoA carboxylase deficiency (propionic acidemia): An organic aciduria with accumulation of propionic acid, which can lead to metabolic crises, developmental delay, and cardiomyopathy.
Diagnosis and Treatment
Diagnostic evaluation of carboxylase disorders typically involves:
- Urinary organic acid analysis (gas chromatography-mass spectrometry)
- Amino acid and acylcarnitine profiling in blood (tandem mass spectrometry)
- Enzyme activity measurements in leukocytes or fibroblasts
- Molecular genetic testing
Treatment depends on the underlying cause. In multiple carboxylase deficiency due to biotinidase deficiency, biotin supplementation (typically 5–20 mg/day) is highly effective and can fully reverse symptoms if initiated early. For inherited enzyme defects, dietary management is the primary approach.
References
- Baumgartner, M. R. et al. (2014): Proposed guidelines for the diagnosis and management of methylmalonic and propionic acidemia. In: Orphanet Journal of Rare Diseases, 9:130.
- Zempleni, J. et al. (2009): Biotin and biotinidase deficiency. In: Expert Review of Endocrinology and Metabolism, 4(4), 385–399.
- Berg, J. M., Tymoczko, J. L., Stryer, L. (2018): Biochemistry. 8th Edition. W. H. Freeman and Company, New York.
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