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MMAB gene

metabolism of cobalamin associated B

Normal Function

The MMAB gene provides instructions for making an enzyme that is involved in the formation of a compound called adenosylcobalamin (AdoCbl). AdoCbl, which is derived from vitamin B12 (also known as cobalamin), is necessary for the normal function of another enzyme known as methylmalonyl CoA mutase. This enzyme helps break down certain proteins, fats (lipids), and cholesterol.

The MMAB enzyme is active in mitochondria, which are specialized structures inside cells that serve as energy-producing centers. Once vitamin B12 has been transported into mitochondria, the MMAB enzyme converts a form of the vitamin called cob(I)alamin to AdoCbl. Studies suggest that this enzyme may also deliver AdoCbl to methylmalonyl CoA mutase.

Health Conditions Related to Genetic Changes

Methylmalonic acidemia

At least 25 mutations in the MMAB gene have been found to cause methylmalonic acidemia, a condition characterized by feeding difficulties, developmental delay, and long-term health problems. Some of these genetic changes delete or duplicate a small amount of genetic material in the MMAB gene. Other mutations change a single protein building block (amino acid) used to make the MMAB enzyme. Researchers believe that nearly all of these mutations lead to the production of a nonfunctional version of the enzyme. As a result, AdoCbl cannot be made properly. A lack of AdoCbl impairs the function of methylmalonyl CoA mutase, which results in the incomplete breakdown of certain proteins and lipids. This defect allows toxic compounds to build up in the body's organs and tissues, causing the signs and symptoms of methylmalonic acidemia.

More About This Health Condition

Other Names for This Gene

  • ATP:Cob(I)alamin Adenosyltransferase
  • ATR
  • cblB
  • CFAP23
  • cob(I)alamin adenosyltransferase
  • methylmalonic aciduria (cobalamin deficiency) cblB type
  • methylmalonic aciduria (cobalamin deficiency) type B

Additional Information & Resources

Tests Listed in the Genetic Testing Registry

Scientific Articles on PubMed

Catalog of Genes and Diseases from OMIM

Gene and Variant Databases


  • Chandler RJ, Venditti CP. Genetic and genomic systems to study methylmalonic acidemia. Mol Genet Metab. 2005 Sep-Oct;86(1-2):34-43. doi: 10.1016/j.ymgme.2005.07.020. Epub 2005 Sep 22. Citation on PubMed or Free article on PubMed Central
  • Dobson CM, Wai T, Leclerc D, Kadir H, Narang M, Lerner-Ellis JP, Hudson TJ, Rosenblatt DS, Gravel RA. Identification of the gene responsible for the cblB complementation group of vitamin B12-dependent methylmalonic aciduria. Hum Mol Genet. 2002 Dec 15;11(26):3361-9. doi: 10.1093/hmg/11.26.3361. Citation on PubMed
  • Horster F, Baumgartner MR, Viardot C, Suormala T, Burgard P, Fowler B, Hoffmann GF, Garbade SF, Kolker S, Baumgartner ER. Long-term outcome in methylmalonic acidurias is influenced by the underlying defect (mut0, mut-, cblA, cblB). Pediatr Res. 2007 Aug;62(2):225-30. doi: 10.1203/PDR.0b013e3180a0325f. Citation on PubMed
  • Leal NA, Olteanu H, Banerjee R, Bobik TA. Human ATP:Cob(I)alamin adenosyltransferase and its interaction with methionine synthase reductase. J Biol Chem. 2004 Nov 12;279(46):47536-42. doi: 10.1074/jbc.M405449200. Epub 2004 Aug 30. Citation on PubMed
  • Leal NA, Park SD, Kima PE, Bobik TA. Identification of the human and bovine ATP:Cob(I)alamin adenosyltransferase cDNAs based on complementation of a bacterial mutant. J Biol Chem. 2003 Mar 14;278(11):9227-34. doi: 10.1074/jbc.M212739200. Epub 2003 Jan 3. Citation on PubMed
  • Lerner-Ellis JP, Gradinger AB, Watkins D, Tirone JC, Villeneuve A, Dobson CM, Montpetit A, Lepage P, Gravel RA, Rosenblatt DS. Mutation and biochemical analysis of patients belonging to the cblB complementation class of vitamin B12-dependent methylmalonic aciduria. Mol Genet Metab. 2006 Mar;87(3):219-25. doi: 10.1016/j.ymgme.2005.11.011. Epub 2006 Jan 10. Citation on PubMed
  • Manoli I, Sloan JL, Venditti CP. Isolated Methylmalonic Acidemia. 2005 Aug 16 [updated 2022 Sep 8]. In: Adam MP, Feldman J, Mirzaa GM, Pagon RA, Wallace SE, Bean LJH, Gripp KW, Amemiya A, editors. GeneReviews(R) [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2024. Available from Citation on PubMed
  • Zhang J, Dobson CM, Wu X, Lerner-Ellis J, Rosenblatt DS, Gravel RA. Impact of cblB mutations on the function of ATP:cob(I)alamin adenosyltransferase in disorders of vitamin B12 metabolism. Mol Genet Metab. 2006 Apr;87(4):315-22. doi: 10.1016/j.ymgme.2005.12.003. Epub 2006 Jan 24. Citation on PubMed

The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.