Both MTRF1L and ICT1 can bind a standard stop codon in the A-site of the ribosome and cause release of the polypeptide. ICT1 can also cause release of ribosomes stalled in non-standard conformations (e.g. non-standard stop codons, mRNA lacking a stop codon) (inferred from pig mitoribosomes in Akabane et al. 2014).
Defects in MTR cause methylcobalamin deficiency type G (cblG; MIM:250940), an autosomal recessive inherited disease that causes mental retardation, macrocytic anemia, and homocystinuria (Leclerc et al. 1996, Gulati et al. 1996, Watkins et al. 2002).
Defects in MTRR cause methylcobalamin deficiency type E (cblE; methionine synthase reductase deficiency; MIM:236270) (Wilson et al. 1999). Patients with cblE exhibit megaloblastic anemia and hyperhomocysteinemia. SAM is used as a methyl donor in many biological reactions and demethylation of SAM produces S-adenosylhomocysteine, which is deadenosylated to form homocysteine. Homocysteine remethylation is carried out by MTR, which requires MTRR to maintain enzyme-bound cobalamin (Cbl) in its active form; but in cblE patients, MTR becomes inactivated and thus homocysteine accumulates.
Cobalamin (Cbl, vitamin B12) is a nutrient essential for normal functioning of the brain and nervous system and for the formation of blood. Cbl-dependent methionine synthase (MTR) is required for conversion of 5-methyltetrahydrofolate (metTHF) to tetrahydrofolate (THF), in addition to its role in conversion of homocysteine to methionine. In Cbl deficiency, and in inborn errors of Cbl metabolism that affect function of methionine synthase, inability to regenerate THF from metTHF results in decreased function of folate-dependent reactions that are involved in 2 steps of purine biosynthesis and thymidylate synthesis. Cbl deficiency results in hyperhomocysteinemia (due to defects in the conversion of homocysteine to methionine which requires Cbl as a cofactor) and increased levels of methylmalonic acid (MMA). Methionine is used in myelin production, protein, neurotransmitter, fatty acid and phospholipid production and DNA methylation. Symptoms of Cbl deficiency are bone marrow promegaloblastosis (megaloblastic anemia) due to the inhibition of DNA synthesis (specifically purines and thymidine) and neurological symptoms. The defective genes involved in Cbl deficiencies are described below (Froese & Gravel 2010, Nielsen et al. 2012, Whitehead 2006, Watkins & Rosenblatt 2011, Fowler 1998).
Methionine synthase (MTR) mediates the continuous shuttling of cobalamin (Cbl) between two forms, cob(I)alamin and MeCbl. In this half reaction, the methyl group from MeCbl is transferred to homocysteine (HYCS) to form methionine and regenerate cob(I)alamin (Hall et al. 2000; Leclerc et al. 1996).
Methionine synthase (MTR) mediates the continuous shuttling of cobalamin (Cbl) between two forms, cob(I)alamin and MeCbl. In this half reaction, the methyl group from 5 methyltetrahydrofolate (5-methyl-THF) is transferred to enzyme bound cob(I)alamin to form MeCbl (Hall et al. 2000). Defects in MTR cause methylcobalamin deficiency type G (cblG, methionine synthase deficiency; MIM:250940), an autosomal recessive inherited disease that causes mental retardation, macrocytic anemia, and homocystinuria (Leclerc et al. 1996).
Methionine synthase (MTR) catalyses the transfer of a methyl group from 5-methyltetrahydrofolate (MTHF) to MTR-bound cob(I)alamin to form methylcobalamin (MeCbl). In the second step, the methyl group from MeCbl is transferred to homocysteine (HCYS), forming methionine (L-Met) (Leclerc et al. 1996). Under normal conditions, the enzyme-bound cobalamin shuttles between the cob(I)alamin and MeCbl forms. Every few hundred cycles, enzyme-bound cob(I)alamin is spontaneously oxidized to cob(II)alamin, inactivating MTR. Active MTR is regenerated by MTRR which, in conjunction with MTR, catalyzes reductive methylation of cob(II)alamin to MeCbl using SAM as a methyl donor.
Defects in MTR cause methylcobalamin deficiency type G (cblG, methionine synthase deficiency; MIM:250940), an autosomal recessive inherited disease that causes mental retardation, macrocytic anemia, and homocystinuria. Mutations causing cblG include P1173L, Ile881, H920D, R585*, E1204* and A1204P (Leclerc et al. 1996, Gulati et al. 1996, Watkins et al. 2002).