Cobalamin as coenzyme: a twisting trail of research

A water channel in the core of the vitamin B(12) RNA aptamer

BACKGROUND

The 3.0 A crystal structure of the vitamin B(12) RNA aptamer revealed an unusual tertiary structure that is rich in novel RNA structural motifs. Important details of the interactions that stabilize noncanonical base pairing and the role of solvent in the structure were not apparent owing to the limited resolution.

RESULTS

The structure of the vitamin B(12) RNA aptamer in complex with its ligand has been determined at 2.3 A resolution by X-ray crystallography. The crystallographic asymmetric unit contains five independent copies of the aptamer-vitamin B(12) complex, making it possible to accurately define well-conserved features. The core of the aptamer contains an unusual water-filled channel that is buried between the three strands of an RNA triplex. Well-ordered water molecules positioned within this channel form bridging hydrogen bonds and stabilize planar base triples that otherwise lack significant direct base-base contacts. The water channel terminates at the interface between the RNA and the bound ligand, leaving a pair of water molecules appropriately positioned to hydrogen bond with the highly polarized cyanide nitrogen of vitamin B(12). Analysis of the general solvation patterns for each nucleotide suggests that water molecules are not precisely positioned, as observed in previous RNA duplex structures, but instead might adjust in response to the varying local environment. Unusual intermolecular base pairing contributes to the formation of three different dimerization contacts that drive formation of the crystal lattice.

CONCLUSIONS

The structure demonstrates the important role of water molecules and noncanonical base pairing in driving the formation of RNA tertiary structure and facilitating specific interactions of RNAs with other molecules.

Cobalamin

Cobalamin (vitamin B12) is an essential nutrient derived exclusively from bacterial sources. It is an essential cofactor for three known enzymatic reactions. Untreated deficiency, caused by either the autoimmune disease pernicious anemia or nutritional lack, results in a macrocytic anemia and/or subacute combined degeneration of the spinal cord and is eventually fatal. Cobalamin in serum is bound to two proteins, transcobalamin and haptocorrin. The former is responsible for the essential delivery of cobalamin to most tissues. Inadequate tissue availability of cobalamin results in increased concentration of methylmalonic acid and homocyst(e)ine due to inhibition of methylmalonyl-CoA mutase and methionine synthase, respectively. Strict vegetarians have long been known to be at risk of cobalamin deficiency, which develops insidiously over many years. It is now clear that a significant number of the elderly and HIV-positive individuals are also at increased risk of deficiency. Any individual with reduced ability to split cobalamin from food-protein may also become deficient even though intrinsic factor is present. Diagnosis of cobalamin deficiency has frequently relied on total serum cobalamin and the Schilling test. Newer approaches such as analysis of methylmalonic acid, homocyst(e)ine, holotranscobalamin, anti-intrinsic factor antibodies, and serum gastrin may provide more cost-effective testing, as well as identify those with a covert deficiency.

Methylcobalamin induces a long-lasting enhancement of the postsynaptic field potential in hippocampal slices of the guinea pig

Methylcobalamin (CH3-B12) enhanced postsynaptic field potential (PSP) elicited from hippocampal slices (around 180% at a maximum), lasting more than 1 h after washing out, while it had no effect on antidromic spike potential (AP). By contrast, cyanocobalamin containing CN- instead of CH3- showed no enhancement in the PSP amplitude, suggesting that CH3- plays an important role for the PSP enhancement by methylcobalamin. The treatment with N-methyl-D-aspartate (NMDA) receptor specific antagonist, DL-2-amino-5-phosphonovaleric acid (APV) inhibited methylcobalamin-induced PSP enhancement by 50%, suggesting that NMDA receptor is partially relevant to the formation of this enhancement. Glutamate release from electrically stimulated slices was not enhanced by the treatment with methylcobalamin, suggesting that methylcobalamin affects the postsynaptic sites in hippocampal neurons. In whole cell voltage clamp recordings using cultured hippocampal neurons, methylcobalamin increased the currents elicited by NMDA time-dependently. These results may indicate that CH3- is required to modulate the PSP amplitude at the postsynaptic site and at least, NMDA receptor is involved in the formation of long-lasting PSP potentiation induced by methylcobalamin.

Vitamin B12 among parturients and their newborns and its relationship with birthweight

Vitamin B12 (Cobalamin) is an essential nutrient in the diet of humans, particularly during pregnancy, nevertheless very few epidemiological studies have been reported, particularly concerning variation factors. The purpose of this study is to assess the role of vitamin B12 on birthweight after taking into account potential confounders, such as obstetrical, socioeconomic, and biological factors which will be identified. This study was conducted on 188 single births occurring during a 5-month period in an obstetrical care unit in Paris. Vitamin B12 plasma levels and folate plasma levels of mothers and cords along with haematological parameters were measured. Social and obstetrical features of the mothers as well as their tobacco exposure and alcohol consumption were recorded. Cord blood vitamin B12 levels were highly correlated with maternal levels (r = 0.63, P less than 0.001) and were 2-3-fold higher. Three variation factors of vitamin B12 were identified: maternal age, ethnicity and tobacco exposure. Negative correlations between birthweight and vitamin B12 levels were observed only among the smoker group (r mothers = -0.46, P less than 0.05 and r cords = -0.42, P less than 0.05). After adjustment for ethnicity and parity, birthweight remained negatively linked to vitamin B12 in smokers. This relationship could suggest that the availability of vitamin B12 for fetal development depends on certain biological factors present only in mothers who smoked. We hypothesized this relationship could be explained, in part, by lipid metabolism, particular to smokers.