Seleno-methionine proteins

Multiwavelength anomalous diffraction Makes use of atoms within a protein that diffract X-rays anomalously. Commonly uses seleno-methionine residues in place of methionines. 

Katarzyna Wrobel, S. S. Kannamkumarath, Kazimierz Wrobel, J. A. Caruso, Hydrolysis of proteins with methanesulfonic acid for improved HPLC-ICP-MS determination of seleno-methionine in yeast and nuts, Anal. Bioanal. Chem., 375 (2003), 133D138. 

Method (1) can not (yet ) be used for proteins. Method (2) can be used to solve any protein structure de novo, but this method requires testing and measuring dozens of heavy atom derivatives, and is hardly used any more. Method (3) allows the fast determination of de, novo protein structures, provided they can be labelled with seleno methionine (expression in E. coli). Extremely promising new methods use the anomalous signal of soaked-in halide ions, or the signal of native sulfurs. When these, methods deliver what they promise, they allow any new structure to be solved in a matter of weeks. Method (4) is very fast it usually takes less than a week, but it requires that the structure of a similar protein be available, which is not always the case. This method is extremely useful in solving the structures of a protein complexed with a series of inhibitors or other ligands. 

The main form of selenium in most foods is protein-bound seleno-methionine. Supplements of selenium are provided by mineral salts containing sodium selenite, slow-release capsules and selenium-enriched yeast. 

Day-old mallard ducklings fed diets for 2 weeks containing 0, 15, or 30 mg Se/kg ration in a 75% wheat diet (22% protein). Selenium given as seleno-DL-methionine, seleno-L-methionine, or selenized yeast... 

Selenium is readily absorbed, especially in the duodenum but also in the caecum and colon. Seleno-amino acids are almost completely absorbed selenomethionine via the gut methionine transporter and selenocysteine probably via the cysteine transporter. Both selenite and selenate are >50% absorbed, selenite more readily so than selenate, and for these forms there is competition with sulphate transport. Selenite is more efficiently retained then selenate because part of the latter is rapidly excreted into the urine. Vitamins A, E, and C can modulate selenium absorption, and there is a complex relationship between selenium and vitamin E that has not been entirely elucidated for man. A combined deficiency of both nutrients can produce increases in oxidative damage markers (malondialdehyde, Ei isoprostanes, and breath hydrocarbons) and in pathological changes that are not seen with either deficiency alone. Inorganic Se is reduced to selenide by glutathione plus glutathione reductase and is then carried in the blood plasma, bound mainly to protein in the very low-density lipoprotein fraction. Selenomethionine is partly carried in the albumin fraction. 

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