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Statistical analysis of amino acid patterns in approximately 160,000 alpha-helices in experimentally determined structures revealed di-, tri-, and tetrapeptides, whose frequencies deviate most from the statistical model. Importantly, some sequences were never found in alpha- helices. This fact was detected initially with tripeptides, where nearly 1% of the possible sequences were never seen in the helical segments. For tetrapeptides, this effect is very strong and significant; almost 43% of the possible sequences never appear in alpha-helices. It is possible that there are some steric and energetic restrictions that do not allow these tetrameric amino acid sequences to form alpha-helical structure.

Introducción. El síndrome de Hunter o mucopolisacaridosis tipo II (MC KUSIK 309900) es causado por la deficiencia de la iduronato 2-sulfato sulfatasa humana (E.C. 3.1.6.13). La enzima no ha sido cristalizada y por tanto sus estructuras no se conocen por deducción experimental. Objetivo. Proponer un modelo computacional para la estructura tridimensional de la iduronato 2-sulfato sulfatasa humana. Materiales y métodos. Se realizó un análisis computacional de esta enzima empleando programas de libre acceso en internet como Comput pI/MW, JaMBW Chapter 3.1.7, SWISS-MODEL, 3D-PSSM, ProSup. Los procesos de minimización de energía se realizaron con el programa Discover 3 del paquete Insight II (2004). Resultados. Se propone un modelo tridimensional de la iduronato 2-sulfato sulfatasa humana que presenta 33,3% en hélice, 7,2% en hoja plegada y 59,5% en enrollamiento al azar (coil). Se hallaron valores de RMS (del inglés Root Mean Square) de 0,78 y 0,86Å al compararla con otras enzimas de la misma familia. El modelo revela cinco sitios potenciales de N-glicosilación y una entrada al bolsillo que contiene los aminoácidos que componen el sitio activo. Usando este modelo se encontró una buena correlación entre el tipo de mutaciones y la gravedad de la enfermedad en 20 pacientes analizados. Conclusión. Los valores de RMS y la correlación genotipo-fenotipo en los pacientes analizados sugieren el modelo puede usarse para predecir ciertos aspectos del comportamiento biológico de la enzima. Palabras clave: mucopolisacaridosis II, iduronato sulfatasa, estructura terciaria de proteína, metodologías computacionales. Computational prediction of the tertiary structure of the human iduronate 2-sulfate sulfatase Introduction. Hunter syndrome (MC KUSIK 309900) or mucopolysacharidosis type II is due to the deficiency of the enzyme iduronate 2 sulfate sulfatase (E.C. 3.1.6.13). This enzyme has not been crystallized, and therefore the experimental structures are not available. Objectives. A computational three-dimensional model was proposed for the iduronate 2 sulfate sulfatase enzyme. Materials and methods. A computational analysis of this enzyme used the following free internet software programs: Comput pI/MW, JaMBW Chapter 3.1.7, SWISS-MODEL, Geno3d, ProSup. Energy minimization was done with Discover 3 and Insight II version 2004. Results. A three-dimensional conformational model was proposed. The model showed 33.3% of helix structure, 7.2% beta sheet, and 59.5% random coil. RMS values (Root Mean Square) (0.78 and 0.86Å) were found when compared with other enzymes of the same family. The model presented 5 exposed N-glycosylation potential sites and an entry to the pocket that