title: Size product modulation by enzyme concentration reveals two distinct levan elongation mechanisms in Bacillus subtilis levansucrase.
abstract: Two levan distributions are produced typically by Bacillus subtilis levansucrase (SacB): a high-molecular weight (HMW) levan with an average molecular weight of 2300 kDa, and a low-molecular weight (LMW) levan with 7.2 kDa. Previous results have demonstrated how reaction conditions modulate levan molecular weight distribution. Here we demonstrate that the SacB enzyme is able to perform two mechanisms: a processive mechanism for the synthesis of HMW levan and a non-processive mechanism for the synthesis of LMW levan. Furthermore, the effect of enzyme and substrate concentration on the elongation mechanism was studied. While a negligible effect of substrate concentration was observed, we found that SacB elongation mechanism is determined by enzyme concentration. A high concentration of enzyme is required to synthesize LMW levan, involving the sequential formation of a wide variety of intermediate size levan oligosaccharides with a degree of polymerization (DP) up to ∼70. In contrast, an HMW levan distribution is synthesized through a processive mechanism producing oligosaccharides with DP <20, in reactions occurring at low enzyme concentration. Additionally, reactions where levansucrase concentration was varied while the total enzyme activity was kept constant (using a combination of active SacB and an inactive SacB E342A/D86A) allowed us to demonstrate that enzyme concentration and not enzyme activity affects the final levan molecular weight distribution. The effect of enzyme concentration on the elongation mechanism is discussed in detail, finding that protein-product interactions are responsible for the mechanism shift.
authors.affiliation: Departamento de Ingeniería Celular y Biocatálisis, Instituto de Biotecnología, UNAM, Cuernavaca, Morelos 62210, Mexico.
authors.forename: Enrique
authors.initials: E
authors.lastname: Raga-Carbajal
authors.affiliation: Laboratorio de Biofisicoquímica, Departamento de Fisicoquímica, Facultad de Química, Universidad Nacional Autónoma de México, México DF 04510, Mexico.
authors.forename: Ernesto
authors.initials: E
authors.lastname: Carrillo-Nava
authors.affiliation: Laboratorio de Biofisicoquímica, Departamento de Fisicoquímica, Facultad de Química, Universidad Nacional Autónoma de México, México DF 04510, Mexico.
authors.forename: Miguel
authors.initials: M
authors.lastname: Costas
authors.affiliation: Departamento de Ingeniería Celular y Biocatálisis, Instituto de Biotecnología, UNAM, Cuernavaca, Morelos 62210, Mexico.
authors.forename: Jaime
authors.initials: J
authors.lastname: Porras-Dominguez
authors.affiliation: Departamento de Ingeniería Celular y Biocatálisis, Instituto de Biotecnología, UNAM, Cuernavaca, Morelos 62210, Mexico.
authors.forename: Agustín
authors.initials: A
authors.lastname: López-Munguía
authors.affiliation: Departamento de Ingeniería Celular y Biocatálisis, Instituto de Biotecnología, UNAM, Cuernavaca, Morelos 62210, Mexico clarita@ibt.unam.mx.
authors.forename: Clarita
authors.initials: C
authors.lastname: Olvera
mesh_terms: D001412:Bacillus subtilis / Q000201:enzymology*; D005630:Fructans / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism; D006602:Hexosyltransferases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008970:Molecular Weight; D009844:Oligosaccharides / Q000737:chemistry / Q000378:metabolism; D013395:Sucrose / Q000737:chemistry / Q000378:metabolism
chemical_list: D005630:Fructans; D009844:Oligosaccharides; D013395:Sucrose; C072599:levan; D006602:Hexosyltransferases; C021513:levansucrase
keywords: enzyme concentration; levansucrase; non-processivity; processivity; substrate concentration