Comparative study of invertases of Streptococcus mutans

Area of Science:

• Microbiology
• Biochemistry
• Molecular Biology

Background:

• Streptococcus mutans is a primary agent of dental caries.
• Sucrase activity, particularly invertase, plays a key role in sucrose metabolism and cariogenicity.
• Understanding variations in sucrase activity across different serotypes is crucial for targeted prevention strategies.

Purpose of the Study:

• To characterize sucrase activity and identify the enzyme responsible in various Streptococcus mutans serotypes.
• To investigate the biochemical and molecular differences in invertase enzymes among different serotypes.
• To evaluate the impact of inorganic orthophosphate on invertase activity across serotypes.

Main Methods:

• Enzyme assays were performed on sucrose-adapted cells from 13 Streptococcus mutans strains representing five Bratthall serotypes.
• Enzyme identification was confirmed through substrate hydrolysis (raffinose and sucrose) and product analysis.
• Molecular weights were determined using molecular exclusion chromatography.
• Electrophoretic mobility was analyzed to assess enzyme differences.
• The effect of inorganic orthophosphate (P(i)) on enzyme kinetics (Vmax and apparent Km) was evaluated.

Main Results:

• Sucrase activity was consistently found in the soluble fraction of all studied strains and identified as invertase (beta-d-fructofuranoside fructohydrolase; EC 3.2.1.26).
• Invertases from serotypes c and e strains showed two activity peaks (45-50 kDa and ~180 kDa), while serotypes a, b, and d exhibited a single peak (45-50 kDa).
• Significant differences in electrophoretic mobility and kinetic responses to inorganic orthophosphate were observed between serotypes, but not within them.
• Melibiase activity was present only in serotypes b and c, suggesting a lack of an intact raffinose permease system in serotype d.

Conclusions:

• The invertase enzymes of Streptococcus mutans exhibit significant variations between serotypes but are consistent within each serotype.
• These enzymatic differences support the established taxonomic schemes of Coykendall and Bratthall.
• The observed variations in invertase and melibiase activity provide insights into the metabolic diversity and potential virulence factors among Streptococcus mutans serotypes.