Biomedical subjects

Anja E M Janssen

Publications and source records attributed to Anja E M Janssen.

3 recordsLinked to original sources

PubMed2006

A kinetic model to explain the maximum in alpha-amylase activity measurements in the presence of small carbohydrates.

The effect of the presence of several small carbohydrates on the measurement of the alpha-amylase activity was determined over a broad concentration range. At low carbohydrate concentrations, a distinct maximum in the alpha-amylase activity versus concentration curves was observed in several cases. At higher concentrations, all carbohydrates show a decreasing alpha-amylase activity at increasing carbohydrate concentrations. A general kinetic model has been developed that can be used to describe and explain these phenomena. This model is based on the formation of a carbohydrate-enzyme complex that remains active. It is assumed that this complex is formed when a carbohydrate binds to alpha-amylase without blocking the catalytic site and its surrounding subsites. Furthermore, the kinetic model incorporates substrate inhibition and substrate competition. Depending on the carbohydrate type and concentration, the measured alpha-amylase activity can be 75% lower than the actual alpha-amylase activity. The model that has been developed can be used to correct for these effects in order to obtain the actual amount of active enzyme.

Binding Sites↗

PubMed2002

Solvent selection for solid-to-solid synthesis.

Thermolysin catalyzed solid-to-solid synthesis of the model peptide Z-L-Phe-L-Leu-NH(2) is practically feasible in water and a range of organic solvents with different physicochemical properties. Excellent overall conversions were obtained in acetonitrile, ethyl acetate, n-hexane, methanol, 2-propanol, tert-amyl alcohol, tetrahydrofuran, toluene and water, while no product precipitation was observed in dichloromethane resulting in a much lower yield. In precipitation driven synthesis the product accumulates both in solution and in the solid phase. It was shown that the highest overall yields (yield in the liquid plus yield in the solid) can be expected in solvents where the substrate solubilities are minimized. The best yields of solid product can be expected in solvents where both product and substrate solubilities are lowest. This was in agreement with experimental observations and should be generally valid.

Alcohols↗

PubMed2002

Reversible acetonitrile-induced inactivation/activation of thermolysin.

Thermolysin is catalytically inactive in mixtures of 10-15 % acetonitrile in aqueous buffer. Unexpectedly, dilution of the inactive enzyme with acetonitrile leads to complete recovery of the catalytic activity in a similar way to dilution with aqueous buffer. Circular dichroism and fluorescence studies of thermolysin in the same solvent mixtures reveal discontinuous changes in the overall secondary and tertiary protein structure that correlate well with the reversible differences in catalytic activity. The spectra on either side of the minimum activity point are different from each other, a fact indicating that the enzyme may be able to access two active conformations which are thermodynamically stable in different solvent environments.

Acetonitriles↗