ISSN:
0887-3585
Keywords:
fatty acid synthetase
;
overall activity
;
viscogens
;
diffusion limited intramolecular motion
;
rate-determining step
;
microviscosity
;
Chemistry
;
Biochemistry and Biotechnology
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Medicine
Notes:
The overall activity of animal fatty acid synthetase at the saturation level of substrate concentration decreased when the solvent viscosity, η, of the reaction mixture was increased with viscogens such as glycerol, sucrose, and polyethylene glycol. The activity of the enzyme changed roughly proportional to η-p, where p = 1.0 for glycerol, p = 0.66 for sucrose, and p 〈 0.6 for polyethylene glycol with different molecular sizes. The thioesterase activity, which catalyzes the final partial reaction in the multifunctional enzyme, was not affected by 5-fold increase of solvent viscosity with sucrose. These results suggested that the rate-determining step of the enzyme other than the thioesterase reaction involves a microscopic transport step, the rate of which is influenced by the solvent viscosity. The microscopic transport step may be related to the transfer of the reaction intermediate from one active site to another or to the motion of a larger part of the enzyme requisite for the catalytic reaction. In the solution containing glycerol, the rate-determining motion was primarily diffusion limited since the inverse of the initial rate was proportional to η, i.e., p = 1. Since the substrate concentration was at a saturation level in this experiment, the viscosity-dependent step cannot be the encounter between the enzyme and substrates, but must be intramolecular in origin, most probably the reaction catalyzed by β-ketoacyl synthetase. In solutions containing other viscogens, however, p was less than 1.0, indicating a significant involvement of chemical steps in the rate-determining step as well. Bovine serum albumin, when used as a proteinic viscogen, also decreased the initial rate. In the living cells therefore, the rate-determining intramolecular motion of fatty acid synthetase could be affected by the presence of high concentrations of cytoplasmic proteins.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/prot.340080310
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