Cooperativity in highly aggregated enzyme systems. A slow transition model for the pyruvate dehydrogenase complex from Escherichia coli.

H Bisswanger

doi:10.1016/s0021-9258(17)43375-8

Abstract

Three models are compared describing cooperative phenomena in enzymatic reactions in order to explain sigmoidal saturation curves found with the pyruvate dehydrogenase complex from Escherichia coli: the concerted model, the sequential model, and the slow transition model. Both the concerted and the sequential model were considered especially with regard to the increasing number of identical interaction subunits (protomers) in order to get close to the situation found with the pyruvate dehydrogenase complex which consists of 24 protomers. Applying the sequential model to a great number of protomers results in a weak increase of the Hill coefficient, while, in addition to this effect, the concerted model drastically shifts the sigmoidal range of the saturation function to very low ligand concentrations. Such shift is seen with saturation curves of pyruvate and thiamine disphosphate with the pyruvate dehydrogenase complex and a good fit with theoretical curves derived from the concerted model is obtained. However, subcomplexes with a reduced number of protomers exhibited no change in saturation behavior, thus providing evidence against concerted conformational changes of all subunits of the enzyme complex. A scheme for the initial reaction of the pyruvate dehydrogenase complex based on slow transitions is presented and a rate equation has been derived. Ordered binding of thiamine diphosphate and pyruvate and a ligand-induced slow transition between a less active and a fully active enzyme form has been assumed. The curves simulated with this model are in agreement with all essential kinetic data, which are observed with the pyruvate dehydrogenase complex: the atypical shape of the saturation curves of pyruvate and thiamine diphosphate, the respective Hill coefficients and Michaelis constants, the hyperbolic binding behavior of thiamine diphosphate, and the inhibition pattern found for acetyl coenzyme A.

Highlights

In the native enzyme complex, each component is present increasingnumber of identicalinteractionsubunits at high multiplicity
The exact composition is not definitely in order to gcleotse to the situationfound with the pyruvate dehydrogenasecomplex which consists of 24 protomers
Models for cooperative behaviorin catalysiswere originally derived to explain the sigmoidal saturation curves observed with protein complexes composed of severalidentical subtion curves of pyruvate and thiamine disphosphate units, such as the bindingof oxygen to hemoglobin

Summary

Cooperativity in Highly Aggregated Enzyme Systems

Hans Bisswanger From thePhysiologisch-chemischesInstitut der Uniuersitat Tubingen, 0-7400 Tubingen, Germany. The curves simulated with this model are in agreement with all essential kinetic data, which are observed with the pyruvate dehydrogenase complex: the atypical shapoef the saturation curveosf pyruvate and thiamine diphosphate, the respective Hill coeffiactive state of the whole enzyme molecule in which the ligand binds preferentially to theactive form. In bothof these models, the transitionbetween the less and more active enzymeforms isviewed as a rapid pre-equilibrium totheactual catalysis.A thirdalternativeto explain the sigmoidal appearance of saturation curves is that aligand inducesa conformationalchangeto amoreactiveenzyme cients andMichaelis constants, the hyperbolic bindingform which is slow with respect to the catalytic steps.

Cooperativity of the Pyruvate Dehydrogenase Complex

RESULTS

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Interpretationsconcerningthemechanism of the acetyl

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