Competitive inhibition in an allosteric enzyme

Enzyme: E. coli's Glucosamine 6-P deaminase

The conformational state of the protein and the underlying physicochemical events are depicted in the lower right corner (T=T conformer, R=R conformer, catálisis=catalysis, transición=transition).

The substrate is shown in green spheres* (GlcN6P), and it binds with low affinity to the T conformer (Kt=14 mM). A competitive inhibitor shown in red spheres also binds with low affinity to that conformer.

The enzyme can undergo a concerted conformational transition to a conformer (R conformer) with high affinity for both the substrate and the inhibitor (Kr=0.6 mM, Kir=0.03 mM).

The energetic contribution of the binding of one molecule of inhibitor is greater than that of one molecule of substrate, but if there are few molecules of inhibitor, remaining subunits are available for substrate binding. All the binding energetic contributions confer a high stability to the R conformer.

Catalysis occurs in each substrate-bound subunit, yielding a total of 5 molecules of product (Fru6P), depicted in purple spheres. The inhibitor is not modified.

In a system in equilibrium where trillions of molecules are present, at null or low concentrations of substrate, unbound and minimally bound T conformers are energetically favored. If low concentrations of inhibitor are added to the system, a subtle activation of catalytic activity is observed, a consequence of the events depicted in the video. At great concentrations of inhibitor, unproductive complexes are favored thus yielding classic competitive inhibition.

The order of events and the dynamic nature of the allosteric transition are computer generated animations based on the extreme states of the system and equilibrium experiments: the exact sequence of events and the allosteric transition is currently under research.

* As enzyme substrates cannot usually be used for X-ray diffraction, the authors of the papers used a non productive substrate analogue, GlcN-ol-6P. Colors were used to distinguish substrate and inhibitor in this animation.
PDB ID: 1FSF, 1FQ0, (2002) Acta Crystallogr.,Sect.D 58: 10-20 1HOR (1995) Structure 3: 1323-1332

Video generated in PyMOL at the LFQIP, UNAM by Diego Alonzo B.Sc. under supervision of Dr. Mario Calcagno, based on the crystallographic data above.

Category:

People & Blogs

Tags:

• competitive inhibition
• allosteric enzyme
• allostery
• paradoxical activation
• monod wyman changeux
• mwc