Conformational transformations of human muscle FBPase

Fructose-1,6-bisphosphatase (FBPase) catalyzes the hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate and is a key enzyme of gluco- and glyconeogenesis and, more generally, of the control of energy metabolism and glucose homeostasis. Vertebrates express two FBPase isoforms. The liver isozyme is expressed in gluconeogenic organs where it functions as a regulator of glucose synthesis. The muscle isoform is expressed in all cells, and its role goes far beyond the enzymatic function, as it can interact with various nuclear and mitochondrial proteins. Even in its enzymatic function, the muscle enzyme is different from the liver isoform, as it is 100-fold more susceptible to allosteric inhibition by AMP and as this effect can be abrogated by complex formation with aldolase. All FBPases are homotetramers composed of two intimate dimers, the upper dimer (C1.C2) and the lower one (C3.C4). They oscillate between two conformational states, the inactive T form when in complex with AMP, and the active R form. The T-to-R transition is correlated with the conformation of a key loop L2, which in the T form is disengaged and unable to participate in the catalytic mechanism. The T states of both isoforms are very similar, with a small clockwise twist of the upper dimer relative to the lower one. Kuba Barciszewski was able to show that at variance with the well studied R form of the liver enzyme, which is flat, the R form of the muscle enzyme is diametrically different, with a perpendicular orientation of the upper and lower dimers. Upon dissociation of the AMP inhibitor (space-filling models), the upper dimer rotates counterclockwise by nearly -110 degrees to reach the R state. In the T state, loop L2 gets locked in the disengaged position by Asp187 within the intimate dimer (e.g. C1.C2, Upon rotation to the cruciform R state, a novel 'leucine lock' is formed, which encages pairs of hydrogen-bonded residues Asp187 across the tetramer (e.g. C1-C3, green-yellow). As the tetramer rotates back from the R to the T state, a profound beta-to-alpha rearrangement of the N-terminus of all subunits takes place, leading to the formation of the binding sites for the AMP inhibitor.

Kuba's results were published in Acta Crystallographica (Acta Cryst. D72, 536-550, 2016).

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Last update: January 22, 2017 (MJ)