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Enzymes in motion Substrate activation of pyruvate decarboxylases

Most yeast pyruvate decarboxylases (PDC) are allosterically activated by their substrate pyruvate. PDC from Kluyveromyces lactis ( Kl PDC) shows an extraordinary activation mechanism, where a thiohemiketal is formed at the regulatory site between the side chain of Cys221 and the carbon atom of the keto carbonyl group of the not convertible substrate surrogate methyl acetylphosphonate (MAP). This causes the fixation of two loop regions, comprising residues 286-304 and 104-115 (including the histidines 114 and 115, being important for the catalysis), respectively. The crystal structure of MAP activated Kl PDC provides a deeper look into the substrate activation mechanism [1]. That of native Kl PDC [2] possesses a half side closed tetramer conformation. MAP activation of Kl PDC gives a complete open tetramer caused by mutual dimer rotation (Fig. 1). Pyruvate activation of two catalytically inactive variants of Sc PDC results in crystal structures almost identical to that of MAP- Kl PDC (unpublished data). However, activation of Sc PDC wild type by pyruvamide (PA), another substrate surrogate, entails a half side closed tetramer conformation after crystallisation [3]. SAXS studies have been performed to analyse the influence of substrate and surrogates on the solution structure of PDC. It is known from previous studies that the radius of gyration (R G ) is increased in the presence of the surrogate PA [4], whereas the extrapolated scattering intensity I(0) remains constant. The authors concluded from these results that enzyme activation is accompanied by significant conformation changes of the catalytically active enzyme state without changing its oligomeric