Catalytic Reaction Mechanism of Drosophila ADH, a Short-Chain Dehydrogenase

2 Catalytic Reaction Mechanism of Drosophila ADH, a Short-Chain Dehydrogenase 

When trying to determine the 3D structure of binary and ternary complexes of Drosophila alcohol dehydrogenase (DADH), researchers initial attempts to soak apo-form crystals with the oxidized coenzyme (NAD+) failed. The crystals cracked after several hours and became unusable. This suggested that the coenzyme, upon binding to the enzyme, induced a conformational change that seriously affected the crystal packing. The same phenomenon prevented solution of the HL-ADH 3D structure for many years. 

Possible roles of the catalytic triad Serl38, Tyrl 51, Lysl55 

The Theorell-Chance mechanism describes the predominating pathway for the conversion of a wide range of primary and also some secondary alcohols by HL-ADH. The same kind of mechanism is also valid for the reaction of DADH with secondary alcohols. As mentioned in the previous section, ternary complexes are kinetically insignificant for the compulsory ordered Theorell-Chance mechanism. 

Upon conversion from alcohol to ketone/aldehyde the active site contains a reduced NADH molecule, which, in conjunction with the hydrophobic nature of the active-site cavity, is believed to raise the pKa of Tyrl51 to above 10. Evidence is provided by studies demonstrating pH-independent binding of acetaldehyde to the E-NADH complex so that the pKa value for the amino acid responsible for interaction with acetaldehyde is higher than 10. 

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