Exploring the Intricacies and Functionalities of Galactose Oxidase: Structural Nuances, Catalytic Behaviors, and Prospects in Bio-electrocatalysis

Abstract

Galactose Oxidase, also known as GOase, is an enzyme found mostly in Fusarium graminearum, Dactylium dendroides, and Gibberella fujikuroi. GOase, containing copper, serves catalytic functions in oxidizing substrates and primary alcohols such as d-galactose, benzyl alcohol derivatives, and dihydroxyacetone. The catalytic property of galactose oxidase that differentiates it from other enzymes is its cofactor consisting of a Cu (II)-bound Cys-Tyr* radical. The cofactor is vital for enabling regioselective oxidation. The application of galactose oxidase (GOase) covers several fields such as enzymatic synthesis, biosensors development, and processes of diagnosis. Galactose oxidase (GOase) was discovered to have a crystallographic structure by X-ray diffraction, which revealed an active site containing copper ions displaying relatively square pyramidal geometry. There are three unique structural and functional domains of the enzyme GOase. The domains include Tyr495 and a covalent bond between Cys228 and Tyr272 serving as equatorial and axial ligands, respectively. The mechanism of catalysis covers three different oxidation states, which include the active state containing Cu (II)-radical, the intermediate state containing Cu (II)-tyrosine, and the Cu(I)-tyrosine state. The cycle of catalysis that has been posited comprises several phases which are: (i) substrate binding, (ii) the transfer of proton (iii) the transfer of hydrogen atom, and (iv) subsequent oxidation steps. These phases eventually yield the synthesis of aldehyde and hydrogen peroxide. Galactose oxidase’s (GOase) mechanism of catalysis has been studied thoroughly via extensive research focusing on explaining the ping-pong mechanism that occurs in both oxidative and reductive half-reactions. The processes of activating and reactivating galactose (GOase) involve the transfer of electrons in which horseradish peroxidase (HRP) serves as an activator. The electrochemical investigations provide evidence of the electrochemical activation and reactivation of GOase in the presence of mediators. This comprehensive review enhances the comprehension of the structural complexities, catalytic mechanisms, and bio-electrocatalytic potential of GOase, thereby establishing a basis for future investigations and developments in technology.