CDH

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Introductions

Cellobiose dehydrogenase (CDH) is an extracellular hemoflavoenzyme secreted by fungi to assist lignocellulolytic enzymes in biomass degradation. Its catalytic flavodehydrogenase (DH) domain is a member of the glucose-methanol-choline oxidoreductase family similar to glucose oxidase. The catalytic domain is linked to an N-terminal electron transferring cytochrome (CYT) domain which interacts with lyticpolysaccharide monooxygenase (LPMO) in oxidative cellulose and hemicellulose depolymerization.

Classifications

Based on CDH sequence analysis, four phylogenetic classes were defined. CDHs in these classes exhibit different structural and catalytic properties in regard to cellulose binding, substrate specificity, and the pH optima of their catalytic reaction or the interdomain electron transfer between the DH and CYT domain. The structure, reaction mechanism and kinetics of CDHs from Class-I and Class-II have been characterized in detail and recombinant expression allows the application in many areas, such as biosensors, biofuel cells biomass hydrolysis, biosynthetic processes, and the antimicrobial functionalization of surfaces.

Biological function

The two-domain structure of CDH, consisting of a flavin-containing dehydrogenase domain (DH) and a heme b containing cytochrome (CYT) linked by a protease-sensitive linker caused speculations about the natural function of the enzyme and its role in lignocellulose depolymerization. The ability of CDH to reduce iron into the ferrous form and its low oxidase activity producing hydrogen peroxide was the basis of the most supported physiological function which suggested that CDH is involved in cellulose degradation by providing the substrates for the Fenton reaction to produce hydroxyl radicals. The formation of reactive hydroxyl radicals was shown to be capable of the degradation of cellulose and is believed to be a key mechanism in the degradation of biomass by brown-rot fungi. However, for all of those proposed mechanisms the activity of the isolated DH domain would be sufficient and the necessity for the presence of the CYT domain is not explained.

Bioelectrochemical applications

The application of enzymes as catalysts in various bioelectrochemical devices, such as biosensors and biofuel cells, has promoted research focused on CDH because of the unique electrocatalytic properties of this class of proteins. The electron transfer between catalytically active enzymes and electrodes can be achieved by using an electrochemically active compound generated by enzyme turnover, which further reacts with the electrode and is referred to as a first-generation biosensor structure. In second generation biosensors, the transfer of electrons is achieved by integrating a redox mediator (MET), either using freely diffusing redox-active species or (usually osmium-modified) polymers. In the third-generation setup, direct electron transfer (DET) is achieved by direct transport of electrons from the redox center of the enzyme to the electrode without intermediates.

CDH as a bioelectrocatalyst

CDH exhibits various properties that make it a suitable electrocatalyst for biosensors or biofuel cells. Unlike other GMC-oxidoreductases for carbohydrate conversion, it can be contacted by mediated electron transfer (MET) or direct electron transfer (DET). CDH-based second- and third-generation biosensors have been developed to quantify commercially or medically relevant molecules, such as glucose or lactose. The advantage of second-generation biosensors is their high sensitivity and the advantage of third generation biosensors is the avoidance of redox mediators in implantable glucose monitoring systems. CDH-based third-generation biosensors offer robust performance because CDH and the mobile CYT structural domain have good thermal and turnover stability, can interact with many electrode materials and surface modifications and provide reasonably high current densities.