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Structures
Published in Thomas M. Nordlund, Peter M. Hoffmann, Quantitative Understanding of Biosystems, 2019
Thomas M. Nordlund, Peter M. Hoffmann
Common forms of heme, shown in Figure 5.8, are heme a and heme b, named for the cytochrome enzymes, cytochrome a and cytochrome b, in which they are found. The overall porphyrin ring is made up of four 5-carbon subrings, numbered I through IV. Figure 5.8c, a space-filling version of the structure, reminds us that these structures do not have empty spaces in them. The cytochromes catalyze electron transfer reactions. Heme b, also called iron protoporphyrin IX, is also present in hemoglobin and myoglobin, both of which bind oxygen. Porphyrin refers to the basic ring structure of heme as well as chlorophyll.
Purified cellobiose dehydrogenase of Termitomyces sp. OE147 fuels cellulose degradation resulting in the release of reducing sugars
Published in Preparative Biochemistry & Biotechnology, 2021
Rishabh Gangwar, Shafaq Rasool, Saroj Mishra
CDHs (EC1.1.99.18; CAZy: AA3.1) belong to the glucose-methanol-choline (GMC) family of the oxidoreductases and are produced by fungi under cellulolytic culture conditions.[4–6] These are monomeric enzymes and consist of two domains, namely, an N-terminal heme b-containing cytochrome domain (CYT), and, a terminal flavin domain containing flavin adenine dinucleotide (FAD). The two domains are linked by a hydroxyl-rich amino acid linker and are in close contact with each other to allow internal electron exchange between the flavin and the heme[6–9] containing domains. CDH has been proposed to fuel the LPMO’s with electrons during the process of cellulose degradation.[6,10–12] Based on a comparison of 677 cdh sequences, these have been proposed to evolve in to four phylogenetic branches.[13] The Class-I CDHs are short in sequence, lack a carbohydrate-binding domain at the C-terminus (but have a cellulose binding site), have strong affinity for cellulose and comprise enzymes from basidiomycetes while the Class II, Class III and Class IV enzymes are found only in ascomycetes. Some Class II enzymes bind to cellulose through a C-terminal family 1 carbohydrate-binding module (CBM), the Class IIA, or, may not possess such CBM, the Class IIB. The Class I and Class II also differ in terms of their pH optimum and substrate specificity. Very little is known about the other two classes. The details of sequence identity within each Class have been reported earlier.[13] In the presence of a suitable electron acceptor, CDHs can catalyze the oxidation of cellobiose and cello-oligosaccharides to their corresponding 1,5-lactones, and subsequently to the carboxylic acids,[14,15] thus preventing the end-product inhibition of cellulose degradation. While cellulose is the most preferred substrate,[16] these are also active on lactose and glucose. CDHs have also been proposed to be involved in the early stages of cellulose and lignin breakdown through Fenton chemistry.[5,16–19]