Team:WLC-Milwaukee/Cellulases

Our team selected three cellulases to begin the breakdown of cellulose: yesZ, bglS, and xynA.

Enzyme xynA

The enzyme xynA is a endo-1,4-beta xylanase or is referred to as beta xylanase. It catalyzes the hydrolysis of the xylan main chain in hemicellulose. Xylan is abundant in the cell wall structures of many plants. Lindner [1] investigated the regulation of xylanase within Bacillus subtilis. It was found that Bacillus subtitis had a slow rate of growth on xylan plates and did not grow on xylose plates. It was also found that the synthesis of xynA is not dependent on the environment, but instead was found to be highly synthesized during the exponential growth phase. {{This is a potential advantage to our project because the probiotic may not have time to reach the stationary phase in the ruminant.}} The presence of glucose also showed to have no effect on the presence of the mRNA sequence for xynA. It is now known that xynA has “glucose resistant synthesis”. This differs from most extracellular catabolic enzymes. Lindner also found that xynA is synthesized constitutively; meaning it is produced in constant amounts regardless of the surrounding environment of the bacterial cell.

Enzyme bglS

The enzyme bglS hydrolyzes linked beta-D-glucans, which are commonly found in lichen. Beta-D-glucans are 5.5% of dry weight in grains, and 75% of carbohydrates in barley endospores. [2] They showed bglS to have an optimum pH of 6.0 and temperature of 50 degrees Celsius (active from 20 degrees Celcius to 70 degrees Celcius). BglS cleaves the beta-1,4 glycosidic linkage that is adjacent to the 3-0 substituted glucopyranose residues. This then releases trisaccharides or tetrasaccharides. [2] The active site for bglS is shown to have a "EIDIEF" motif. The two glutamic acid residues participate in acid and base nucleophile hydrolysis.

Structure of bglS

"The active site cleft of the enzyme presents a negatively charged crevice surrounded by a number of aromatic residues (Fig. 2B). A single molecule of bis–tris-propane was found in the active- site cleft, forming hydrogen bonds with the nucleophile Glu133 (2.62A ̊),theacidcatalystGlu137(2.56A ̊),Tyr151(2.83A ̊)andwater mediated hydrogen bonds with Asn210, Asn56, Asn149, Gln147, Glu159 (Fig. 2C). On the opposite face of the active site, a single cal- cium ion (Fig. 2A) was coordinated by backbone carbonyl oxygen atoms from Pro37, Gly73, Asp235, a carboxylate oxygen of Asp235 and two water molecules, as previously observed in the B. licheni- formis homologue [18]. These amino-acids are located in the 􏰑1–􏰑2 loop, in the 􏰑3–􏰑4 loop and in the 􏰑15 strand respectively, show- ing that the stabilizing effect of the calcium stabilization is due to cross-linking of these regions." [2]

Enzyme yesZ

The enzyme yesZ is homologous to a beta-galactosidase. Beta-galactosidases are found in four distinct CAZY (carbohydrate-active enzymes): GH1, GH2, GH35 and GH42. YesZ falls under GH42 and as it is in the clan GHA, it is is thought to have a two step, double displacement reaction. This forms a covalent glycosyl enzyme intermediate with is then hydrolyzed via a oxocarbenium like transition state. [3] The active site contains two key carboxylic type residues that are 5.5 Angstroms apart. This catalytic nucleophile attacks the sugar anomeric centre. This then covalently attaches to the galactosyl moiety as well as the acid/base residue that protonates a departing aglycone oxygen. The identification of the carboxylic acids that takes place in this is key to proposing this mechanism. Shaikh was able to identify this as Glutamate-295. Glutamate-145 was also determined as a possible acid/base catalyst.

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Written by: Sierra Tackett