3-37: The structural role of O-linked glycosylation on the linker peptide of the Trichoderma reesei family 7 cellobiohydrolase

Despite the significant importance of protein glycosylation, its effects on the structure, function, and stability of proteins is poorly understood at the molecular-level. Fungi and bacteria, for example, secrete large amounts of glycoprotein cocktails to accomplish cellulose turnover in the biosphere. These types of cellulose-degrading enzymes are typically modular in nature with catalytic domains and carbohydrate-binding modules connected by O-linked glycosylated linker domains rich in serine and threonine. Although multiple molecular biology studies have been conducted to ascertain the function of glycosylation on linkers for intracellular trafficking, secretion, and proteolysis prevention, relatively few biophysical studies have probed the role that the linker and the O-linked glycosylation play in enzymatic catalysis of recalcitrant carbohydrates. Since different expression organisms glycosylate proteins to different extents, understanding the molecular-level structure-function relationship that the glycosylation imparts to linker domains is relevant to understanding the evolution of cellulose-degrading organisms as well as the optimal design of cellulose-degrading enzymes for industrial use. Here, the glycosylated linker domain from a well-characterized fungal enzyme, the Trichoderma reesei Family 7 cellobiohydrolase, is examined with advanced molecular simulation techniques. The excluded volume and stiffness of the linker arising from glycosylation is quantified. It is shown that the O-linked glycosylation significantly impacts the structure of the linker, and therefore the glycosylation likely plays a significant role in structure and the enzymatic catalysis of cellulose by Cel7A.