The Biosynthesis of Cellulose

Abstract

Cellulose is one of the major commercial products of Sweden and constitutes the most abundant of the natural polymer systems. Thus, it is of interest to review the molecular design and architecture of cellulose with particular reference to the controls of its biosynthesis. The bioassembly process is highly ordered and structured, reflecting the intricate series of events which must occur to generate a thermodynamically metastable crystalline submicroscopic, ribbonlike structure. The plant cell wall is an extremely complex composite of many different polymers. Cellulose is the “reinforcing rod” component of the wall. True architectural design demands a polymer which can withstand great flexing and torsional strain. Using comparative Hydrophobic Cluster Analysis of a bacterial cellulose synthase and other glycosyl transferases, the multidomain architecture of glycosyl transferases has been analyzed. All polymerization reactions which are processive require at least three catalytic sites located on two different domains. In contrast, retaining reactions with glycosyl transferases require only a single domain and two sites. Cellulose synthase appears to have evolved a mechanism to simultaneously bind at least three UDP-glucoses and to polymerize, by double addition, two UDP-glucoses in such a manner that the 2-fold screw axis of the β-1,4-glucan chain is maintained. Thus, no primer is required as the glucose monomers are added two-by-two to the growing chain. At the next higher level of assembly, the catalytic sites simultaneously polymerize parallel glucan chain polymers in close proximity so that they will favorably associate to crystallize into the metastable cellulose I allomorph. Recent energy analysis suggests that the first stage of this association is the formation of a minisheet through van der Waals forces, followed by layering of these minisheets to form the crystalline microfibril. In native cellulose biogenesis, the microfibril shape and size appear to be determined by a multimeric enzyme complex (TC) which resides in the plasma membrane. This complex, known as a terminal complex, was discovered through electron microscopy of freeze fracture replicas. The entire complex moves in the plane of the fluid plasma membrane as the result of polymerization/crystallization reactions. The assembly stages for native cellulose I are coordinated on a spatial/temporal scale, and they are under the genetic control of the organism. This might lead one to conclude that cellulose I could only be assembled with Nature's indigenous machinery; however, this is not the case. Recently, in collaboration with Professor Kobayashi and his colleagues in Sendai and Tokyo, we have synthesized cellulose I abiotically under conditions very different from those in the living cell or from isolated cell components. Purification of an endoglucanase from Trichoderma which serves as the catalyst and the addition of β-cellobiosyl fluoride as the substrate in acetonitrile/acetate buffer has led to the assembly of synthetic cellulose I. Although natural and synthetic assembly pathways are very different, there are similar, underlying fundamental mechanisms common to both. These mechanisms will be discussed in relation to the more thermodynamically stable allomorph of cellulose (cellulose II) first demonstrated by Professor Rånby in 1952. The evolution of cellulose biosynthesis will be summarized in terms of the demands for maintaining optimal cellular environments to generate the complex macromolecular assemblies for cell wall biogenesis. Nature provides an exceptional model for cellulose biosynthesis that will lead us toward the biotechnological production of improved natural cellulose as well as synthetic cellulose and its derivatives.