Enzyme processive action

Ribonuclease II [EC 3.1.13.1], also called exoribo-nuclease II, catalyzes the exonucleolytic cleavage of the polynucleic acid, preferring single-stranded RNA, in the 3 - to 5 -direction to yield 5 -phosphomononucleotides. The enzyme processes 3 -terminal extra-nucleotides of monomeric tRNA precursors, following the action of ribonuclease P. Similar enzymes include RNase Q, RNase BN, RNase PHI, and RNase Y. Ribonuclease T2 [EC 3.1.27.1] is also known as ribonuclease II. 

Structural features met in some cellulases include an a,a barrel111 similar to that of glucoamylase (Fig. 2-29) and, in a cellobiohydrolase,101 a 5-nm-long tunnel into which the cellulose chains must enter. Ten well-defined subsites for glycosyl units are present in the tunnel.101 A feature associated with this tunnel is processive action, movement of the enzyme along the chain without dissociation,105 a phenomenon observed long ago for amylases (see Section 9) and often observed for enzymes acting on nucleic acids. 

The mechanism of any process can be discussed at several levels of sophistication. This is especially pertinent to the definition of the mechanism of action of enzymes and, in particular, those that require several components to accomplish the catalysis, such as the nitrogenases. In such systems, we can define a mechanism merely as that order in which the various components associate and dissociate. However, this in no way addresses the problems of the underlying chemistry of the enzyme s action. In the following discussion of the mechanism of action... 

During the last two decades, the mechanisms of many enzymic processes have been established, and model systems have been developed that effectively mimic their action. In particular, the roles of thiamin, NAD, pyridoxal, flavins, Bl2, ferridoxin, and metals in many enzymic processes now are understood. Model systems have been developed to imitate the action of decarboxylases and esterases, to imitate the action of enzymes in binding their substrates, and to approach the stereospecificity of enzymes. Our laboratory recently has found phosphorylating agents that release monomeric methyl metaphosphate, which in turn carries out phosphorylation reactions, including some at carbonyl oxygen atoms, that suggest the actions of ATP. The ideas of biomimetic chemistry are illustrated briefly in terms of the processes mentioned above. 

Of course, the mechanisms of these enzymic processes are at present unknown. G. Lowe and B. S. Sproat (78) have suggested that the action of pyruvate kinase involves monomeric metaphosphate as an intermediate. His evidence concerns the effect of the enzyme in scrambling the oxygen atoms of ATP in the absence of pyruvate, Equation 15. This scrambling process is enzyme catalyzed, and is interpreted most easily, but not exclusively, in terms of the reversible formation of monomeric metaphosphate. 

Topoisomerase reactions have been shown to proceed via both distributive and processive modes (Wang and Liu, 1979). Distributive action involves the dissociation of the enzyme from the DNA after each catalytic cycle whereas processive action requires several catalytic cycles to occur before the enzyme dissociates. In the relaxation of supercoiled DNA, the appearance of fully relaxed products, while some supercoiled substrate still remains, indicates a processive mode. Alternatively, conversion of all supercoiled substrate to partially relaxed intermediates before the appearance of fully relaxed products indicates a distributive mode. 

D-Glucose can also be obtained by enzymic conversion of starch or cellulose. Two separate enzymic processes, liquefaction and saccharification, are generally used in the production of D-glucose from starch. The liquefaction process solubilizes the molecules in the starch granules and decreases the viscosity of the starch. In the saccharification step, the liquefied starch is further hydrolyzed to D-glucose by the action of glucoamylase. The majority of starches used for the manufacture of D-glucose contain up to 80% amylopectin 99... 

Bruggink A (2000), Green solutions for chemical challenges biocatalysis in the synthesis of semi-synthetic antibiotics. In Zwanenburg B, Mikolajczyk M, Kielbasinsky P (eds) Enzymes in action. NATO sciences series 1/33, Kluwer Academic, pp 449-458 Bruggink A, Roos EC, de Vroom E (1998) Penicillin acylase in the industrial production of lactam antibiotics.Org Process Res Dev2 128-133... 

---