Multifunctional enzyme systems

By 1960 it was clear that acetyl CoA provided its two carbon atoms to the to and co—1 positions of palmitate. All the other carbon atoms entered via malonyl CoA (Wakil and Ganguly, 1959 Brady et al. 1960). It was also known that 3H-NADPH donated tritium to palmitate. It had been shown too that fatty acid synthesis was very susceptible to inhibition by p-hydroxy mercuribenzoate, TV-ethyl maleimide, and other thiol reagents. If the system was pre-incubated with acetyl CoA, considerable protection was afforded against the mercuribenzoate. In 1961 Lynen and Tada suggested tightly bound acyl-S-enzyme complexes were intermediates in fatty acid synthesis in the yeast system. The malonyl-S-enzyme complex condensed with acyl CoA and the B-keto-product reduced by NADPH, dehydrated, and reduced again to yield the (acyl+2C)-S-enzyme complex. Lynen and Tada thought the reactions were catalyzed by a multifunctional enzyme system. 

One of the most exciting areas in the field of multifunctional enzyme systems is the synthesis of a wide array of organic molecules by polyketide and nonribosomal protein synthetases. These enzymes are generally characterized by multiple subunits which themselves consist of individual domains with distinct enzymatic activities (Fig. 9.11a). The range of natural products synthesized by these mega-synthetases includes a considerable number of important antibiotics, antifungals, antitumor and cholesterol-lowering compounds, immunosuppressants, and siderophores. 

The finding of multifunctional enzyme systems for biosynthetic pathways, such as fatty acid synthesis, suggests that such organized systems may have importance elsewhere. Whether they exist in photosynthesis and how they operate if they do exist is a very important question for the future. The answer will come from a combination of tracer studies, investigation of the enzymes isolated by a variety of sophisticated techniques, and the gathering of better and more detailed knowledge of the structure of the chloroplast through the application of electron microscopy and various techniques of chemical and physical analysis. 

The Fatty Acid Synthase Enzyme Systems The FAS enzyme systems are central for the biosynthesis of saturated fatty acids [1] and share many similarities with the polyketide synthases (PKSs) involved in the biosynthesis of many polyketides [6] see Section 2.1 for details. Common precursors, functional gronps, and reactions are involved in each biosynthetic step. Saturated fatty acids, such as palmitic acid (1, Scheme 3.2), represent a rather small and structurally simple class of natural products produced by the FAS enzyme systems. The FAS enzyme system in animals is a large multifunctional protein with seven individnal... 

The failure in increasing residence time of mucoadhesive systems in the human intestinal tract has led scientists to the evaluation of multifunctional mucoadhesive polymers. Research in the area of mucoadhesive drug delivery systems has shed light on other properties of some of the mucoadhesive polymers. One important class of mucoadhesive polymers, poly(acrylic acid) derivatives, has been identified as potent inhibitors of proteolytic enzymes [72-74]. The interaction between various types of mucoadhesive polymers and epithelial cells has a direct influence on the permeability of mucosal epithelia by means of changing the gating properties of the tight jrmctions. More than being only adhesives, some mucoadhesive polymers can therefore be considered as a novel class of multifunctional macromolecules with a number of desirable properties for their use as delivery adjuvants [72,75]. 

This enzyme [EC 2.4.2.18], also referred to as phospho-ribosyl-anthranilate pyrophosphorylase, catalyzes the reaction of anthranilate with phosphoribosylpyrophos-phate to produce A-5 -phosphoribosylanthranilate and pyrophosphate. In certain species, this enzyme is part of a multifunctional protein, together with one or more other components of the system for the biosynthesis of tryptophan (i.e., indole-3-glycerol-phosphate synthase, anthranilate synthase, tryptophan synthase, and phos-phoribosylanthranilate isomerase). 

Also, it should be pointed out in this connection that the reaction mechanism (see Fig. 4 and Section III,D,5,a) postulated on the basis of kinetic observations may be an oversimplification of the actual situation. For example, the possibility exists that separate phosphoryl-enzyme intermediates may be formed with various phosphate substrates (10). If such intermediates were interconvertible with a single phosphoryl-enzyme complex which was common to reactions involving all such substrates, and if the rates of these interconversions were very rapid relative to the other steps in the catalytic process, the system would be kinetically indistinguishable from that depicted in Fig. 4. Further work relating to the mechanism of reaction, and the experimental assessment of the possible involvement of this multifunctional catalyst in intra- and extracellular glucose transport, would appear to be in order. 

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