GDP-D-glucose

Early reports of such an enzyme system, tightly bound and occurring in a particulate fraction of mung bean, were made by Hassid and associates.292,293 The glycosyl donor was reported to be GDP-D-glucose and the product was characterized as a / -(l— 4)-linked D-glucan,292,293 Liu and Hassid294 solubilized, and partially purified, this enzyme. 

Hassid and coworkers293 reported that this mung-bean enzyme-system catalyzes the incorporation of D-glucose from GDP-D-glucose into a polysaccharide having the characteristics of cellulose and that, in the presence of GDP-D-mannose, this incorporation was stimulated. The same enzyme-system, when provided with GDP-D-mannose as the sole substrate, catalyzed the incorporation of GDP-D-mannose into a glucoman-... 

Heller and Villemez299,300 solubilized, but did not separate, the D-mannosyltransferase and D-glucosyltransferase activities from this particulate-enzyme system, and found that, when only GDP-D-mannose is provided as the substrate, a/ -(1— 4)-linked D-mannan is synthesized and, when only GDP-D-glucose is present, ap-(l— 4)-linked D-glucan is produced. They further noted, with this transferase system, that incorporation of D-glucose from GDP-D-glucose into polysaccharide is a shortlived reaction that can be greatly extended if GDP-D-mannose is also included in the reaction mixture. 

However, an enzyme that uses GDP-D-glucose as a substrate in order to synthesize a cellulose-like product has been reported to be present in developing cotton fibers during the period when primary-wall cellulose synthesis normally occurs, but is absent from older fibers undergoing active deposition of secondary-wall cellulose.303... 

It has also been shown that mung beans, peas, and other plants contain a pyrophosphorylase which forms GDP-D-glucose from a-D-glucose 1-P and GTP. Based on the data obtained with enzymic plant preparations, we proposed the following mechanism for cellulose synthesis ... 

The fact that glucomannan is obtained from GDP-D-mannose as substrate suggests that the particulate enzyme contains an epimerase which converts GDP-D-mannose to GDP-D-glucose. However, such an epimerase has not been observed. In this connection cellulose isolated from wood contains a considerable amount of D-mannose. 

Fig. 3-4. Simplified representation of the formation of hemicellulose precursors from UDP-d-glucose or GDP-D-glucose. Note that NDP (nucleotide diphosphate) means either UDP or GDP.

Ray and coworkers found that a particulate preparation from peas could synthesize cellulose from both UDP-D-glucose and GDP-D-glucose. It is noteworthy that the synthetase particles consisted of Golgi fragments and membranes bearing vesicles (see p. 341) this aspect of the synthesis of cellulose will be discussed later (see p. 341). 

It has been shown that soluble, enzyme preparations from mung beans, peas, and other plants show pyrophosphorylase activity, and can form GDP-D-glucose from GTP and a-D-glucosyl phosphate. These enzyme preparations also contain various carbohydrate compounds, including small proportions of cellulose, that could act as acceptor. Therefore, it was postulated that cellulose in higher plants is formed through the following reactions. ... 

Elbein had found that an enzyme system from Streptomyces hygroscopicus and a number of other Streptomyces species catalyzes the transfer of D-glucose- C from GDP-D-glucose- C to D-glucose 6-phosphate, affording a,a-trehalose- C 6-phosphate. The reaction appears to be specific for GDP-D-glucose. 

However, for other actinomycetes, principally Mycobacteria and Nocardia species, crude extracts were found to utilize D-glucose 6-phosphate and either UDP-D-glucose or GDP-D-glucose for the formation of Q ,Q -trehalose 6-phosphate according to the following two reactions. ... 

A cell-free extract of Mycobacterium smegmatis was separated into the two fractions by chromatography on 0-(2-diethylaminoethyl)-cellulose. Fraction A catalyzed synthesis of Q ,a-trehalose from GDP-D-glucose, but was relatively inactive with UDP-D-glucose. However, when Fraction B was added to Fraction A, UDP-D-glucose was able to serve as an effective D-glucopyranosyl donor for synthesis of a,a-trehalose. Under these conditions, GDP-D-glucose was still... 

Examination of the polymers synthesized by particulate enzyme preparations from mung beans Phaseolus aureus) and those of the Lupinus albus variety, by use of GDP-D-glucose and UDP-D-glucose, showed that a (1 4)-/3-D-glucan is formed only from GDP-D-glu-cose, " and a (1 3)-/3-D-glucan when UDP-D-glucose is the substrate. 

On the other hand, when GDP-D-glucose was used as the substrate, both the particulate and the digitonin-solubilized enzyme systems from mung beans and from Lupinus albus yielded only (1 — 4)-/3-D-gIucan (cellulose), practically all insoluble. 

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