Enzymic Synthesis of Dextran

As stated earlier (see p. 342), study of the cell-free, enzymic synthesis of dextran was initiated by Hehre and Sugg They demonstrated the ability of a cell-free extract of Leuconostoc mesenteroidea to catalyze the formation of dextran. The over-all reaction was represented by equation (1). In addi-... 

The following mechanism is a slight modification of the scheme proposed by Bovey for explaining the enzymic synthesis of dextran. The step con-... 

Thus, it was determined that the mechanism of action of the acceptors is one of terminating dextran synthesis by the release of the glucosyl and dextranyl units from the covalent enzyme intermediate rather than one of priming the synthesis of dextran. 

For use as a blood-plasma substitute, the dextran should have a molecular weight in the range of 50,000 to 100,000. This criterion has occasioned a concerted effort to produce dextrans in the correct range. Partial, acid hydrolysis of native dextran followed by fractionation with various solvents, or enzymic production of dextran of low molecular weight are methods which have been used. In addition, ultrasonics has been suggested as a means of depolymerizating native dextran to the correct size for clinical use. Introduction of alternative acceptors into the reaction mixture for enzymic synthesis has also been employed for this purpose it is described in an earlier Section of this Chapter. 

DAHP synthetase, 251, 255 Dehydroquinase, 258 Dehydroshikimate riductase, 259 Dextrans, 341 acid hydrolysis of, 349 chain lengths of, 345, 346 cuprammonium complexes of, 355 electron microscope studies on, 349 enzymic synthesis of, 342, 345, 355 from sucrose, 342 flow birefringence studies on, 349 fructose-containing, 359 infrared absorption spectra of, 352 isolation of, 343... 

Strain NRRL B-512(F) produces large proportions of the extracellular enzyme dextransucrase,339 which is responsible for the synthesis of linear sequences ofa-D-(l —> 6)-linked D-glucosyl residues. The enzyme transfers the D-glucosyl group from a sucrose molecule to an enlarging dextran chain and liberates the D-fructose portion. As dextransucrase is an extracellular enzyme, production of dextran by cell-free, culture filtrates can result in enhanced yield and quality, and ease of purification of the product. By suitable adjustment of the conditions, products in a chosen molecular-weight range can be obtained. Formation of branches is not yet well understood, but the enzymes responsible will certainly be found. 

McCabe and Smith" have observed that the cell-associated dextransucrase elaborated by S. mutans strain Kl-R is not bound to the cell-wall structure, but is derived from a water-soluble form of the enzyme that becomes incorporated into the (dextran) cell-capsule during dextran synthesis. As synthesis of dextran proceeds, the initially soluble enzyme is converted into two dextran-bound enzyme fractions, one bound reversibly and the other bound irreversibly to the water-insoluble polysaccharide. The soluble dextransucrase is progressively changed to the irreversibly bound enzyme and is inactivated. In terms of the reaction... 

A reducing disaccharide, consisting of D-glucose and D-fructose, named levjcrose 214) is formed in the reaction mixture to the extent of about 3 % during the synthesis of dextran from sucrose by an enzyme isolated from the microorganism Leuconostoc mesenteroides. Its specific rotation is [a]o —6.8° (in water) m.p. 161-162°C. Methylation studies show that its structure is 5-0-a-D-glucopyranosyl-D-fructopyranose. 

A comparison has been made of the methods commonly used to estimate antibodies to the D-glucosyltransferases of S. mutans OMZ 176. A radioenzyme assay showed that the D-glucosyltransferases are composed of many multi-reactive molecules, which enable the enzymes to act as cross-linking reagents. A correlation has been demonstrated between inhibition of the synthesis of dextran by cells of S. mutans and inhibition of the adherence by antibodies, which may be brought about by a reduction in the synthesis of dextran. The synthesis of soluble D-glucans by a number of serotypes of... 

The synthesis of dextran from sucrose by a cell-free bacterial culture filtrate was first reported by Hehre in 1941 [152]. The genera of bacteria that are recognized to produce enzymes capable of synthesizing polysaccharides from sucrose are principally Leuconostoc and Streptococcus. These genera are gram positive, facultatively anaerobic cocci that are very closely related to each other. One notable difference between them is that the L. mesenteroides strains required sucrose in the growth medium to induce the formation of the enzyme(s), whereas the Streptococcus species did not require sucrose in the medium to form the enzymes [153]. 

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