Dextransucrases [EC 18.104.22.168] are enzymes elaborated by various strains of Leuconostoc mesenteroides, Streptococcus sp., and Lactobacillus sp. that synthesize dextrans and related glucans from sucrose. Dextrans are polysaccharides composed of D-glucose units that are α-(1,6)-linked in the main chains and are branched by α -(1,3) linkages, and for some other strains have α-(1,2) or α -(1,4) branch linkages.
The branches are composed of different arrangements of D-glucose units and α-(1,6)-linked dextran chains attached to main dextran chains, depending on the specific dextransucrase elaborated by individual bacterial species. Kim and Robyt mutated L. mesenteroides B-512F to give the B-512FMC strain that was constitutive and did not require sucrose in the culture medium to elaborate dextransucrase. It also gave much higher amounts of dextransucrase and did not produce levansucrase and invertase, in contrast to the native B-512F strain.
Commercial dextrans of different molecular weights have been produced, ranging from about 5000 Da to 500,000 Da. Dextrans of relatively low molecular weights (clinical dextrans, MWn = 26,500–39,000 Da) have been produced as blood plasma extenders. Even lower molecular weight dextrans (MWn = 6000–8000 Da) also have been used to form complexes with iron to treat severe anemia. High-molecular-weight dextrans (MWn = 1–2 X 106 Da) have been approved by the European Union as a food ingredients in bakery products. Relatively high-molecular-weight dextrans have been crossed-linked to varying extents to give different size-exclusion chromatography materials (Sephadex products), which have been used as gel-filtration materials for over 50 years. Low-molecular-weight dextran sulfate has been used as substitute for heparin for aiding the clotting of blood. With the development of methods for directly producing a wide range of different molecular sizes of dextrans, an even wider range of applications for dextrans in foods, pharmaceuticals, and various dextran derivatives will be needed.
It is shown that the molecular weights of synthesized dextrans are inversely proportional to the concentration of dextransucrase and directly proportional to the concentration of sucrose and the temperature. It is concluded that the best method for obtaining dextrans of different molecular weights (low, medium, and high) is the controlled, direct synthesis by B-512FMC dextransucrase, using specific selections of dextransucrase concentrations, sucrose concentrations, and temperatures in which it is shown that dextrans were synthesized, with different number-average molecular weights, ranging from 20,600 Da to 1,645,700 Da. The results of the inverse relationship of the concentrations of dextransucrase to the molecular weights of the synthesized dextrans gives further confirmation to the mechanism for dextransucrase, by the processive addition of D-glucose to the growing reducing-ends of the dextran chains, covalently attached to the active site of the enzyme. A combination of the three variables of dextransucrase concentration, sucrose concentration, and temperature can be used to control the number-average molecular weight of synthesized dextrans. It is further concluded that the most accurate and correct method for determining the average molecular sizes of dextrans (polysaccharides) is a number-average molecular weight method, rather than the weight-average molecular weight methods that are obtained by physical methods such as light scattering.
|Dextran Type||MWw (Da)||MWn (Da)|
Table for comparison of the weight-average molecular weights (MWw) and number-average molecular weights (MWn) of commercial dextrans.