UPTAKE OF UNTRIENTS BY YEASTS II. MALTOTRIOSE PERMEASE AND THE UTILIZATION OF MALTOTRIOSE BY YEASTS

Influence of wheat flour and Lactobacillus strains on the dynamics of by-products from amylolytic activities / Influencia de la harina de trigo y de la especie de Lactobacilo en la dinámicade subproductos de la actividad amilolítica

Amylolytic activity of six flours from three European wheat cultivars (Obelisk, Camp Remy and Fresco, at 70 and 100% extraction level), and five lactobacilli strains (Lactobacillus plantaracm B-39, L-73; L. brevis 25a, L-62, and L. sanfrancisco L-99), as well as the dynamics of by-products from amylolytic degrading action in mixed flour-lactobacilli systems have been investigated. α-Amylase activity of flours depended on ash content; whole flours from the three cultivars had similar values, which were of the order of three to ten times that of white flours. L. sanfrancisco showed the lowest α-amylase activity, and L.plantarum the highest. At the concentrations of lactobacilli used in sourdough processes, flour was the main contributor to final α-amylase action in flour-lactobacilli mixtures. Production of monosaccharides followed variable trends, depending on the extraction level of the flour and on the Lactobacillus strain. The dynamics of monosaccha rides, due to their active and variable involvement in metabolic pathways, could not be adjusted to model regression equations. Maltose levels increased during fermentation, although an effec tive incorporation in Lactobacillus metabolism was observed. A second order polynomy could be used to describe changes in maltose. Low molecular weight dextrins (LMWD) with degrees of polymerization (DP) from three to seven, only appeared and accumulated during the late stages of fermentation. LMWD with a DP of three was the only oligosaccharide capable of being incor porated by lactobacilli, and, of the strains tested, only by L. brevis (25a). The larger dextrins seemed to be involved in further degrading enzymic reactions yielding smaller size fragments. Dynamics of LMWD followed exponential curves, with highly significant regression coefficients.

Maltotriose utilization by industrial Saccharomyces strains: characterization of a new member of the alpha-glucoside transporter family

Maltotriose utilization by Saccharomyces cerevisiae and closely related yeasts is important to industrial processes based on starch hydrolysates, where the trisaccharide is present in significant concentrations and often is not completely consumed. We undertook an integrated study to better understand maltotriose metabolism in a mixture with glucose and maltose. Physiological data obtained for a particularly fast-growing distiller's strain (PYCC 5297) showed that, in contrast to what has been previously reported for other strains, maltotriose is essentially fermented. The respiratory quotient was, however, considerably higher for maltotriose (0.36) than for maltose (0.16) or glucose (0.11). To assess the role of transport in the sequential utilization of maltose and maltotriose, we investigated the presence of genes involved in maltotriose uptake in the type strain of Saccharomyces carlsbergensis (PYCC 4457). To this end, a previously constructed genomic library was used to identify maltotriose transporter genes by functional complementation of a strain devoid of known maltose transporters. One gene, clearly belonging to the MAL transporter family, was repeatedly isolated from the library. Sequence comparison showed that the novel gene (designated MTY1) shares 90% and 54% identity with MAL31 and AGT1, respectively. However, expression of Mty1p restores growth of the S. cerevisiae receptor strain on both maltose and maltotriose, whereas the closely related Mal31p supports growth on maltose only and Agt1p supports growth on a wider range of substrates, including maltose and maltotriose. Interestingly, Mty1p displays higher affinity for maltotriose than for maltose, a new feature among all the alpha-glucoside transporters described so far.

Molecular analysis of maltotriose transport and utilization by Saccharomyces cerevisiae

Efficient fermentation of maltotriose is a desired property of Saccharomyces cerevisiae for brewing. In a standard wort, maltotriose is the second most abundant sugar, and slower uptake leads to residual maltotriose in the finished product. The limiting factor of sugar metabolism is its transport, and there are conflicting reports on whether a specific maltotriose permease exists or whether the mechanisms responsible for maltose uptake also carry out maltotriose transport. In this study, radiolabeled maltotriose was used to show that overexpression of the maltose permease gene, MAL61, in an industrial yeast strain resulted in an increase in the rate of transport of maltotriose as well as maltose. A strain derived from W303-1A and lacking any maltose or maltotriose transporter but carrying a functional maltose transport activator (MAL63) was developed. By complementing this strain with permeases encoded by MAL31, MAL61, and AGT1, it was possible to measure their specific transport kinetics by using maltotriose and maltose. All three permeases were capable of high-affinity transport of maltotriose and of allowing growth of the strain on the sugar. Maltotriose utilization from the permease encoded by AGT1 was regulated by the same genetic mechanisms as those involving the maltose transcriptional activator. Competition studies carried out with two industrial strains, one not containing any homologue of AGT1, showed that maltose uptake and maltotriose uptake were competitive and that maltose was the preferred substrate. These results indicate that the presence of residual maltotriose in beer is not due to a genetic or physiological inability of yeast cells to utilize the sugar but rather to the lower affinity for maltotriose uptake in conjunction with deteriorating conditions present at the later stages of fermentation. Here we identify molecular mechanisms regulating the uptake of maltotriose and determine the role of each of the transporter genes in the cells.

Maltotriose transport and utilization in baker's and brewer's yeast

Maltotriose is metabolized by baker's and brewer's yeast only oxidatively, with a respiratory quotient of 1.0, the QCO2Ar being, depending on the strain used, 0-11, as compared with QCO2air of 6-42 microL CO2 per h per mg dry substance. The transport appeared to proceed by facilitated diffusion (no effects of NaF, iodoacetamide and 3-chlorophenylhydrazonomalononitrile) with a KT of more than 50 mM and was inhibited by maltose greater than maltotriose greater than methyl-alpha-D-glucoside greater than maltotetraose greater than D-fructose greater than D-glucose. The transport was present constitutively in both Saccharomyces cerevisiae (baker's yeast) and in S. uvarum (brewer's yeast) and it was not significantly stimulated by preincubation with glucose or maltose. The pH optimum was 4.5-5.5, the temperature dependence yielded an activation energy of 26 kJ/mol.