Computer-aided baker's yeast fermentations.

The economics of yeast production depend heavily upon the cellular yield coefficient on the carbon source and the volumetric productivity of the process. The application of an on-line computer to maximize these two terms during the fermentation requires a continuous method of measuring cell density and growth rate. Unfortunately, a direct sensor for biomass concentration suitable for use in industrial fermentations is not available. Material balancing, with the aid of on-line computer monitoring, offers an indirect method of measurement. Laboratory results from baker's yeast production in a 14-liter fermentor (with a PDP-11/10 computer for on-line analyses) show this indirect measurement technique to be a viable alternative. From the oxygen uptake and carbon dioxide production data, gas flow rate, and ammonia addition rate, the cell density during the fermentation has been estimated and found to compare well with actual fermentation data.     

Cell biology: Alternatives to baker's yeast

Saccharomyces cerevisiae is an excellent model organism for addressing questions in cell biology, but other yeast systems are also providing new insights into several fundamental cellular processes.     

Active subunits of transketolase from baker's yeast.

Transketolase from baker's yeast was covalently bound to Sepharose via one subunit. Storage in glycine buffer pH 11 entailled loss of half of the protein and a 50% decrease in the activity of the immobilized enzyme. Addition to the system of free subunits of transketolase doubled the amount of the protein attached to the matrix and restored the catalytic activity to the initial level. Thermoinactivation of the initial immobilized dimer of transketolase and the renatured dimer formed on reassociation of the immobilized subunits with the free ones was the same and considerably differed from the thermoinactivation of the immobilized subunits. The conclusion is made that the individual subunits of transketolase are catalytically active.     

Mutarotase in galactose-induced baker's yeast

Cell-free extracts of baker's yeast possess mutarotase activity only after induction of cells in the presence of galactose. The mutarotase activity appears 1 h after transfer to a galactose-containing medium and rises in synchrony with the utilization of galactose. Cycloheximide blocks the induction completely at a concentration of 100 μg/ml. InSaccharomyces fragilis the mutarotase is constitutive but its activity is strikingly increased after growth on galactose. The yeast mutarotase resembles in some respects analogous enzymes from other cells (pH dependence, substrate specificity, heat lability). Its affinity ford-galactose is substantially greater than ford-glucose. There may exist a coupling between mutarotase activity and the anomer-specific galactokinase.     

Preparation of cytochrome c peroxidase from baker's yeast.

A simple and readily reproducible procedure is presented for the preparation and purification of cytochrome c peroxidase from baker's yeast. Following autolysis of the yeast and extraction, the enzyme is collected on DEAE-cellulose at moderately high ionic strength, cluted, concentrated, and subjected to gel filtration in 0.1 m sodium acetate buffer, pH 5.0. The properties of the crude preparation make gel filtration in this buffer suitable for near-final purification of the heme protein. The enzyme is then easily crystallized by dialysis.     

Isolation of hexokinase from baker's yeast

1. A method is described for the isolation of hexokinase from baker''s yeast. The method is based mainly on fractionation with alcohol and results See PDF for Structure in a 30-fold increase in specific activity. The final product could be crystallized from ammonium sulfate without change in specific activity. 2. The enzyme catalyzes a transfer of phosphate from adenosinetriphosphate to glucose, fructose, or mannose, the relative rates with these three sugars being 1:1.4:0.3. 3. With glucose as substrate, the turnover number for the crystalline enzyme is 13,000 moles of substrate per 10⁵ gm. of protein per minute at 30° and pH 7.5. The temperature coefficient (Q _10°) between 0 and 30° is 1.9. 4. Magnesium ions are necessary for the activity, the dissociation constant for the Mg⁺⁺ -protein complex being 2.6 x 10^–3. Fluoride in concentrations as high as 0.125 M has no inhibitory effect on the enzyme when the Mg⁺⁺ and orthophosphate concentrations are 6.5 x 10^–3 M and 1 x 10^–3 M, respectively. 5. The crystalline enzyme shows a loss in activity when highly diluted. This loss in activity can be prevented by diluting in the presence of small amounts of other proteins. Of the various protective proteins tested, insulin was the most effective, providing complete protection in a concentration of 6 micrograms per cc.; with serum albumin, a concentration of 60 micrograms per cc. was necessary. Thiol compounds (cysteine, glutathione) exerted no protective action. 6. The inactivation of the crystalline enzyme on incubation with trypsin can be prevented to a marked degree by the presence of glucose. The instability of crude preparations of yeast hexokinase may be attributed to the presence of proteolytic enzymes, since glucose or fructose has a remarkable protective effect on such preparations.