Yeasts cells

Cells of microorganisms have constituted a portion of human food siace ancient times. Yeast-leavened baked products contain the residual nutrients from the yeast cells destroyed duriag bakiag (see Bakery processes and leavening agents). Cultured dairy products, such as yogurt, buttermilk, and sour cream, contain up to lO cells of lactic acid bacteria per gram (19) (see Milk and milkproducts). Other examples of fermented foods consumed siace early times iaclude fermented meats, fish, and soybean products. 

The presence of diacetyl at any stage of the process does not necessarily iadicate an infection by pediococci, because diacetyl is normally formed duting fermentation by oxidation of the precurser 2-acetolactate, which reaches a peak (1—1.2 ppm) at 24—36 h fermentation. The concentration of 2-acetolacetate is usually reduced to values of 0.01 ppm or less, and the diacetyl is reabsorbed by the yeast cells and en2ymatically transformed through acetoia to butanediol. It is extremely important that 2-acetolactate as diacetyl is reduced below the threshold of 0.05—0.10 ppm (ia terms of diacetyl). 

Himdin [8001-27-2] is a polypeptide of 66 amino acids found ia the saUvary gland secretions of the leech Himdo medicinalis (45). It is a potent inhibitor of thrombin and biads to y-thrombia with a dissociation constant of 0.8 x 10 ° M to 2.0 x lO " M. Himdin forms a stable noncovalent complex with free and bound thrombin completely iadependent of AT-III. This material has now been cloned and expressed ia yeast cells (46,47). Its antigenic poteatial ia humans remains to be estabUshed. 

In 1878 the term enzyme, Greek for "in yeast," was proposed (8). It was reasoned that chemical compounds capable of catalysis, ie, ptyalin (amylase from sahva), pepsin, and others, should not be called ferments, as this term was already in use for yeast cells and other organisms. However, proof was not given for the actual existence of enzymes. EinaHy, in 1897, it was demonstrated that ceU-free yeast extract ("zymase") could convert glucose into ethanol and carbon dioxide in exactiy the same way as viable yeast cells. It took some time before these experiments and deductions were completely understood and accepted by the scientific community. 

Because enzymes can be intraceUularly associated with cell membranes, whole microbial cells, viable or nonviable, can be used to exploit the activity of one or more types of enzyme and cofactor regeneration, eg, alcohol production from sugar with yeast cells. Viable cells may be further stabilized by entrapment in aqueous gel beads or attached to the surface of spherical particles. Otherwise cells are usually homogenized and cross-linked with glutaraldehyde [111-30-8] to form an insoluble yet penetrable matrix. This is the method upon which the principal industrial appHcations of immobilized enzymes is based. 

Yeasts are one land of fungi. They are unicellular organisms surrounded by a cell wall and possessing a distinct nucleus. With veiy few exceptions, yeasts reproduce by a process known as budding, where a small new cell is pincEed off the parent cell. Under certain conditions, an individual yeast cell may become a fruiting body, producing spores. 

TBP mutants lacking the N-terminal region are fully functional in promoter binding and stimulation of basal transcription and therefore these two functions must be provided by the C-terminal domain. Furthermore, the C-terminal domain of yeast TBP contains all the functions essential for normal yeast cell growth and for responses to specific transcriptional activators with a net negative charge. This C-terminal domain contains two homologous... 

Another specialized cell line that has been utilized for functional drug screening are yeast cells. A major... 

Xenopus laevis melanophores, 82 yeast cells used in, 81-82 Furchgott method, 92, 95, 97-98, 261 Furosemide, 150, 151f... 

Since 673 kcal/mole could be released by complete oxidation, we might wonder why the yeast cells (and muscle) extract only 20 kcal/mole and leave so much of the potentially available energy untouched. This extra energy is there in ethanol and lactic acid and could be released if these compounds were oxidized further to C02. 

When excess substrate interferes with growth and/or product formation. One example is the production of baker s yeast. It is known that relatively low concentrations of certain sugars repress respiration and this will make the yeast cells switch to fermentative metabolism, even under aerobic conditions. This, of course, has a negative effect on biomass yield. When maximum biomass production is aimed at, fed batch cultures are the best choice, since the concentration of limiting sugar remains low enough to avoid repression of respiration. 

Very few optically active cyanohydrins, derived from ketones, are described in the literature. High diastcrcosclectivity is observed for the substrate-controlled addition of hydrocyanic acid to 17-oxosteroids27 and for the addition of trimethyl(2-propenyl)silane to optically active acyl cyanides28. The enantioselective hydrolysis of racemic ketone cyanohydrin esters with yeast cells of Pichia miso occurs with only moderate chemical yields20. 

---