Yeast functional properties

T21. Tougard, P., Le, T. H., Minard, P Desmadril, M., Yon, J. M., Bizebard, T., Lebras, G and Dumas, C., Structural and functional properties of mutant Arg203Pro from yeast phosphoglycerate kinase, as a model of phosphoglycerate kinase-Uppsaia. Protein Eng. 9, 181-187... 

Initial attempts at producing the Thr mutant of Pro-35 were unsuccessful in producing sufficient quantities of the mutant protein for functional analysis [129], suggesting that cytochrome c is significantly more sensitive to modifications at this prolyl residue than at Pro-71. Either the Thr-35 mutant is not properly processed to mature cytochrome c, it is less thermally stable than wild-type iso-2-cytochrome c, or its functional properties are sufficiently perturbed that it cannot function adequately under physiological conditions to support yeast growth. 

While the results of this work are encouraging, it is clear that the structural definition of mutant proteins of this type is critical to development of rational interpretation of the results if for no other reason than that the structural perturbation introduced is presumably greater than for simple point mutations. Moreover, it would be particularly interesting to compare the functional properties of mutants compared in this manner in assays involving protein-protein reactions relevant to the species of cytochrome c on which the mutagenesis is based. For example, comparison of the activities of wild-type yeast cytochrome c with that of a loop-insertion mutant modelled on a photosynthetic cytochrome c in the reaction with the photosynthetic reaction center could help define the structural elements involved in the cytochrome c binding domain for the reaction center. 

Huang, Y.T. and Kinsella, J.E. 1986. Functional properties of phosphorylated yeast proteins solubility, water holding capacity and viscosity. J. Agric. Food Chem. 34, 670-674. 

For adoption as an ingredient in foods, the isolated protein should have appropriate functional properties (111). Currently we are examining some functional properties of decitraconylated yeast proteins. 

The 12 investigations compiled here deal mostly with functional investigations in the food area. Three chapters discuss soy proteins, relating the effects of heat, specific solvents, and enzyme activity on structure and functional properties. Single chapters are concerned with the proteins of wheat, milk, and yeast, respectively, as well as specific crosslinking effects and, enzyme-carrier interactions. Other chapters contrast general functional properties of different proteins. 

Chemical Modification lor Improving Functional Properties of Plant and Yeast Proteins... 

There are large resources of potential food proteins (oilseed, yeast, leaf) which are presently unexploited. With the application of innovative scientific and technological methods these can become significant sources of food protein. In developing ingredient protein from plant sources, research emphasis must include studies to determine the physicochemical or functional properties of these proteins. 

In the remainder of this paper we review the available information on the effects of chemical modification on the functional properties of plant proteins and report on the use of this approach for preparing functional proteins from yeast. 

Ideally microbial cells should be consumable directly as food or food ingredients. However, because of their nucleic acid content the presence of undesirable physiologically active components the deleterious effects of cell wall material on protein bioavailability and the lack of requisite and discrete functional properties, rupture of cells and extraction of the protein is a necessary step. Importantly, for many food uses (particularly as a functional protein ingredient) an undenatured protein is required. For these reasons and for many potential applications of yeast protein(s) it is very desirable to separate cell wall material and RNA from the protein(s) for food applications. Much research is needed to develop a practical method for isolation of intact, undenatured yeast proteins from the yeast cell wall material to ensure the requisite nutritional and functional properties. 

Intact dried microbial cells have limited functional properties. Labuza et al. (58) reported that spray drying of yeast cells at higher temperatures improved the functional performance of yeast cells in bread making but they were still inferior to control samples. Spray drying of yeast cells at 75 and 124°C... 

In conjunction with research on protein extraction from yeast, we investigated methods for the maximum recovery of protein possessing good functional properties but low in nucleic acid. Therefore, we examined the feasibility of making the yeast protein resistant to proteolysis during extraction and nucleic acid reduction. Using established extraction procedures (76), we observed... 

Solubility is a critical functional characteristic because many functional properties depend on the capacity of proteins to go into solution initially, e.g. gelation, emulsification, foam formation. Data on solubility of a protein under a variety of environmental conditions (pH, ionic strength, temperature) are useful diagnostically in providing information on prior treatment of a protein (i.e. if denaturation has occurred) and as indices of the potential applications of the protein, e.g. a protein with poor solubility is of little use in foams). Determination of solubility is the first test in evaluation of the potential functional properties of proteins and retention of solubility is a useful criterion when selecting methods for isolating and refining protein preparations (1). Several researchers have reported on the solubility of extracted microbial proteins (69,82,83,84). In many instances yeast proteins demonstrate very inferior solubility properties below pH 7.5 because of denaturation. 

The limited data indicate that yeast proteins extracted by the current, more conventional methods lack the requisite functional properties for many applications. 

During isolation by conventional methods yeast proteins frequently become denatured, insolubilized and display poor functional properties. These proteins can be rendered more soluble by limited hydrolysis with acid, alkali or proteolytic enzymes. Protein hydrolyzates are most commonly prepared by partial acid hydrolysis and yeast hydrolyzates are popular as food flavorings and ingredients (66). Acid hydrolyzates have flavors resembling cooked meats and are widely used by earners to impart brothy, meaty flavors to soups, gravies, sauces, canned meats. 

Alkali treatment has been used to improve the functional properties of the insoluble protein prepared by heat precipitation of an alkaline extract of broken yeast cells (63). Heating yeast protein at pH 11.8 followed by acid precipitation (pH 4.5) yielded a preparation composed of polypeptides with increased aqueous solubility. It also increased foaming capacity of the protein 20-fold. The emulsifying capacity of the modified protein was good whereas the original insoluble protein was incapable of forming an emulsion. Alkali treatment must be carefully controlled to avoid its possible deleterious effects (24,75), e.g. alkaline treatment of yeast protein resulted in a loss (60%) of cysteine (63). 

Enzyme hydrolysis is occasionally used to modify the functional properties of proteins and yeast autolyzates are used commercially as food flavorants (66,86). Partial proteolysis of... 

Chemical modification of yeast protein has received limited attention though as described above it has potential as a method for facilitating recovery of yeast protein. Current studies are concerned with determination of the functional properties of proteins succinylated during the extraction. The composition of yeast proteins prepared by different methods is shown (Table 8). Noteworthy is the protein and nucleic acid concentration in the yeast isolate which differed from the concentrate in that cell wall material was removed by centrifugation. 

To achieve success as protein ingredients for food formulation and fabrication, novel proteins should possess a range of functional properties. Frequently during extraction, refining and drying, plant and yeast proteins, intended for food uses, become denatured or altered and subsequently display poor functional properties which render them of limited use. Chemical modification provides a feasible method for improving the functional properties of plant and yeast proteins and potentially may make it possible to tailor proteins with very specific functional properties. In this review the information on modified plant proteins is reviewed and the use of succinylation for the recovery of yeast proteins with low nucleic acid is described. 

Jamas, S., C.-K. Rha, A.J. Sinskey, "Directed biosynthesis of yeast glucans with known structure-function properties", paper presented at the ACS 190th Annual Meeting, Chicago, IL, September 9-13, 1985. 

---