Pyrazine binding protein

Baldaccini NE, Gagliardo A, Pelosi P, Topazzini A. Occurrence of a pyrazine binding protein in the nasal mucosa of some vertebrates. Compar. Biochem. Physiol. B. 1986 84 249-253. 

Cavaggioni, A., Sorbi, R.T., Keen, J.N., Pappin, D.J.C. Findlay, J.B.C. 1987. Homology between the pyrazine-binding protein from nasal mucosa and major urinary proteins. FEBS Letters, 212, 225—228. 

Cavaggioni, A., Findlay, J.B.C. Tirindelli, R. 1990. Ligand binding characteristics of homologous rat and mouse urinary proteins and pyrazine-binding protein of calf Comp. Biochem. Physiol, 96B, 513-520. 

The stoichiometry and energetics of alkyl pyrazines binding to soy proteins were determined from the free, [L], and bound, u, ligand concentration and the amount of protein (12). A double reciprocal plot of 1/u vs 1/[L], a Klotz plot, yielded a straight line with the equation ... 

In this study the physical parameters involved in interaction of a major class of meat flavorants, methyl pyrazines, with soy proteins were determined at meat roasting temperatures. Beef diffusate, the water soluble, low molecular weight fraction that constitutes about IX of beef, was shown to contain the necessary precursors to obtain a desirable, thermally generated meat aroma (8). Diffusate was heated under controlled conditions and generated volatiles were transferred to a gas chromatograph for separation and quantitation. Methyl pyrazines, either from heated diffusate or from standard solutions, were measured in the presence of purified soy proteins and the thermodynamics of binding were determined. 

Binding Study. Concentrations of soy and diffusate were equivalent to ground beef fortified with 5-20% hydrated soy protein. The diffusate/soy mixtures were heated under the same conditions and generated pyrazines were separated and quantitated by gas chromatography with an alkali flame ionization detector (afid). Triplicate results were averaged for each level of vegetable protein and the concentration of pyrazines generated in the presence of soy proteins was determined relative to the control, diffusate heated neat. 

In a second experiment designed to determine thermodynamic binding constants of pyrazines with the protein types, only a 400 mg level of protein was used. This was equivalent to a 16% soy substituted ground beef. The protein was preheated in the microreactor prior to injecting 2 microliters of a standard solution of methyl substituted pyrazines in heptane. Six concentrations from 3mM to 0.03mM were tested at 120° and 145°C. The standard solutions were heated under the same conditions as was the diffusate. Replicate values were averaged and binding parameters were determined from Klotz plots of the results. 

When standard solutions of methyl pyrazines were used in binding studies with the soy proteins instead of thermally generated pyrazines, the same relationships were observed. Table II is a listing of percentage of the control concentration of the methyl pyrazine congeners from diffusate and standard solutions in the presence of 400 mg soy 11S protein. 

Measurable loss of pyrazine concentration in the presence of soy proteins was a function of the type of protein. The number of binding sites and intrinsic binding constants on 7S, 11S, and whole soy proteins for the series of pyrazines are given in Table III. 

Most of the pyrazines had only one binding site available. Methyl pyrazine was the exception because, while only one binding site was available on whole soy, the 7S and 11S fractions had multiple sites. Since binding sites for 7S and 11S are on the interior of cylindrical shaped proteins, as the ligand size increased, it became increasingly difficult to fit inside the protein (6). 

The problem of developing desirable meat flavor in the presence of vegetable protein has been clearly demonstrated in the literature. Physical measurements after heating a meat model system with soy proteins have shown a dramatic reduction in the concentration of alkyl pyrazine compounds due to interaction with the soy proteins. These interactions have been defined in terms of stoichiometry and binding energies from measurements on pure standards of the methyl pyrazines. 

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