Structural and functional properties of a partially purified cowpea (Vigna unguiculata) globulin modified with protein kinase and glycopeptidase

The major globulin fraction in cowpea seed was partially purified by selective ammonium sulfate precipitation and gel chromatography. The partially purified protein was enzymatically phosphorylated or deglycosylated. Phosphate content increased from 5.81 mu g/mg in the untreated protein to 10.55 mu g/mg in the protein kinase treated protein. Fluorescence intensity and protein solubility were significantly (p less than or equal to 0.05) increased at pH 3-8 as a result of phosphorylation, while susceptibility to heat-induced coagulation was significantly (p less than or equal to 0.05) decreased. At 50% deglycosylation, fluorescence intensity was significantly (p less than or equal to 0.05) higher for the untreated protein than the treated protein. The intensity of the near-UV circular dichroism spectra of the deglycosylated protein was lower than that of the untreated protein at pH 3, 4, and 6, whereas the reverse was observed at pH 7 and 8. The deglycosylated protein was less soluble at pH 3, 4, 7, and 8 and was more susceptible to heat coagulation when compared to the untreated protein. Modification by protein kinase would allow use in foods where greater solubility is needed, while deglycosylation could enhance utilization in foods that require heat-induced coagulation.