Covalent interactions between proteins and oxidation products of caffeoylquinic acid (chlorogenic acid)

Abstract

BACKGROUND: The interactions between phenolic compounds and proteins can modify protein properties important in the food industry. To understand the effects of these interactions, the covalent interactions between caffeoylquinic acid (chlorogenic acid, CQA) oxidised by polyphenol oxidase (PPO) at acidic pH 6 (pH 6) and αlactalbumin, lysozyme and bovine serum albumin (BSA) were compared with non-enzymatically induced covalent interactions at alkaline pH (pH 9). The effects of these modifications on protein properties were examined.RESULTS: Both ways of modification seemed to result in protein modification mainly via dimeric rather than monomeric CQA quinones. These modifications led to a decrease in the number of free primary amino groups of the proteins. Modification with CQA alone induced a low degree of protein dimerisation, which also occurred through the action of PPO alone. Modification drastically reduced the solubility of lysozyme over a broad pH range, whereas that of α-lactalbumin was strongly reduced only at pH values close to its pI. The solubility of BSA was much less affected than that of the other proteins and only at acidic pH. CONCLUSION: These results indicate some similarities between modifications at pH 6 and 9 and that both modifications clearly change the functional properties of globular proteins.

Stability of hydroxycinnamic acids and caffeine from green coffee extracts after heating in food model systems

“…Covalent interactions of proteins and polyphenols oxidized to quinones decrease the number of free primary amino groups of the proteins, cause protein dimerization, and reduce their solubility. Side chains of lysine and tyrosine in proteins appeared the most susceptible residues to react with oxidized polyphenols [22][23][24]. Non-covalent interactions including hydrogen and ionic bonds and hydrophobic interactions cause an increase in proteins denaturation enthalpy and temperature and this type of bonding may be digested by human digestive tract enzymes and the released hydroxycinnamic acids may be active as antioxidants in the body [25].…”

Sugar beet leaves: from biorefinery to techno-functionality

“…A white leaf protein fraction (WLPF) can be obtained after removal of the green protein fraction (GLPF) using a two-step fractionation. The first step is typically achieved by heating at 50 -60 C (29)(30)(31)(32), use of flocculants (33) or use of organic solvents (34). The final products of this step are an insoluble fraction (GLPF), which is typically obtained after centrifugation or filtration, and a soluble fraction.…”

“…Step 3 fraction is obtained by precipitation, which is commonly achieved by heating at high temperatures (80-85 C) (29), use of acid (32) or combination of the above (33,34) ( Table 1.2). Table 1.2 Examples of the main methods described in literature for the separation or fractionation (step 3) of proteins from leaves.…”

“…Table 1.2 Examples of the main methods described in literature for the separation or fractionation (step 3) of proteins from leaves. Data obtained from (1, 4, 29, 30,(33)(34)(35).a Expressed as N6.25 b not determinedRegardless of the isolation method, and the source of the leaves, the main protein recovered in the final concentrate/isolate is Rubisco (3, 20,32). The isolation method does, however, affect both the protein recovery and the protein purity of the final protein concentrate (Table 1.2).…”

“…Table 1.2 Examples of the main methods described in literature for the separation or fractionation (step 3) of proteins from leaves. Data obtained from (1, 4, 29, 30,(33)(34)(35).…”