Bsk

Figure 7. Reaction of /^-elimination in alkali degradation of proteins: (1) dehydroalanine, (2) persulfide product, (3) lysinoalanine.

^-Elimination

Protein Structure and Functionality

For a food chemist the structure of proteins is quite often viewed in the context of functionality in a food system. For example, the chemistry of muscle fibers and the mechanism of muscle contraction are related to rigor mortis and postmortem tenderness of meat (39). The onset of rigor mortis follows rapid depletion of ATP and breakdown in the regulatory system that controls the calcium level in muscle. Increasing concentration of calcium in the sarco-plasm induces contraction, whereas lack of ATP in the system stops the dissociation of the actin-myosin complex formed. The muscle loses its natural extensibility and this postmortem change is known as rigor mortis. Postmortem tenderness, however, is related to proteolysis of the muscle proteins. The acid proteases, such as the cathepsins, have received much attention in this respect. The calcium-dependent proteinase, calcium activated factor, has been linked to the causes of postmortem tenderization (40). In the last decade, there have been significant advances in our understanding of the molecular structure as well as the morphology and mechanism of muscle cytoskeletal proteins (41-43). These new developments in the basic knowledge of muscle proteins will inevitably affect the way their functionality in food systems is interpreted.

Despite the importance of cereal seed proteins, until recently, their molecular structure has been little understood. In the last several years, the complete amino acid sequence (or sequence deduced from cDNA) of gliadin (city pe) has been determined (44). The low molecular weight (LMW) glutenin subunits have been mapped by two-dimensional gel electrophoresis and sequenced (45). The high molecular weight (HMW) glutenin genes have been sequenced and expressed in Escherichia coli (46). Most recently, wheat transformation with genes for HMW sub-

units has been achieved, and the expression of one or two transgenic proteins in wheat results in stepwise increase in dough elasticity (47). Structural analysis of the amino acid sequence of the HMW glutenins is especially revealing. The protein molecule contains a large central repetitive region rich in glutamine, forming a loose spiral structure (48). Several cysteine residues are located at the a-helical region near the N- and C-terminal ends. Intermolecular disulfide bonds between the terminal cysteine residues cross-link the glutenin subunits into gluten polymers with the spiral regions in between. Hydrogen bonding could form among the side chains of glutamine residues, as well as the peptide backbone. However, attempt to relate the elastic mechanism of HMW subunits to a model consisting of spiral motifs that can extend and reform has not been entirely convincing (49). The continuing efforts on the investigation of protein sequences and structures in combination with genetic engineering studies are of immense value in providing the molecular basis for the specific role of HMW glutenins.

Bovine casein micelles exist in large spherical colloidal particles of 500 to 3000 A in diameter and 107 to 3 x 1010 in particle weight. It has long been postulated that a micelle is assembled from submicelles containing a mixture of various casein molecules. However, the supramolecular structure of casein submicelles and micelles is unclear. One model suggests that submicelles are bound by electrostatic interaction via colloidal calcium phosphate through their ester phosphate groups (50). Because «-casein is almost phosphate free, binding occurs only among the other caseins in the submicelle. Submicelles with a low level of /c-casein are oriented in the interior, and the surface of the micelle is covered entirely with submicelles having a high content of /c-casein. A more recent model depicts the struc ture of micelles as a protein gel without the formation of discret submicelles (51). The core of a micelle is a gel matrix with casein proteins held by microgranules of calcium phosphate. On the surface of the micelle is a "hairy" layer composed of a uniform density of macropeptide segments of «-casein. The highly flexible and hydrated polar polypeptide chains provide steric stabilization to the micelle. Cleavage of the macropeptides by chymosin changes the surface characteristics of the micelle resulting in aggregation.

The cDNA sequences for the four major caseins (asl, as2, /?, and k) are known. Suggestions have been made to improve the functionality of casein using genetic engineering techniques (52). These include (1) alteration of the proportion of «--casein to enhance the stability of casein micelles, (2) construction of an additional cleaving site in casein for chymosin, resulting in a change of rheological effects of proteolysis, (3) dephosphorylation of casein, creating additional phosphate groups for stabilization, (4) deletion of a polar segment from the otherwise nonpolar JV-terminus of fc-casein, thereby enhancing its amphiphilicity.

Similar strategy has been applied to the whey protein /Mactoglobulin. It has been postulated that the thermal instability of this protein is due to the unfolding of the polypeptide segment (residues 115-125) containing the free cysteine-121, and the subsequent sulfhydryl-disulfide exchange with the disulfides (65-160 and 106-143) or with /c-casein in a milk system. Hence, deletion or substitution of the cys-121 by site-directed mutagenesis is expected to enhance the thermal stability and the functional properties of /Mactoglobulin.

Sleeping Sanctuary

Sleeping Sanctuary

Salvation For The Sleep Deprived The Ultimate Guide To Sleeping, Napping, Resting And  Restoring Your Energy. Of the many things that we do just instinctively and do not give much  of a thought to, sleep is probably the most prominent one. Most of us sleep only because we have to. We sleep because we cannot stay awake all 24 hours in the day.

Get My Free Ebook


Post a comment