Synthese eines labempfindlichen pentadecapeptids aus kuh--casein

Factors affecting the retention of rennet in cheese curd

The coagulant retained in cheese curd is a major contributor to proteolysis during ripening. The objective of this study was to quantify the effects of several milk-related factors and parameters during cheese manufacture on the retention of coagulant in cheese curd. The amount of coagulant retained in curd was determined by its activity on a synthetic heptapeptide (Pro-Thr-Glu-Phe-[NO2-Phe]-Arg-Leu) using reversed-phase HPLC. The retention of chymosin in cheese curd increased significantly when the pH of milk was reduced at rennet addition below pH 6.1, the pH at whey drainage below pH 5.7, or the average casein micelle size in milk and when the ionic strength of milk was increased. The casein content of milk and the quantity of chymosin added to milk had no significant effect on the retention of chymosin in curd; the quantity of coagulant bound per gram of casein remained unchanged.

Sensitive fluorometric assay for the activity of chymosin

Fluorogenic substrates for chymosin [Dns-Leu-Ser-Phe-Trp-Ala-Leu-OCH2Py (I), Dns-Leu-Ser-Phe-Met-Trp-Leu-OCH2Py (II), Dns-Leu-Ser-Leu-Trp-Ala-Leu-OCH2Py (III), Dns-Leu-Ala-Phe-Trp-Ala-Leu-OCH2Py (IV), Dns-Leu-Ser-Phe-Leu-Ala-Leu-OCH2Py (V) and Dns-Leu-Ser-Phe-Phe-Ala-Leu-OCH2Py (VI)] were synthesized by a solution method. The obtained substrates I-VI were cleaved specifically (between the Phe and Trp residues for substrates I and IV, the Phe and Met residues for substrate II, the Leu and Trp residues for substrate III, the Phe and Leu residues for substrate V, and the Phe and Phe residues for substrate VI) by chymosin. The fluorescence of substrates I-IV (345 nm) increased with their hydrolysis, and hydrolysis rates were obtained by measuring the increase in fluorescence. The minimum detectable chymosin concentrations for substrates I and IV were about 1 nM; those for substrates II and III were about 4 and 2 nM. This assay method is very sensitive, and it is possible to determine the chymosin activity rapidly and easily. Substrates I and IV-VI were hydrolyzed by chymosin two times faster than substrates II and III. The effect of the amino-acid residues of the substrates on the hydrolysis rate is discussed.

The involvement of one of the three histidine residues of cow kappa-casein in the chymosin-initiated milk clotting process

Cow kappa-casein has been modified by photo-oxidation in the presence of rose bengal and by the chemical reagents diethyl pyrocarbonate, 2-hydroxy-5-nitro-benzyl bromide and iodoacetic acid. Photo-oxidation resulted in the destruction of histidine and tryptophan residues and all of the histidines could be ethoxy-formylated by treatment with diethyl pyrocarbonate. Both procedures caused a loss in the susceptibility of the Phe-Met linkage of kappa-casein to chymosin hydrolysis. Treatment of kappa-casein with 2-hydroxy-5-nitrobenzyl bromide and iodoacetic acid caused the loss of tryptophan and methionine residues respectively but, in both cases, the susceptibility of the modified protein to chymosin hydrolysis remained unaffected. Of the amino acids examined it is concluded that only the histidine residues of cow kappa-casein are important for the hydrolytic action of chymosin and, furthermore, the treatment with diethyl pyrocarbonate suggests that only one of the three histidines plays an essential role.

Peptide substrates for chymosin (rennin). Kinetic studies with peptides of different chain length including parts of the sequence 101-112 of bovine k-casein

Kinetic parameters have been determined for the reaction between milk-clotting chymosin (EC 3.4.23.4) and a series of peptides (or their methyl esters) including the amino acid sequence around the enzyme-sensitive Phe(105)-Met (106) bond the bovine k-casein. In particular, the influence of the substrate's chain length on the kinetic parameters has been studied. Evidence is presented that in the model peptides studied the sequence -Ser-Phe-Met-Ala with a further residue added to either end (in casu Leu(103) or Ile(108)) is necessary to induce any cleavage by the enzyme. When both the Leu(103) and Ile(108) residues form part of the peptide chain, a marked improvement of the substrate properties is observed. It is suggested that prolyl residues on either side of the sensitive peptide bond form additional sites for secondary enzyme-substrate interactions.

Nomemclature of the proteins of cow's milk: fourth revision

This report reviews the nomenclature of the milk proteins of cow's milk in light of more recent advances in our knowledge. With the establishment of the primary structures of a number of these proteins, we now have a definite identification of alphas1-, kappa-, beta-, and the gamma-caseins as well as beta-lactoglobulin and alpha-lactalbumin. On the basis of new information on their primary structures and relationship to beta-casein polymorphs, changes in nomenclature have been recommended for proteins of the gamma-casein fraction. Although the primary structure serves as the unambiguous definition of proteins for which it is known, a more practical identification is necessary. We recommend that their behavior in gel electrophoresis under suitable conditions be employed for this purpose for all of the "major" milk proteins of raw skim milk except the immunoglobulins where, because of their heterogeneity and molecular genetics, physical parameters are less useful and their identification must be based upon antigenic determinants and their homology with their human counterparts. More work is needed and, with the accumulation of more information, additional changes in nomenclature can be expected for such proteins as the minor components of alphas- and kappa-caseins, alpha-lactalbumin, and the proteose-peptone fraction as well as further confirmation of the presence of immunoglobulins IgE and additional IgG subclasses. Additional components and genetic variants also can be expected.

Structural aspects of the milk clotting process. Comparative features with the blood clotting process

The enzyme chymosin and its substrate, a casein fraction called k-casein, are involved in the milk clotting process. Recent data concerning the structure (peptide and sugar moieties) of various k-caseins and their role in casein micelles formation and stabilization are presented. The molecular events occurring during the primary phase of chymosin action on k-casein are discussed. Finally some structural features concerning more particularly the caseinoglycopeptides and the fibrinopeptides as well as the action of chymosin and thrombin involved in the milk and blood clotting processes are compared. Three examples of sequences of portions of k-caseins and fibrinogen presenting homology are presented.

A rennin-sensitive bond in alpha-s1 B-casein

Rennin acts on a specially sensitive bond in αs1B-casein to produce a basic peptide containing residues 1–23 of the original protein. At pH 6·4 and 30°C, the action is specific and rapid, the kinetic constants beingKm4·5×10−4M,Kcat3·8 s−1, andkcat/Km0·85×104s−1M−1. Pepsin, and a protease impurity in the acid phosphatase from wheat germ, have a similar action.