Investigations on the activation of bovine prochymosin

Biochemical and technological properties of moose (<i>Alces alces</i>) recombinant chymosin

Recombinant chymosins (rСhns) of the cow and the camel are currently considered as standard milk coagulants for cheese-making. The search for a new type of milk-clotting enzymes that may exist in nature and can surpass the existing “cheese-making” standards is an urgent biotechnological task. Within this study, we for the first time constructed an expression vector allowing production of a recombinant analog of moose chymosin in the expression system of Escherichia coli (strain SHuffle express). We built a model of the spatial structure of moose chymosin and compared the topography of positive and negative surface charges with the correspondent structures of cow and camel chymosins. We found that the distribution of charges on the surface of moose chymosin has common features with that of cow and camel chymosins. However, the moose enzyme carries a unique positively charged patch, which is likely to affect its interaction with the substrate. Biochemical and technological properties of the moose rChn were studied. Commercial rСhns of cow and camel were used as comparison enzymes. In some technological parameters, the moose rChn proved to be superior to the reference enzymes. Сompared with the cow and camel rСhns, the moose chymosin specific activity is less dependent on the changes in CaCl2 concentration in the range of 1–5 mM and pH in the range of 6–7, which is an attractive technological property. The total proteolytic activity of the moose rСhn occupies an intermediate position between the rСhns of cow and camel. The combination of biochemical and technological properties of the moose rСhn argues for further study of this enzyme.

Recombinant expression and characterization of Oryctolagus cuniculus chymosin in Komagataella phaffii (Pichia pastoris)

This study was conducted for investigating expression and enzymatic characteristics of recombinant Oryctolagus cuniculus chymosin (ROCC) expressed in Pichia pastoris. SDS-PAGE of partially purified supernatant displayed two distinct molecular bands approximately at the sizes of 40 kDa and 45 kDa corresponding to chymosin and partially glycosylated chymosin, respectively. Proteolysis assay demonstrated that rabbit chymosin was more specific compared to bovine and camel chymosins when it comes to hydrolyzing α, β, and κ-casein. Rabbit chymosin kept its stability in a wide pH range (3.0-6.0) at 37 °C for 8 h. Active chymosin exhibited maximum enzymatic activity at 40 °C and pH 4.0 with the addition of 75 mM CaCl₂. The ROCC clotting activity on donkey, cow, goat, lamb, camel milk was determined as 40, 10, 5.7, 3.07, and 2.66 IMCU/mL, respectively. These results revealed that ROCC might possess a potential for incorporation into cheese manufacture technology as a milk-clotting enzyme.

Characterization of the Altai Maral Chymosin Gene, Production of a Chymosin Recombinant Analog in the Prokaryotic Expression System, and Analysis of Its Several Biochemical Properties

For the first time, the chymosin gene (CYM) of a maral was characterized. Its exon/intron organization was established using comparative analysis of the nucleotide sequence. The CYM mRNA sequence encoding a maral preprochymosin was reconstructed. Nucleotide sequence of the CYM maral mRNA allowed developing an expression vector to ensure production of a recombinant enzyme. Recombinant maral prochymosin was obtained in the expression system of Escherichia coli [strain BL21 (DE3)]. Total milk-coagulation activity (MCA) of the recombinant maral chymosin was 2330 AU/ml. The recombinant maral prochymosin relative activity was 52955 AU/mg. The recombinant maral chymosin showed 100-81% MCA in the temperature range 30-50°C, thermal stability (TS) threshold was 50°C, and the enzyme was completely inactivated at 70°C. Preparations of the recombinant chymosin of a single-humped camel and recombinant bovine chymosin were used as reference samples. Michaelis-Menten constant (K_m), turnover number (k_cat), and catalytic efficiency (k_cat/K_m) of the recombinant maral chymosin, were 1.18 ± 0.1 µM, 2.68 ± 0.08 s^-1 and 2.27± 0.10 µm M^-1·s^-1, respectively.

Temporal changes of abomasal contents and volumes in calves fed milk diluted with oral rehydration salt solution

Several manufacturers recommend to feed mixture comprising equal amounts of oral rehydration salt (ORS) solution and milk for diarrheic calves after milk withdrawal. Such a feeding method is expected to supply more nutrients and energy compared to feeding only the ORS solution. However, little is known about the effects of feeding milk diluted with ORS solution on calves' digestive process. This study examined the abomasal contents, volumes, and emptying rates in calves fed whole milk, milk diluted by 50% with ORS solution (50% ORS-milk), and ORS solution. Ultrasonography identified curds in the milk-fed calves, but not in the 50% ORS-milk-fed or the ORS-fed calves. The abomasal fluid of the 50% ORS-milk-fed calves contained not only β-lactoglobulin but also α-casein (CN), β-CN, and κ-CN, which were used for curd formation and undetectable in the milk-fed calves. Abomasal pH was relatively higher in the 50% ORS-milk-fed than that in the milk-fed calves. Abomasal emptying rates were significantly faster in the ORS-fed than in the 50% ORS-milk-fed and the milk-fed calves. These data indicate that the formation of abomasal curd is inhibited in the 50% ORS-milk-fed calves due to the resultant high abomasal pH and low κ-CN concentration. The 50% ORS-milk may not provide rehydration as quickly as the ORS solution. In conclusion, we do not recommend feeding 50% ORS-milk to calves.

Microbial Production of Recombinant Rennet

Rennet, traditionally obtained from calves, is non-vegeterian and unethical due to the slaughter of unweaned animals. Chymosin is highly specific to the Phe105-Met106 bond of κ-casein and has low proteolytic activity. Microbial aspartic proteases can partly replace chymosin. However, recombinant DNA technology has allowed chymosin itself to be produced by bacteria, yeast, and molds. Not only rennet from calf, but from animals like goat kid, lamb, buffalo, camel, and others can be used in cheesemaking. Chymosins of these animals can be cloned and successfully expressed in microorganisms and can be employed in the production of novel as well as traditional cheese products from the milk of camel, goat, and even horse and donkey. This chapter outlines the recombinant DNA techniques applied over the past few years to improve the microbial production of recombinant rennet, from animals and plants.

Production of Bioactive Recombinant Bovine Chymosin in Tobacco Plants

Chymosin (also known as rennin) plays an essential role in the coagulation of milk in the cheese industry. Chymosin is traditionally extracted from the rumen of calves and is of high cost. Here, we present an alternative method to producing bovine chymosin from transgenic tobacco plants. The CYM gene, which encodes a preprochymosin from bovine, was introduced into the tobacco nuclear genome under control of the viral 35S cauliflower mosaic promoter. The integration and transcription of the foreign gene were confirmed with Southern blotting and reverse transcription PCR (RT-PCR) analyses, respectively. Immunoblotting analyses were performed to demonstrate expression of chymosin, and the expression level was quantified by enzyme-linked immunosorbent assay (ELISA). The results indicated recombinant bovine chymosin was successfully expressed at an average level of 83.5 ng/g fresh weight, which is 0.52% of the total soluble protein. The tobacco-derived chymosin exhibited similar native milk coagulation bioactivity as the commercial product extracted from bovine rumen.

Cloning and Expression of Yak Active Chymosin in Pichia pastoris

Rennet, a complex of enzymes found in the stomachs of ruminants, is an important component for cheese production. In our study, we described that yak chymosin gene recombinant Pichia pastoris strain could serve as a novel source for rennet production. Yaks total RNA was extracted from the abomasum of an unweaned yak. The yak preprochymosin, prochymosin, and chymosin genes from total RNA were isolated using gene specific primers based on cattle chymosin gene sequence respectively and analyzed their expression pattern byreal time-polymerase chain reaction. The result showed that the chymosin gene expression level of the sucking yaks was 11.45 times higher than one of adult yaks and yak chymosin belongs to Bovidae family in phylogenetic analysis. To express each, the preprochymosin, prochymosin, and chymosin genes were ligated into the expression vector pPICZαA, respectively, and were expressed in Pichia pastoris X33. The results showed that all the recombinant clones of P. pastoris containing the preprochymosin, prochymosin or chymosin genes could produce the active form of recombinant chymosin into the culture supernatant. Heterologous expressed prochymosin (14.55 Soxhlet unit/mL) had the highest enzyme activity of the three expressed chymosin enzymes. Therefore, we suggest that the yak chymosin gene recombinant Pichia pastoris strain could provide an alternative source of rennet production.

Bioprocess and downstream optimization of recombinant bovine chymosin B in Pichia (Komagataella) pastoris under methanol-inducible AOXI promoter

A clone of the methylotrophic yeast Pichia pastoris strain GS115 transformed with the bovine prochymosin B gene was used to optimize the production and downstream of recombinant bovine chymosin expressed under the methanol-inducible AOXI promoter. Cell growth and recombinant chymosin production were analyzed in flask cultures containing basal salts medium with biodiesel-byproduct glycerol as the carbon source, obtaining values of biomass level and milk-clotting activity similar to those achieved with analytical glycerol. The effect of biomass level at the beginning of methanol-induction phase on cell growth and chymosin expression was evaluated, determining that a high concentration of cells at the start of such period generated an increase in the production of chymosin. The impact of the specific growth rate on chymosin expression was studied throughout the induction stage by methanol exponential feeding fermentations in a lab-scale stirred bioreactor, achieving the highest production of heterologous chymosin with a constant specific growth rate of 0.01h(-1). By gel filtration chromatography performed at a semi-preparative scale, recombinant chymosin was purified from exponential fed-batch fermentation cultures, obtaining a specific milk-clotting activity of 6400IMCU/mg of chymosin and a purity level of 95%. The effect of temperature and pH on milk-clotting activity was analyzed, establishing that the optimal temperature and pH values for the purified recombinant chymosin are 37°C and 5.5, respectively. This study reported the features of a sustainable bioprocess for the production of recombinant bovine chymosin in P. pastoris by fermentation in stirred-tank bioreactors using biodiesel-derived glycerol as a low-cost carbon source.

A carrier fusion significantly induces unfolded protein response in heterologous protein production by Aspergillus oryzae

In heterologous protein production by filamentous fungi, target proteins are expressed as fusions with homologous secretory proteins, called carriers, for higher production yields. Although carrier fusion is thought to overcome the bottleneck in transcriptional and (post)translational processes during heterologous protein production, there is limited knowledge of its physiological effects on the host strain. In this study, we performed DNA microarray analysis by comparing gene expression patterns of two Aspergillus oryzae strains expressing either carrier- or non-carrier-fused bovine chymosin (CHY). When CHY was expressed as a fusion with α-amylase (AmyB), the production level increased by approximately 2-fold as compared with the non-carrier-fused CHY. DNA microarray analysis revealed that the carrier fusion significantly up-regulated many genes involved in endoplasmic reticulum (ER) protein-folding and secretion. Consistently, hacA transcripts were efficiently spliced in the strain expressing the carrier-fused CHY, indicating an unfolded protein response (UPR). The carrier-fused CHY was detected intracellularly without processing at the Kex2 cleavage site, which is likely recognized in the Golgi, and the carrier fusion delayed extracellular CHY production in the early growth phase as compared with the non-carrier-fused expression. Taken together, our data suggest a proposal that the carrier fusion temporarily accumulates the carrier-fused CHY in the ER and significantly induces UPR.

Salting-in characteristics of globular proteins

Protein solubility, and the formation of various solid phases, is of interest in both bioprocessing and the study of protein condensation diseases. Here we examine the the phase behavior of three proteins (chymosin B, β-lactoglobulin B, and pumpkin seed globulin) previously known to display salting-in behavior, and measure their solubility as a function of pH, ionic strength, and salt type. Although the phase behavior of the three proteins is quantitatively different, general trends emerge. Stable crystal nucleation does not occur within the salting-in region for the proteins examined, despite the crystal being observed as the most stable solid phase. Instead, two types of amorphous phases were found within the salting-in region; additionally, an analog to the instantaneous clouding curve was observed within the salting-in region for chymosin B. Also, protein solutions containing sulfate salts resulted in different crystal morphologies depending on whether Li(2)SO(4) or (NH(4))(2)SO(4) was used.

Cloning and expression of buffalo active chymosin in Pichia pastoris

To date, only recombinant chymosin has been obtained in its active form from supernatants of filamentous fungi, which are not as good candidates as yeasts for large-scale fermentations. Since Bos taurus chymosin was cloned and expressed, the world demand for this protease has increased to such an extent that the cheesemaking industry has been looking for novel sources of chymosin. In this sense because buffalo chymosin has properties that are more stable than those of B. taurus chymosin, it may occupy a space of its own in the chymosin market. The main objective of the present work was the production of active recombinant buffalo chymosin in the culture supernatant of Pichia pastoris . This yeast has demonstrated its usefulness as an excellent large-scale fermentation tool for the secretion of recombinant foreign proteins. RNA was extracted from the abomasum of a suckling calf water buffalo ( Bubalus arnee bubalis ). Preprochymosin, prochymosin, and chymosin DNA sequences were isolated and expressed into P. pastoris. Only the recombinant clones of P. pastoris containing the prochymosin sequence gene were able to secrete the active form of the chymosin to the culture supernatant. This paper describes for the first time the production of active recombinant chymosin in P. pastoris without the need of a previous in vitro activation. The new recombinant yeast strain could represent a novel and excellent source of rennet for the cheesemaking industry.

Pepsinogens and pepsins from mandarin fish (Siniperca chuatsi)

Four pepsinogens (PG-I, PG-II, PG-III(a), and PG-III(b)) were highly purified from the stomach of the freshwater fish mandarin fish (Siniperca chuatsi) by ammonium sulfate fractionation, anion exchange, and gel filtration. The molecular masses of the four purified PGs were 36, 35, 38, and 35 kDa, respectively. All the pepsinogens converted into their active form pepsins within a few minutes under pH 2.0. The optimum pH and temperature of the four enzymes were 3.0-3.5 and 45-50 degrees C, using hemoglobin as a substrate. The N-terminal amino acid sequences of PG-I and PG-II were determined to the 12th and 17th amino acid residues, respectively. Western blot analysis using antisea bream polyclonal antibodies cross reacted with PG-I, PG-II, and PG-III(b) while no cross reaction with PG-III(a) was detected, suggesting the diversity of pepsinogens in fish.

Kinetic analysis of a general model of activation of aspartic proteinase zymogens

Starting from a simple general reaction mechanism of activation of aspartic proteinase zymogens involving an uni- and a bimolecular simultaneous route, the time course equation of the concentration of the zymogen and of the activated enzyme have been derived. From these equations, an analysis quantifying the relative contribution to the global process of the two routes has been carried out for the first time. This analysis suggests a way to predict the time course of the relative contribution as well as the effect of the initial zymogen and activating enzyme concentrations, on the relative weight. An experimental design and kinetic data analysis is suggested to estimate the kinetic parameters involved in the reaction mechanism proposed. Finally, we apply some of our results to experimental data obtained by other authors in experimental studies of the activation of some aspartic proteinase zymogens.

Effect of orally administered omeprazole on abomasal luminal pH in dairy calves fed milk replacer

The objective of this study was to determine whether oral administration of omeprazole, a proton-pump inhibitor, increased abomasal luminal pH in calves fed milk replacer. Four male dairy calves with cannulae in the abomasal body suckled milk replacer (60 ml/kg body weight every 12 h) and were administered a non-enteric-coated omeprazole (4 mg/kg body weight every 24 h) in a paste formulation for five successive days. Abomasal luminal pH was continuously measured using miniature glass pH electrodes. On the first day of omeprazole administration, there was a significant (P<0.05) increase in mean 24-h pH from 2.89 to 4.17. The mean 24-h pH on days 2, 3, 4 and 5 of omeprazole administration were 3.85, 4.02, 3.97 and 3.39 respectively. We conclude that oral administration of non-enteric-coated omeprazole increased abomasal luminal pH in calves fed milk replacer, but that the effect may decrease over time.

Molecular cloning and expression in yeast of caprine prochymosin

We cloned and characterized a preprochymosin cDNA from the abomasum of milk-fed kid goats. This cDNA contained an open reading frame that predicts a polypeptide of 381 amino acid residues, with a signal peptide and a proenzyme region of 16 and 42 amino acids, respectively. Comparison of the caprine preprochymosin sequence with the corresponding sequences of lamb and calf revealed 99 and 94% identity at the amino acid level. The cDNA fragment encoding the mature portion of caprine prochymosin was fused in frame both to the killer toxin signal sequence and to the alpha-factor signal sequence-FLAG in two different yeast expression vectors. The recombinant plasmids were transformed into Kluyveromyces lactis and Saccharomyces cerevisiae cells, respectively. Culture supernatants of both yeast transformants showed milk-clotting activity after activation at acid pH. The FLAG-prochymosin fusion was purified from S. cerevisiae culture supernatants by affinity chromatography. Proteolytic activity assayed toward casein fractions indicated that the recombinant caprine chymosin specifically hydrolysed kappa-casein.

Chaperone characteristics of PDI-related protein A from Aspergillus niger

The functional properties of a novel protein, protein disulfide isomerase-related protein A (PRPA) from Aspergillus niger T21, have been characterized. (1) PRPA possesses disulfide isomerase activity. (2) In Hepes buffer, at substoichiometric concentrations, PRPA facilitates the formation of inactive lysozyme aggregates associated with PRPA (anti-chaperone activity); while at a high molar excess, PRPA inhibits aggregation by maintaining lysozyme in a soluble, yet inactive, state (chaperone-like activity). However, PRPA only exhibits chaperone-like activity during lysozyme refolding in phosphate buffer. (3) Experiments have indicated that disulfide cross-linkage is not required for the interaction between PRPA and lysozyme, and hydrophobic interaction may be responsible for PRPA effect on lysozyme. (4) Co-expression of PRPA and prochymosin in Escherichia coli leads to reduction of inclusion bodies, rendering part of prochymosin molecules soluble yet inactive. The structural and functional characteristics of PRPA suggest that PRPA may play an important role in protein folding, aggregation, and retention in the endoplasmic reticulum.

Bovine chymosin: production by rDNA technology and application in cheese manufacture

Bovine chymosin, an aspartyl protease extracted from abomasum of suckling calves, is synthesized in vivo as preprochymosin and secreted as prochymosin which is autocatalytically activated to chymosin. Chymosin is bilobular, with Asp 32 and Asp 215 acting as the catalytic residues. Chymosin A and chymosin B have pH optima of 4.2 and 3.8, respectively, and act to initiate milk clotting by cleaving kappa-casein between Phe 105 and Met 106. The gene encoding chymosin has been cloned and expressed in suitable bacteria and yeast hosts under the control of lac, trp, trp-beta, gly A genes, and serine hydroxymethyl-transferase promoters. Protein engineering of chymosin has also been attempted. A number of companies are now producing recombinant chymosin for commercial use in cheese manufacture.

Expression of a synthetic copy of the bovine chymosin gene in Aspergillus awamori from constitutive and pH-regulated promoters and secretion using two different pre-pro sequences

A copy of the bovine chymosin gene (chy) with a codon usage optimized for its expression in Aspergillus awamori was constructed starting from synthetic oligonucleotides. To study the ability of this filamentous fungus to secrete bovine prochymosin, two plasmids were constructed in which the transcriptional, translational, and secretory control regions of the A. nidulans gpdA gene and pepB genes were coupled to either preprochymosin or prochymosin genes. Secretion of a protein enzymatically and immunologically indistinguishable from bovine chymosin was achieved in A. awamori transformants with each of these constructions. In all cases, the primary translation product (40.5 kDa) was self-processed to a mature chymosin polypeptide having a molecular weight of 35.6 kDa. Immunological assays indicated that most of the chymosin was secreted to the extracellular medium. Hybridization analysis of genomic DNA from chymosin transformants showed chromosomal integration of prochymosin sequences and, in some transformants, multiple copies of the expression cassettes were observed. Expression from the gpdA promoter was constitutive, whereas expression from the pepB promoter was strongly influenced by pH. A very high expression from the pepB promoter was observed during the growth phase. The A. awamori pepB gene terminator was more favorable for chymosin production than the S. cerevisiae CYC1 terminator.

Short communication: Identification and characterization of multiple splicing forms of bovine prochymosin mRNA

Bovine prochymosin (bPC) is an inactive precursor of the milk clotting enzyme chymosin (EC 3.4.23.4), that is present in the abomasum of suckling calves. We investigated the pattern of bPC mRNA expression in the calf stomach tissues by RT-PCR assay and sequence analysis of cloned RT-PCR products. We identified multiple isoforms appearing due to alternative splicing of bPC mRNA. Alternative mRNA forms were generated by skipping one to four full exons within the bPC gene. Various splicing events resulted in seven bPC transcripts, which are 99, 114, 213, 237, 336, 351 and 450 nucleotides shorter compared to full-length mRNA. Analysis of amino acid sequences deduced from alternatively spliced mRNA sequences showed no amino acid transversions and no protein reading frame shift for any splice forms.

Effect of feeding frequency and route of administration on abomasal luminal pH in dairy calves fed milk replacer

The aim of this study was to determine the effect of feeding frequency and route of administration on abomasal luminal pH in suckling calves. Six male dairy calves with cannulae in the abomasal body were administered the following six treatments in a randomized crossover design: 24 h fasting, suckling of a high-quality milk replacer (all-milk protein; 12% of body weight [BW]/d) at 12-h (2x), 8-h (3x), 6-h (4x), and 3-h (8x) intervals, and ruminal intubation of milk replacer (12% of body weight/day) at a 12-h (2x) interval. Abomasal luminal pH was measured every second for 24 h with miniature glass pH electrodes. Least squares mean 24-h fasting abomasal luminal pH was 1.73, whereas mean 24-h pH after suckling and intubation of milk replacer every 12 h were higher at 3.44 and 3.17, respectively. Increasing the frequency of milk replacer suckling to 3x, 4x, and 8x increased mean 24-h abomasal luminal pH; however, there was no difference in mean 24-h pH between 3x (3.69), 4x (3.64), and 8x (3.67) suckling. The percentage of the 24-h recording period that abomasal luminal pH was > 3.0 was 0, 49, 53, 61, 61, and 71% for fasting, 2x intubation of milk replacer, and 2x, 3x, 4x, and 8x suckling of milk replacer, respectively. Increasing the frequency of milk replacer suckling may be efficacious in the prophylaxis of abomasal ulceration in milk-fed calves.

Effect of an orally administered antacid agent containing aluminum hydroxide and magnesium hydroxide on abomasal luminal pH in clinically normal milk-fed calves

OBJECTIVE

To determine the effects of a commercially available orally administered antacid agent containing aluminum hydroxide and magnesium hydroxide on abomasal luminal pH in clinically normal milk-fed calves.

DESIGN

Randomized trial.

ANIMALS

5 male dairy calves.

PROCEDURE

Throughout the study, calves were fed milk replacer at 7:30 AM and 7:30 PM. Cannulae for pH electrodes were placed in the abomasal body and pyloric antrum. Treatments consisted of oral administration of a high (50 ml) or low (25 ml) dose of the antacid agent and oral administration of milk replacer alone (control). Antacid was given at 7:30 AM, 3:30 PM, and 11:30 PM, and luminal pH was monitored continuously for 24 hours, beginning 15 minutes before administration of the first dose of antacid.

RESULTS

Administration of the first dose of antacid at the time of the morning feeding resulted in an increase in mean abomasal body luminal pH of < 1 pH unit, whereas administration of the second and third doses of the antacid caused transient (< 3 hours) increases in mean luminal pH of approximately 1.5 (low dose) and 2.5 (high dose) pH units.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that clinically normal milk-fed calves given a commercially available antacid agent, PO, will have a transient increase in abomasal luminal pH. Such agents may, therefore, have a role in the treatment of abomasal ulceration in calves; however, the long-term effects of orally administered antacid agents in milk-fed calves and the clinical efficacy of such agents in treating abomasal ulceration remain to be determined.

A basic residue at position 36p of the propeptide is not essential for the correct folding and subsequent autocatalytic activation of prochymosin

Position 36p in the propeptides of gastric aspartic proteinases is generally occupied by lysine or arginine. This has led to the conclusion that a basic residue at this position, which interacts with the active-site aspartates, is essential for folding and activation of the zymogen. Lamb prochymosin has been shown by cDNA cloning to possess glutamic acid at 36p. To investigate the effect of this natural mutation which appears to contradict the proposed role of this residue, calf and lamb prochymosins and their two reciprocal mutants, K36pE and E36pK, respectively, were expressed in Escherichia coli, refolded in vitro, and autoactivated at pH 2 and 4.7. All four zymogens could be activated to active chymosin and, at both pH values, the two proteins with Glu36p showed higher activation rates than the two Lys36p forms. Glu36p was also demonstrated in natural prochymosin isolated from the fourth stomach of lamb, as well as being encoded in the genomes of sheep, goat and mouflon, which belong to the subfamily Caprinae. A conserved basic residue at position 36p of prochymosin is thus not obligatory for its folding or autocatalytic activation. The apparently contradictory results for porcine pepsinogen A [Richter, C., Tanaka, T., Koseki, T. & Yada, R.Y. (1999) Eur. J. Biochem. 261, 746-752] can be reconciled with those for prochymosin. Lys/Arg36p is involved in stabilizing the propeptide-enzyme interaction, along with residues nearer the N-terminus of the propeptide, the sequence of which varies between species. The relative contribution of residue 36p to stability differs between pepsinogen and prochymosin, being larger in the former.

Mass spectrometric characterisation of proteins in rennet and in chymosin-based milk-clotting preparations

The protein composition of natural rennet and of chromatographic and crystalline chymosin preparations has been defined by on-line reverse-phase high performance liquid chromatography/electrospray ionisation mass spectrometry (RP-HPLC/ESI-MS) and by tandem mass spectrometry (MS/MS). Natural rennet was found to consist of six chymosin species, corresponding to chymosin A and B genetic variants, each of which comprised a mixture of two other forms differing at theN-terminal end, with one being three residues longer, and the other two residues shorter, than the mature chymosin. Two main tissue proteins were also identified as lysozyme (isozyme 2 plus a novel isozyme labelled 4) and bovine serum albumin. In addition to the proteins, chymosin fragments 247-323 and 288-323 were consistently present in natural rennet. Conversely, chromatographic and crystalline chymosin preparations lacked bovine serum albumin and/or lysozyme, although they contained the same six chymosin species as natural rennet. Since these tissue-specific contaminating proteins each possess specific functions in terms of stabilising enzyme solutions and protecting proteins from proteolytic enzymes, oxidising agents and bacterial proliferation, the rennet may be considered as a functional enzyme preparation that is effectively and naturally adapted to the purposes of cheesemaking. In practice, the highly complex protein composition inherent to natural rennet provided the possibility to differentiate the natural product from other bovine chymosin-based milk-clotting preparations examined in this work.

The plant aspartic proteinase-specific polypeptide insert is not directly related to the activity of oryzasin 1

Many plant aspartic proteinases (APs) are different from animal and microbial APs in that they contain a polypeptide insert, approximately 100 amino acids in length, in the C-terminal region. To interpret the significance of this insert, we constructed an expression system for rice AP oryzasin 1 by linking a pro-oryzasin 1 downstream of glutathione S-transferase (GST). GST-proOS1 expressed the highest degree of hemoglobin-hydrolytic activity when treated at pH 3.3 and incubated for 24 h at room temperature. We carried out a similar experiment using an insert-lacking proOS1 mutant, GST-DeltaproOS1, as the fusion protein, and found it to show similar activity. This result indicates that the insert is not involved in the production of AP activity. We then investigated the autolysis of the two proteins by Western blot analysis. GST-proOS1 was autolyzed into 67- and 64-kDa fragments, while GST-DeltaproOS1 autolyzed to 54- and 52-kDa products. GST-DeltaproOS1 clearly produced two molecular species early in the autolytic process, and not later than 3 h from the start, but no such clear result was observed in the case of GST-proOS1. This suggests that, although the presence of the plant AP-specific insert does not influence the enzyme activity by itself, it apparently has an effect on the autolysis of OS1.

In vivo processing of nonanchored Yapsin 1 (Yap3p)

A C-terminally truncated form of yapsin 1 (yeast aspartic protease 3) was overexpressed in yeast and its processing through the secretory pathway was followed by pulse-labeling and immunoprecipitation studies. In the soluble cell extract, three forms of yapsin 1-87, 74, and 18 kDa-were found. Identification of these forms of yapsin 1 using different antisera suggests that the 87-kDa form is pro-yapsin 1, which is processed into two subunits, alpha (18 kDa) and beta (74 kDa), by cleavage at a loop region not found in traditional aspartic proteases. By use of a temperature-sensitive mutant strain, sec18, the generation of the two subunits was found to occur in the endoplasmic reticulum. An active site-mutated yapsin 1 was not processed into the two subunits, suggesting that this process occurs in an autocatalytic manner.

Mechanisms and kinetics of procathepsin D activation

In vitro, procathepsin D is activated to pseudocathepsin D by incubation at low pH. To investigate the mechanism of this activation, recombinant human procathepsin D and two mutants were generated in a baculovirus expression system. One mutant carried a point mutation within the catalytic domain, which resulted in a catalytically inactive enzyme form (D77A). The other carried a point mutation within the propeptide, which prevented activation by processing at the 'autoproteolysis-site' (L26P). Neither mutant is capable of processing itself to form pseudocathepsin D, and L26P is not able to process D77A. Despite the inability of L26P to cleave either its own or a wild-type prosequence, it did exhibit activity against a synthetic peptide substrate. The ability of intact precursor (zymogen) to cleave a peptide, but not a protein substrate, offers new insights into the mechanism of inhibition by the propeptide. Mature cathepsin D can process the inactive D77A mutant to the pseudoform, demonstrating that processed species are capable of cleaving zymogen molecules in an intermolecular interaction. In addition, kinetic studies provide evidence for a two-phase mechanism for the conversion of procathepsin D to pseudocathepsin D, one phase where the first molecules of pseudocathepsin D are formed at a low rate and a second phase where the process is autocatalytically accelerated by newly formed pseudocathepsin D molecules. Finally, with the help of the mutants L26P and D77A it was observed that at least two additional proteinase activities, found in conditioned media from insect cell culture, are capable of activating procathepsin D by cleaving it within the proregion. This observation suggests that there are likely to be multiple proteinases in the extracellular matrix that are capable of activating procathepsin D, thereby triggering the second autocatalytic phase. This may also be important for solid tumors, where the presence of cathepsin D has been correlated with tumor growth and invasion.

Mechanism of activation of the gastric aspartic proteinases: pepsinogen, progastricsin and prochymosin

The gastric aspartic proteinases (pepsin A, pepsin B, gastricsin and chymosin) are synthesized in the gastric mucosa as inactive precursors, known as zymogens. The gastric zymogens each contain a prosegment (i.e. additional residues at the N-terminus of the active enzyme) that serves to stabilize the inactive form and prevent entry of the substrate to the active site. Upon ingestion of food, each of the zymogens is released into the gastric lumen and undergoes conversion into active enzyme in the acidic gastric juice. This activation reaction is initiated by the disruption of electrostatic interactions between the prosegment and the active enzyme moiety at acidic pH values. The conversion of the zymogen into its active form is a complex process, involving a series of conformational changes and bond cleavage steps that lead to the unveiling of the active site and ultimately the removal and dissociation of the prosegment from the active centre of the enzyme. During this activation reaction, both the prosegment and the active enzyme undergo changes in conformation, and the proteolytic cleavage of the prosegment can occur in one or more steps by either an intra- or inter-molecular reaction. This variability in the mechanism of proteolysis appears to be attributable in part to the structure of the prosegment. Because of the differences in the activation mechanisms among the four types of gastric zymogens and between species of the same zymogen type, no single model of activation can be proposed. The mechanism of activation of the gastric aspartic proteinases and the contribution of the prosegment to this mechanism are discussed, along with future directions for research.

Functional implications of the 21-24 loop in recombinant prochymosin

To investigate the role of the 21-24 (pepsin numbering) loop in prochymosin, the amino acid residues GTPP at positions 21 through 24 were replaced with GG, the equivalent loop residues from its homologous protein, penicillopepsin, or SG, GS by site-directed mutagenesis. The mutants except GTPP(21-24)GS could be expressed in Escherichia coli. Activation studies indicated that the refolded prochymosin mutants were capable of undergoing autocatalytic activation to produce pseudochymosin by cleaving its N-terminal 27 amino acid residues at pH 2. The resulting pseudochymosin mutants were able to convert into chymosin at pH 5.5 by further autocatalytic cleavage to remove additional 15 amino acid residues. These results demonstrate that the prochymosin analogs can fold into an active state from an unfolded state and that the pseudochymosin analogs can proceed in the transformation from one active form into another active form. Spectroscopic analyses revealed that after mutation the far UV CD spectrum of prochymosin was considerably modified, showing less negative ellipticity values, and the fluorescence emission intensities of prochymosin and pseudochymosin were remarkably reduced. The stabilities of prochymosin and pseudochymosin, especially, were dramatically decreased. The stabilization energy of prochymosin was reduced by 7-8 kJ/mol. The inactivation temperature of pseudochymosin was decreased by 15-20 degrees C. The wild-type pseudochymosin was stable at pH 1.5 and 6.5, whereas the mutants were completely inactivated at the same pH values. Taken together, it is reasonable to conclude that the 21-24 loop (GTPP) plays an important role in determining the stability of prochymosin and pseudochymosin, although the mutants with mutated loop (GG or SG) still can refold into an active conformation.

Activation and processing of non-anchored yapsin 1 (Yap3p)

A C-terminally truncated form of yapsin 1 (yeast aspartic protease 3), the first member of the novel sub-class of aspartic proteases with specificity for basic residues (designated the Yapsins), was overexpressed and purified to apparent homogeneity, yielding approximately 1 microg of yapsin 1/g of wet yeast. N-terminal amino acid analysis of the purified protein confirmed that the propeptide was absent and that the mature enzyme began at Ala68. The mature enzyme was shown to be composed of approximately equimolar amounts of two subunits, designated alpha and beta, that were associated to each other by a disulfide bond. C-terminally truncated proyapsin 1 was also expressed in the baculovirus/Sf9 insect cell expression system and secreted as a zymogen that could be activated upon incubation at an acidic pH with an optimum at approximately 4.0. When expressed without its pro-region, it was localized intracellularly and lacked activity, indicating that the pro-region was required for the correct folding of the enzyme. The activation of proyapsin 1 in vitro exhibited linear kinetics and generated an intermediate form of yapsin 1 or pseudo-yapsin 1.

Functional implications of disulfide bond, Cys45-Cys50, in recombinant prochymosin

Bovine prochymosin (chymosin) contains three disulfide bonds: Cys45-Cys50, Cys206-Cys210 and Cys250-Cys283 (pepsin numbering). We have demonstrated that Cys250-Cys283 is not intimately involved in the catalytic mechanism of chymosin but is required for the correct refolding of prochymosin. To elucidate the functional implications of another disulfide bond of Cys45-Cys50, these two cysteine residues were replaced separately or simultaneously by site-directed mutagenesis. Like the wild-type prochymosin all the seven mutants generated (C45A, C50A, C45A/C50A, C45D, C50S, C45D/C50S, C45A/C50S) exhibit the activity of autocatalytic activation after refolding, indicating that Cys45-Cys50 is dispensable in prochymosin refolding. Spectroscopic analyses and urea-induced denaturation studies of prochymosin and four mutants tested (C45A, C50A, C45A/C50A, C45D/C50S) show that: (1) they share similar far-UV CD spectra and similar fluorescence emission spectra; (2) mutation results in a perturbance of tryptophan environment and somewhat destabilization of prochymosin conformation. However, quenching studies reveal that the only one tryptophan residue inaccessible to acrylamide is still buried in the mutated molecules. All these results suggest that the overall conformation of prochymosin is maintained after mutation. As for the enzymatic properties of pseudochymosin, the activation product of prochymosin, it has been found that elimination of Cys45-Cys50 causes a marked drop of thermostability and an alteration of substrate specificity.

Prochymosin polymorphism in calves of black-and-white cattle and their crosses with Simental bulls

Polymorphism of prochymosin was observed in individual calf abomasa, using agarose gel electrophoresis followed by detection of proteolytic activity. Abomasum samples were randomly collected during slaughtering from 239 and 146 calves (3-5 weeks old) of Black-and-White cattle and their crosses with Simental bulls, respectively. Four distinct prochymosins were found and, according to their decreasing electrophoretic mobility in alkaline agarose gel, termed as prochymosin A, D, B and C which occurred singly and in pairs (then with equal proteolytic activities of both components). Prochymosin A, B and C (designation according to FOLTMANN, 1966) activated at pH 4.7 was transformed into electrophoretically distinct chymosin. When prochymosin D was activated at this pH, chymosin D showed similar mobility as chymosin B both at alkaline and acidic pHs. Prochymosin variants occurred at genetical equilibrium in nine and ten phenotypes in the first and second genetic group. The distribution of phenotypes in the two groups differed significantly (P < 0.05). The gene frequencies of prochymosin A, D, B and C were 0.35, 0.11, 0.52 and 0.02 in Black-and-White calves, and 0.39, 0.08, 0.47 and 0.06 in crosses, respectively. These prochymosins were controlled by four pairs of codominant alleles. A possible correlation of the results obtained by FOLTMANN (1966) with ours and those of ASATO and RAND (1972, 1977) was discussed.

Strain improvement of chymosin-producing strains of Aspergillus niger var. awamori using parasexual recombination

Parasexual recombination was used to obtain improved chymosin-producing strains and to perform genetic analysis on existing strains. Chlorate resistance was used to select for a variety of spontaneous nitrate assimilation pathway mutations in strains previously improved for chymosin production using classical strain improvement methods including mutation and screening, and selection for 2-deoxyglucose resistance (dgr). Diploids of these improved strains were generated via parasexual recombination and were isolated on selective media by complementation of nitrate assimilation mutations. A preliminary genetic analysis of diploid and haploid segregants indicated that the dgr trait, resulting in overexpression of chymosin, was recessive. Also, mutations in two different dgr genes resulted in an increased level of chymosin production. When these mutations were combined via parasexual recombination, the resulting haploid segregants produced about 15% more chymosin than either parental strain. CHEF gel electrophoresis was used to determine the chromosomal location of the integrated chymosin DNA sequences, and to verify diploidy in one case where the chromosome composition of two haploid parents differed.

Occurrence of prochymosin variants in the abomasum of bovine foetuses and calves

Prochymosin variants were analysed in single abomasa from 67 foetuses (3-9 months of gestation) and 33 calves (about 6 weeks old) of Black-and-White cattle collected in a slaughter house. The method of agarose gel electrophoresis followed by detection of proteolytic activity was used. Three distinct prochymosins A, B and C that occurred singly or in pairs (with equal proteolytic activities of both components) were found. Chymosin A, B and C obtained after conversion of corresponding prochymosins, demonstrated similar electrophoretical mobilities like the three chymosin fractions contained in commercial rennin (Sigma, USA). Our chymosin B showed identical mobility as the amidated form of recombined chymosin B contained in Chymogen (Chr. Hansen's Lab., Denmark A/S). Prochymosin A, B and C in the examined animals were precursors of corresponding chymosins and were controlled by three separate codominant alleles. The following prochymosin phenotypes were found: AA (30), AB (32), AC (4), BB (29), BC (4) and CC (1). Chi-square analysis demonstrated significant differences between the observed and expected numbers of phenotypes. The gene frequencies of prochymosin A, B and C were 0.48, 0.47 and 0.05, respectively.

Activation of porcine pepsinogen A. The stability of two non-covalent activation intermediates at pH 8.5

Reaction products formed during activation of porcine pepsinogen A at pH 2 were characterized by native agar-gel electrophoresis and by denaturing SDS/PAGE. The results revealed the presence of non-covalent intermediates between prosegment peptides and pepsin. The complexes Leu1p-Leu44p/pepsin and Leu1p-Leu16p/pepsin were isolated (the prosegment residues are characterized by the suffix p; numbering of residues starts again from the N-terminus of pepsin). Relative to mature pepsin, the inherent milk-clotting activities of the intermediates were 3% and 18%, respectively. The intermediates were incubated at pH 8.5 for 20 min at 28 degrees C and the residual proteolytic activities were tested at pH 2. The stabilities at pH 8.5 were between those of pepsinogen and pepsin, Leu1p-Leu44p/pepsin being most stable. The implications of these findings for the conformational changes that occur during the activation of pepsinogen are discussed.

Procathepsin D cannot autoactivate to cathepsin D at acid pH

The amino acid sequence of the propart of bovine procathepsin D was determined at the protein level. Incubation of the isolated procathepsin D at pH 3.5-5.0 for 30-120 min leads to a 2 kDa reduction in its molecular mass, as seen by SDS-PAGE. The activation product is pseudocathepsin D and is the result of a proteolytic cleavage between LeuP26 and IleP27 in the propart. Incubation at pH 5.0 for 20 h of either procathepsin D or pseudocathepsin D results in both cases in approximately equal amounts of pseudocathepsin D and a further processed intermediate, nine amino acids shorter than pseudocathepsin D. No reaction products corresponding to cathepsin D with a mature amino terminus were observed, showing that autoproteolysis alone cannot generate the mature form found in the lysosomes.

Expression and characterisation of chymosin pH optima mutants produced in Trichoderma reesei

The production of chymosin mutants designed to have altered pH optima using the cellulolytic filamentous fungus Trichoderma reesei is described. The strong promoter of the gene encoding the major cellulase, cellobiohydrolase I (CBHI) has been used for the expression and secretion of active calf chymosin. Structural analysis of the hydrogen bonding network around the two active site aspartates 32 and 215 in chymosin have suggested that residues Thr 218 and Asp 303 may influence the rate and pH optima for catalysis. The chymosin mutants Thr218Ala and the double mutant Thr218Ala/Asp303Ala have been made by site-directed mutagenesis and expressed in T. reesei. Enzyme kinetics of the active enzyme T218A indicate a pH optimum of 4.2 compared to 3.8 for native chymosin B using a synthetic octa-peptide substrate, confirming the previous analysis undertaken in E. coli. The double mutant T218A/D303A exhibits a similar optimum of 4.4 to that reported for the D303A, indicating that the combination of these changes is not additive. The application of protein engineering in the rational design of specific modifications to tailor the properties of enzymes offers a new approach to the development of industrial processes.

Isolation and characterization of a stable activation intermediate of the lysosomal aspartyl protease cathepsin D

Procathepsin D is the intracellular aspartyl protease precursor of cathepsin D, a major lysosomal enzyme. Procathepsin D is rapidly processed inside the cell, and, thus, examination of its proteolyic activation and structure has been difficult. To study this proenzyme, a nonglycosylated form of the human fibroblast procathepsin D was expressed in Escherichia coli, refold in vitro, and purified by affinity chromatography on pepstatinyl agarose. Sequence analysis of the refolded, autoactivated enzyme allowed determination of the autoproteolytic cleavage site. The sequence surrounding this cleavage site between residues LeuP26 and IleP27 (in the "pro" region) resembled the first cleavage site found during activation of other aspartyl proteases. Thus, the autoactivated procathepsin D is analogous to the pepsin activation intermediate, which has been termed pseudopepsin. The enzymatic activity, thermal and pH stability, and fluorescence spectra of pseudocathepsin D were compared to mature, predominantly two-chain, cathepsin D isolated from human placenta. The results indicated that pseudocathepsin D and mature enzyme have a similar Km toward a peptide substrate and cleave a protein substrate at identical sites. Temperature stability of the recombinant enzyme was similar to that of the tissue-derived enzyme. However, the recombinant enzyme had increased stability at low pH when compared to the glycosylated tissue-derived two-chain cathepsin D. Fluorescence spectra of the recombinant and tissue-derived enzymes were identical. Thus, the absence of asparagine-linked oligosaccharides and the presence of the remaining segment of propeptide did not significantly alter the structural and enzymatic properties of the enzyme.