Intact Enzymes Research Articles

Pressure Regulation of Vascular L-type Calcium Channels

Myogenic tone is dependent upon the activation of L-type Ca2+ channels and the rise in cytosolic [Ca2+]i. While L-type Ca2+ channel activation is often singularly linked to arterial depolarization, sparse evidence suggests that pressure itself may also enhance functional activity, through cooperative gating and trafficking. In this context, this study explored whether intravascular pressure indeed tune the myogenic response by enhancing the L-type Ca2+ channel trafficking and amplifying cooperative gating.Experimentation began in isolated smooth muscle cells (whole cell/single-channel patch clamp; immunohistochemistry; proximity ligation assay) and extended to intact arteries (pressure myography and Ca2+ measurements). Whole-cell electrophysiology revealed the ability of pressure stimuli to increase the L-type Ca2+ current, a response dependent on the intact cytoskeleton and enzymes tied to functional coupling (PKCα, AKAP150, PKA). Cell-attached electrophysiology confirmed whole-cell observations, registering increased coupling among L-type Ca2+ channels when cells were pressurized. Ensuing experiments on isolated arteries subsequently revealed linkage between intravascular pressure and enhanced L-type Ca2+ channel activity, independent of membrane depolarization.In closing, our results suggest there is more to L-type Ca2+ channel regulation than voltage gating and that pressure itself can enhance functional activity. Work continues to precisely define the pressure-sensitive signaling complexes within vascular smooth muscle. Supported by the Canadian Institute for Health Research, National Institute of Health and the Rorabeck Chair in Vascular Biology and Neuroscience. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Safety evaluation of a food enzyme containing trypsin and chymotrypsin from porcine pancreas.

The food enzyme is a serine protease complex containing trypsin (EC 3.4.21.4) and chymotrypsin (EC 3.4.21.1) obtained from porcine pancreas by Paninkret Chem.‐Pharm. Werk GmbH. The food enzyme is currently only used in protein processing to hydrolyse milk proteins. Milk protein hydrolysates and peptides are mainly used in formulae intended to have reduced allergenicity. Based on the recommended use level and the high consumption of formulae in very young babies, dietary exposure to the food enzyme–total organic solids (TOS) was estimated to be 180 mg TOS/kg body weight (bw) per day for infants and toddlers. Toxicological evaluation was based on the available clinical studies with pancreatic enzymes. Hypersensitivity to the product was identified as the major side effect. However, the intact enzymes are inactivated during preparation of food products; therefore, the Panel considered that the likelihood of adverse effects of the intact enzyme to occur is low. The Panel considered that a risk of allergic sensitisation to these protein hydrolysates after consumption cannot be excluded, but the likelihood of occurrence was considered to be low. Based on the origin of the food enzyme from edible parts of animals, the data provided and the evaluation of clinical studies with pancreatic enzymes and the estimated dietary exposure, the Panel concluded that the food enzyme does not give rise to safety concerns when used in the production of infant formulae based on milk protein hydrolysates.

Safety evaluation of a food enzyme containing trypsin, chymotrypsin, elastase and carboxypeptidase from porcine pancreas.

The food enzyme is a protease complex, containing trypsin (EC 3.4.21.4), chymotrypsin (EC 3.4.21.1), pancreatic elastase (EC 3.4.21.36) and carboxypeptidase B (EC 3.4.17.2), obtained from porcine pancreas by Neova Technologies Inc. The food enzyme is intended to be used for hydrolysis of whey proteins employed as ingredients of infant formulae, follow‐on formulae and in food for special medical purposes (tube feeding). Based on maximum use levels and the maximum permitted protein content in infant formula, dietary exposure to the food enzyme–total organic solids (TOS) was estimated to be 36 mg TOS/kg body weight (bw) per day for infants. The Panel estimates that this value covers all population groups. In the toxicological evaluation, clinical studies with pharmaceutical preparations containing pancreatic enzymes were considered. Hypersensitivity to the pharmaceuticals was identified as the major side effect. However, the intact enzymes are removed during preparation of food products; therefore, the Panel considers that the likelihood of adverse effects of the intact enzyme to occur is small. Low molecular weight peptides derived from the enzyme are still likely to be present in the protein hydrolysate. The Panel considered that a risk of allergic sensitisation to these peptides after consumption of products prepared by hydrolysis of milk, cannot be excluded in infants, but the likelihood to occur is considered to be low. Based on the origin of the food enzyme from edible parts of animals, the data provided by the applicant, supported by the evaluation of clinical studies with pharmaceutical preparations based on pancreatic enzymes, the Panel concluded that the porcine pancreatic enzymes do not give rise to safety concern under the intended conditions of use.

Interfacial molecular interactions of cellobiohydrolase Cel7A and its variants on cellulose

BackgroundMolecular-scale mechanisms of the enzymatic breakdown of cellulosic biomass into fermentable sugars are still poorly understood, with a need for independent measurements of enzyme kinetic parameters. We measured binding times of cellobiohydrolase Trichoderma reesei Cel7A (Cel7A) on celluloses using wild-type Cel7A (WTintact), the catalytically deficient mutant Cel7A E212Q (E212Qintact) and their proteolytically isolated catalytic domains (CD) (WTcore and E212Qcore, respectively). The binding time distributions were obtained from time-resolved, super-resolution images of fluorescently labeled enzymes on cellulose obtained with total internal reflection fluorescence microscopy.ResultsBinding of WTintact and E212Qintact on the recalcitrant algal cellulose (AC) showed two bound populations: ~ 85% bound with shorter residence times of < 15 s while ~ 15% were effectively immobilized. The similarity between binding times of the WT and E212Q suggests that the single point mutation in the enzyme active site does not affect the thermodynamics of binding of this enzyme. The isolated catalytic domains, WTcore and E212Qcore, exhibited three binding populations on AC: ~ 75% bound with short residence times of ~ 15 s (similar to the intact enzymes), ~ 20% bound for < 100 s and ~ 5% that were effectively immobilized.ConclusionsCel7A binding to cellulose is driven by the interactions between the catalytic domain and cellulose. The cellulose-binding module (CBM) and linker increase the affinity of Cel7A to cellulose likely by facilitating recognition and complexation at the substrate interface. The increased affinity of Cel7A to cellulose by the CBM and linker comes at the cost of increasing the population of immobilized enzyme on cellulose. The residence time (or inversely the dissociation rates) of Cel7A on cellulose is not catalysis limited.

Infrared laser ablation and capture of enzymes with conserved activity

Infrared (IR) laser ablation at 3 μm wavelength was used to extract enzymes from tissue and quantitatively determine their activity. Experiments were conducted with trypsin, which was ablated, captured and then used to digest bovine serum albumin (BSA). BSA digests were evaluated using matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) and sequence coverage of 59% was achieved. Quantification was performed using trypsin and catalase standards and rat brain tissue by fluorescence spectroscopy. Both enzymes were reproducibly transferred with an efficiency of 75 ± 8% at laser fluences between 10 and 30 kJ/m2. Trypsin retained 37 ± 2% of its activity and catalase retained 50 ± 7%. The activity of catalase from tissue was tested using three consecutive 50 μm thick rat brain sections. Two 4 mm2 regions were ablated and captured from the cortex and cerebellum regions. The absolute catalase concentration in the two regions was consistent with previously published data, demonstrating transfer of intact enzymes from tissue.

Enzyme Stability and Activity in Non-Aqueous Reaction Systems: A Mini Review

Enormous interest in biocatalysis in non-aqueous phase has recently been triggered due to the merits of good enantioselectivity, reverse thermodynamic equilibrium, and no water-dependent side reactions. It has been demonstrated that enzyme has high activity and stability in non-aqueous media, and the variation of enzyme activity is attributed to its conformational modifications. This review comprehensively addresses the stability and activity of the intact enzymes in various non-aqueous systems, such as organic solvents, ionic liquids, sub-/super-critical fluids and their combined mixtures. It has been revealed that critical factors such as Log P, functional groups and the molecular structures of the solvents define the microenvironment surrounding the enzyme molecule and affect enzyme tertiary and secondary structure, influencing enzyme catalytic properties. Therefore, it is of high importance for biocatalysis in non-aqueous media to elucidate the links between the microenvironment surrounding enzyme surface and its stability and activity. In fact, a better understanding of the correlation between different non-aqueous environments and enzyme structure, stability and activity can contribute to identifying the most suitable reaction medium for a given biotransformation.

Glucoamylases from Penicillium verruculosum and Myceliophthora thermophila: analysis of differences in activity against polymeric substrates based on 3D model structures of the intact enzymes.

Two glucoamylases, a recombinant enzyme from Penicillium verruculosum (PvGla) heterologously expressed in Penicillium canescens RN3-11-7 (niaD-) strain and a native glucoamylase from Myceliophthora thermophila (MtGla), were purified and their properties were studied. MtGla displayed 2-5-fold higher specific activities against soluble starch, amylose and amylopectin than PvGla. MtGla also provided higher glucose yields in extended hydrolysis of the polymeric substrates. Analysis of 3D model structures of the intact PvGla and MtGla, which were built using the 2vn7.pdb crystal structure of the intact Trichoderma reesei glucoamylase (TrGla) as a template, showed that the reason for lower hydrolytic performance of PvGla in comparison to MtGla may be less strong interactions between the enzyme domains as well as a longer (by 17 residues) linker in the first enzyme.

Pressure effects reveal that changes in the redox states of the heme iron complexes in the sensor domains of two heme-based oxygen sensor proteins, EcDOS and YddV, have profound effects on their flexibility.

The catalytic activity of a heme-based oxygen sensor phosphodiesterase from Escherichiacoli (EcDOS) towards cyclic diGMP is regulated by the redox state of the heme iron complex in the enzyme's sensing domain and the association of external ligands with the iron center. Specifically, the Fe(II) complex is more active towards cyclic diGMP than the Fe(III) complex, and its activity is further enhanced by O2 or CO binding. In order to determine how the redox state and coordination of the heme iron atom regulate the catalytic activity of EcDOS, we investigated the flexibility of its isolated N-terminal heme-binding domain (EcDOS-heme) by monitoring its spectral properties at various hydrostatic pressures. The most active form of the heme-containing domain, i.e. the Fe(II)-CO complex, was found to be the least flexible. Conversely, the oxidized Fe(III) forms of EcDOS-heme and its mutants had relatively high flexibilities, which appeared to be linked to the low catalytic activity of the corresponding intact enzymes. These findings corroborate the suggestion, made on the basis of crystallographic data, that there is an inverse relationship between the flexibility of the heme-containing domain of EcDOS and its catalytic activity. The Fe(II)-CO form of the heme domain of a second heme-based oxygen sensor, diguanylate cyclase (YddV), was also found to be quite rigid. Interestingly, the incorporation of a water molecule into the heme complex of YddV caused by mutation of the Leu65 residue reduced the flexibility of this heme domain. Conversely, mutation of the Tyr43 residue increased its flexibility.

A microfluidic device containing membrane-immobilized antibodies for successively capturing cytosolic enzymes

A microfluidic device containing membrane-immobilized anti-esterase (ES) antibodies and anti-lactate dehydrogenase (LDH) antibodies was prepared. The membrane was prepared by transferring antibodies that had been separated by non-denaturing two-dimensional electrophoresis to a polyvinylidene difluoride membrane, which was then stained and cut into small pieces (16mm2). In this microfluidic device, >0.014UnitmL−1 of the purified porcine carboxylesterase was specifically captured by membrane-immobilized anti- ES antibodies and >147UnitmL−1 of purified porcine LDH was specifically captured by membrane-immobilized anti-LDH antibodies. Furthermore, ES and LDH in micro-scale aliquots of porcine liver cytosol were successively captured by membrane-immobilized antibodies in the device, and the enzyme activities were quantitatively analyzed by spectrofluorometry. The results indicate that the microfluidic device containing membrane-immobilized antibodies can be used to investigate the activities of several types of intact enzymes.

Context dependency of Set1/COMPASS-mediated histone H3 Lys4 trimethylation

The stimulation of trimethylation of histone H3 Lys4 (H3K4) by H2B monoubiquitination (H2Bub) has been widely studied, with multiple mechanisms having been proposed for this form of histone cross-talk. Cps35/Swd2 within COMPASS (complex of proteins associated with Set1) is considered to bridge these different processes. However, a truncated form of Set1 (762-Set1) is reported to function in H3K4 trimethylation (H3K4me3) without interacting with Cps35/Swd2, and such cross-talk is attributed to the n-SET domain of Set1 and its interaction with the Cps40/Spp1 subunit of COMPASS. Here, we used biochemical, structural, in vivo, and chromatin immunoprecipitation (ChIP) sequencing (ChIP-seq) approaches to demonstrate that Cps40/Spp1 and the n-SET domain of Set1 are required for the stability of Set1 and not the cross-talk. Furthermore, the apparent wild-type levels of H3K4me3 in the 762-Set1 strain are due to the rogue methylase activity of this mutant, resulting in the mislocalization of H3K4me3 from the promoter-proximal regions to the gene bodies and intergenic regions. We also performed detailed screens and identified yeast strains lacking H2Bub but containing intact H2Bub enzymes that have normal levels of H3K4me3, suggesting that monoubiquitination may not directly stimulate COMPASS but rather works in the context of the PAF and Rad6/Bre1 complexes. Our study demonstrates that the monoubiquitination machinery and Cps35/Swd2 function to focus COMPASS's H3K4me3 activity at promoter-proximal regions in a context-dependent manner.

Rotary ATPases—dynamic molecular machines

Recent work has provided the detailed overall architecture and subunit composition of three subtypes of rotary ATPases. Composite models of F-type, V-type and A-type ATPases have been constructed by fitting high-resolution X-ray structures of individual components into electron microscopy derived envelopes of the intact enzymes. Electron cryo-tomography has provided new insights into the supra-molecular arrangement of eukaryotic ATP synthases within mitochondria. An inherent flexibility in rotary ATPases observed by different techniques suggests greater dynamics during operation than previously envisioned. The concerted movement of subunits within the complex might provide means of regulation and information transfer between distant parts of rotary ATPases thereby fine tuning these molecular machines to their cellular environment, while optimizing their efficiency.

Cellulose hydrolysis and binding with Trichoderma reesei Cel5A and Cel7A and their core domains in ionic liquid solutions

Ionic liquids (ILs) dissolve lignocellulosic biomass and have a high potential as pretreatment prior to total enzymatic hydrolysis. ILs are, however, known to inactivate cellulases. In this article, enzymatic hydrolysis of microcrystalline cellulose (MCC) and enzyme binding onto the cellulosic substrate were studied in the presence of cellulose-dissolving ILs. Two different ILs, 1,3-dimethylimidazolium dimethylphosphate ([DMIM]DMP) and 1-ethyl-3-methylimidazolium acetate ([EMIM]AcO), and two monocomponent cellulases, Trichoderma reesei cellobiohydrolase Cel7A and endoglucanase Cel5A, were used in the study. The role and IL sensitivity of the carbohydrate-binding module (CBM) were studied by performing hydrolysis and binding experiments with both the intact cellulases, and their respective core domains (CDs). Based on hydrolysis yields and substrate binding experiments for the intact enzymes and their CDs in the presence of ILs, the function of the CBM appeared to be very IL sensitive. Binding data suggested that the CBM was more important for the substrate binding of endoglucanase Cel5A than for the binding of cellobiohydrolase Cel7A. The CD of Cel7A was able to bind well to cellulose even without a CBM, whereas Cel5A CD had very low binding affinity. Hydrolysis also occurred with Cel5A CD even if this protein had very low binding affinity in all the studied matrices. Binding and hydrolysis were less affected by the studied ILs for Cel7A than for Cel5A. To our knowledge, this is the first systematic study of IL effects on cellulase substrate binding.

Rotary ATPases

Rotary ATPases are molecular rotary motors involved in biological energy conversion. They either synthesize or hydrolyze the universal biological energy carrier adenosine triphosphate. Recent work has elucidated the general architecture and subunit compositions of all three sub-types of rotary ATPases. Composite models of the intact F-, V- and A-type ATPases have been constructed by fitting high-resolution X-ray structures of individual subunits or sub-complexes into low-resolution electron densities of the intact enzymes derived from electron cryo-microscopy. Electron cryo-tomography has provided new insights into the supra-molecular arrangement of eukaryotic ATP synthases within mitochondria and mass-spectrometry has started to identify specifically bound lipids presumed to be essential for function. Taken together these molecular snapshots show that nano-scale rotary engines have much in common with basic design principles of man made machines from the function of individual “machine elements” to the requirement of the right “fuel” and “oil” for different types of motors.

Toxic misfolding of Arabidopsis cellulases in the secretory pathway of Pichia pastoris

Plants produce a large number of cellulases that are either secreted or anchored in the plasma membrane where they likely function in various aspects of cellulose synthesis, modification and degradation during plant growth and development. Very few of these enzymes have been characterized in any detail, however. Here we attempted to produce two Arabidopsis modular cellulases, which contain a catalytic domain belonging to glycoside hydrolase family 9 (GH9) and a carbohydrate binding module (CBM), in the yeast Pichia pastoris. Neither of the intact modular enzymes was detectably produced, although the independently expressed GH9 catalytic domain of one enzyme was secreted when the protein was expressed at low temperature. Expression of intact and truncated cellulases at the standard temperature caused extensive cell lysis, with release of high concentrations of endogenous proteins into the culture medium. Cell lysis appeared to result from misfolding of cellulase proteins within the Pichia secretory pathway. The toxicity of these misfolded cellulases potentially could be exploited to derive host strains with enhanced capability to fold recombinant secretory proteins.

Membrane fractionation of a β-lactoglobulin tryptic digest: Effect of the pH

The tryptic digestion of β-lactoglobulin leads to the release of a large range of biologically active peptides. Ultrafiltration/nanofiltration technology can be used to fractionate protein hydrolysates in order to obtain permeate products with increased functionality and free from intact proteins and enzymes. The influence of the pH in the fractionation of the hydrolysate through a polyethersulfone membrane (MWCO 5kDa) was investigated in this work. In this case peptide transmission was mainly governed by charge mechanisms and reached its maximum value when the pH value is close to the peptide isoelectric point. Almost complete rejection of acidic peptides was achieved at basic pH values due to electrostatic repulsive forces with the negatively charged membrane and, in agreement with Donnan theory, positively charged peptides shown lower transmission than the neutral species. The highest peptide recovery and the best separation factor between bioactive and non-bioactive peptides were obtained following nanofiltration at pH 8.0.

Targeting Proteases in Cardiovascular Diseases by Mass Spectrometry-Based Proteomics

Proteases hydrolyze peptide bonds, thereby controlling the function of proteins and peptides on the posttranslational level. In the cardiovascular system, proteases play pivotal roles in the regulation of blood pressure, coagulation and other essential physiological processes. Accordingly, proteases are prime targets for therapeutic interventions and diagnostics. Proteases are part of complex proteolytic networks comprised of enzymes, inhibitors, activators, substrates and cleavage products. Analyzing these networks on a system-wide level is essential to understanding cardiovascular function and how dysregulation can lead to pathological conditions. Mass spectrometry-based quantitative and dynamic proteomics approaches are leading the way to enhance our knowledge of proteolytic networks such as the renin-angiotensin-system. Here, we critically review proteomics tools utilized in protease biology and provide an overview on how these methods can be used to characterize and validate protease function.

Molecular Identification of β-Citrylglutamate Hydrolase as Glutamate Carboxypeptidase 3

β-Citrylglutamate (BCG), a compound present in adult testis and in the CNS during the pre- and perinatal periods is synthesized by an intracellular enzyme encoded by the RIMKLB gene and hydrolyzed by an as yet unidentified ectoenzyme. To identify β-citrylglutamate hydrolase, this enzyme was partially purified from mouse testis and characterized. Interestingly, in the presence of Ca(2+), the purified enzyme specifically hydrolyzed β-citrylglutamate and did not act on N-acetyl-aspartylglutamate (NAAG). However, both compounds were hydrolyzed in the presence of Mn(2+). This behavior and the fact that the enzyme was glycosylated and membrane-bound suggested that β-citrylglutamate hydrolase belonged to the same family of protein as glutamate carboxypeptidase 2 (GCP2), the enzyme that catalyzes the hydrolysis of N-acetyl-aspartylglutamate. The mouse tissue distribution of β-citrylglutamate hydrolase was strikingly similar to that of the glutamate carboxypeptidase 3 (GCP3) mRNA, but not that of the GCP2 mRNA. Furthermore, similarly to β-citrylglutamate hydrolase purified from testis, recombinant GCP3 specifically hydrolyzed β-citrylglutamate in the presence of Ca(2+), and acted on both N-acetyl-aspartylglutamate and β-citrylglutamate in the presence of Mn(2+), whereas recombinant GCP2 only hydrolyzed N-acetyl-aspartylglutamate and this, in a metal-independent manner. A comparison of the structures of the catalytic sites of GCP2 and GCP3, as well as mutagenesis experiments revealed that a single amino acid substitution (Asn-519 in GCP2, Ser-509 in GCP3) is largely responsible for GCP3 being able to hydrolyze β-citrylglutamate. Based on the crystal structure of GCP3 and kinetic analysis, we propose that GCP3 forms a labile catalytic Zn-Ca cluster that is critical for its β-citrylglutamate hydrolase activity.

Comparison of monolithic and microparticulate columns for reversed-phase liquid chromatography of tryptic digests of industrial enzymes in cleaning products

Enzymes of several classes used in the formulations of cleaning products were characterized by trypsin digestion followed by HPLC with UV detection. A polymeric monolithic column (ProSwift) was used to optimize the separation of both the intact enzymes and their tryptic digests. This column was adequate for the quality control of raw industrial enzyme concentrates. Then, monolithic and microparticulate columns were compared for peptide analysis. Under optimized conditions, the analysis of tryptic digests of enzymes of different classes commonly used in the formulation of cleaning products was carried out. Number of peaks, peak capacity and global resolution were obtained in order to evaluate the chromatographic performance of each column. Particulate shell–core C18 columns (Kinetex, 2.6 μm) showed the best performance, followed by a silica monolithic column (Chromolith RP-18e) and the conventional C18 packings (Gemini, 5 μm or 3 μm). A polymeric monolithic column (ProSwift) gave the worst performances. The proposed method was satisfactorily applied to the characterization of the enzymes present in spiked detergent bases and commercial cleaners.

Preparation and enzymatic behavior of surfactant-enveloped enzymes for glycosynthesis in nonaqueous aprotic media

Abstract Surfactant-enveloped enzymes (SEEs) were prepared from pure cellulases, cellobiohydrolase I and endoglucanase I (Cel7A and Cel7B, respectively), via simply freeze-drying water-in-oil emulsions, wherein the aqueous phase containing each cellulase was stabilized with the nonionic surfactant, dioleyl- N - d -glucona- l -glutamate. The enzymatic tolerance of SEEs to various nonaqueous solvents was investigated, aiming at a novel synthetic approach in biocatalytic glycoengineering. SEE-Cel7A preserved ca . 67% of the original activity after 3 h incubation in lithium chloride (LiCl)/dimethylacetamide (DMAc) that is a good solvent for carbohydrates but completely deactivates intact enzymes. This excellent enzymatic durability depended on the preparation conditions of SEEs, e.g. pH and salt species of the aqueous phase during SEE preparation. SEE-Cel7A or SEE-Cel7B was applied as a biocatalyst to synthesize cellulose, a sugar polymer which is insoluble in common solvents but dissolves in LiCl/DMAc. Both SEEs could catalyze the direct dehydration of cellobiose without any activation of the anomeric carbon, a property that is indispensable for conventional chemo-enzymatic synthesis. The SEE-Cel7A provided short-chain cellulose with the degree of polymerization (DP) ca . 20, and longer-chain cellulose with DP ca . 60 was preferentially obtained by the SEE-Cel7B, possibly through preferential reverse hydrolysis instead of inherent hydrolysis. Nonaqueous SEE-mediated biocatalysis using inexpensive glycohydrolases and sugars that do not need to be chemically modified beforehand would have potentially wide applications in glycoengineering.

Mechanism of initial rapid rate retardation in cellobiohydrolase catalyzed cellulose hydrolysis

Despite intensive research, the mechanism of the rapid retardation in the rates of cellobiohydrolase (CBH) catalyzed cellulose hydrolysis is still not clear. Interpretation of the hydrolysis data has been complicated by the inability to measure the catalytic constants for CBH-s acting on cellulose. We developed a method for measuring the observed catalytic constant (k(obs)) for CBH catalyzed cellulose hydrolysis. It relies on in situ measurement of the concentration of CBH with the active site occupied by the cellulose chain. For that we followed the specific inhibition of the hydrolysis of para-nitrophenyl-beta-D-lactoside by cellulose. The method was applied to CBH-s TrCel7A from Trichoderma reesei and PcCel7D from Phanerochaete chrysosporium and their isolated catalytic domains. Bacterial microcrystalline cellulose, Avicel, amorphous cellulose, and lignocellulose were used as substrates. A rapid decrease of k(obs) in time was observed on all substrates. The k(obs) values for PcCel7D were about 1.5 times higher than those for TrCel7A. In case of both TrCel7A and PcCel7D, the k(obs) values for catalytic domains were similar to those for intact enzymes. A model where CBH action is limited by the average length of obstacle-free way on cellulose chain is proposed. Once formed, productive CBH-cellulose complex proceeds with a constant rate determined by the true catalytic constant. After encountering an obstacle CBH will "get stuck" and the rate of further cellulose hydrolysis will be governed by the dissociation rate constant (k(off)), which is low for processive CBH-s.