Thermal denaturation of erythrocyte carbonic anhydrase

Amino Acid-Mediated Synthesis of the ZIF-8 Nanozyme That Reproduces Both the Zinc-Coordinated Active Center and Hydrophobic Pocket of Natural Carbonic Anhydrase

The zeolitic imidazolate framework-8 (ZIF-8) nanozyme has been synthesized using hydrophobic amino acid (AA) to regulate crystal growth. The as-synthesized ZIF-8 reproduces both the structural and functional properties of natural carbonic anhydrase (CA). Structurally, Zn²⁺/2-methylimidazole coordinated units mimic very well the active center of CA while the hydrophobic microdomains of the adsorbed AA simulate the CA hydrophobic pocket. Functionally, the nanozymes show excellent CA-like esterase activity by giving specific enzyme activity of 0.22 U mg^-1 at 25 °C in the case of Val-ZIF-8. More strikingly, such nanozymes are superior to natural CA by having excellent hydrothermal stability, which can give highly enhanced esterase activity with increasing temperature. The specific enzyme activity of Val-ZIF-8 at 80 °C is about 25 times higher than that at 25 °C. In addition, AA-ZIF-8 also shows an excellent catalytic efficiency toward carbon dioxide (CO₂) hydration. This study puts forward the important role of hydrophobic microdomains in biomimetic nanozymes for the first time and develops a facile and mild method for the synthesis of nanozymes with controlled morphology and size to achieve excellent catalytic efficiency.

Brain Metabolite, N-Acetylaspartate Is a Potent Protein Aggregation Inhibitor

Deposition of toxic protein inclusions is a common hallmark of many neurodegenerative disorders including Alzheimer's disease, Parkinson disease etc. N-acetylaspartate (NAA) is an important brain metabolite whose levels got altered under various neurodegenerative conditions. Indeed, NAA has been a widely accepted biological marker for various neurological disorders. We have also reported that NAA is a protein stabilizer. In the present communication, we investigated the role of NAA in modulating the aggregation propensity on two model proteins (carbonic anhydrase and catalase). We discovered that NAA suppresses protein aggregation and could solubilize preformed aggregates.

Protein-caged zinc porphyrin as a carbonic anhydrase mimic for carbon dioxide capture

Zinc tetraphenylporphyrin (Zn-TPP) solubilized by GroEL protein cage was prepared as a supramolecular mimic of carbonic anhydrase (CA) for CO₂ capture. It is shown that the soluble Zn-TPP-GroEL complex can be formed easily by detergent dialysis. The Zn-TPP/GroEL binding ratio was found to increase with their dialysis ratio until reaching the maximum of about 30 porphyrins per protein cage. Moreover, the complex showed hydrase activity that catalyzes the CO₂ hydration in HCO₃⁻ and H⁺. It is further seen that the catalytic activity of Zn-TPP-GroEL was about one-half of that of a bovine CA at 25 °C. On the other hand, as the temperature was increased to 60 °C close to an industrial CO₂ absorption temperature, the natural enzyme lost function while Zn-TPP-GroEL exhibited better catalytic performance indicative of a higher thermal stability. Finally, we demonstrate that the GroEL-solubilized Zn-TPP is able to accelerate the precipitation of CO₂ in the form of CaCO₃ and has better long-term performance than the bovine CA. Thus a new type of nano-caged system mimicking natural CAs for potential applications in carbon capture has been established.

In vitro application of carbonic anhydrase to accelerate the equilibration of 18O between H2O and CO2 for the rapid measurement of 18O/16O isotope ratios in aqueous samples

A novel method of the accelerated equilibration of ¹⁸O between CO₂ and H₂O for the measurement of the ¹⁸O/¹⁶O isotope ratios in aqueous samples with natural isotope abundances is presented. This rapid equilibrium method is based on the in vitro application of the enzyme carbonic anhydrase (CA). The CA from bovine erythrocytes was adsorptively fixed to 3-mm glass beads with an etched surface. After the addition of this carrier-fixed CA catalyst to the water sample, the isotope equilibrium was already reached after 1 h. The previously used non-catalysed ¹⁸O isotope exchange in water samples needs about 24 h. Whole blood samples also showed fast ¹⁸O isotope equilibration, which definitely results from the native presence of CA in erythrocytes. By shortening the time for sample preparation, the CA catalysed technique can significantly increase the throughput of the samples to be measured, and also ¹⁸O and ²H measurement by means of isotope ratio mass spectrometry (IRMS) may be synchronized. The ²H and ¹⁸O sample preparation can be performed in the same reaction vessel because cross-effects at the simultaneous use of Pt and CA catalysts do not occur.

Salt mediated unusual switching in the aggregation kinetic profile of human carbonic anhydrase

Aggregation pathway of human carbonic anhydrase II in the presence of salt.

Macromolecular crowding decelerates aggregation of a β-rich protein, bovine carbonic anhydrase: a case study

The majority of in vitro investigations concerning protein aggregation have been performed in dilute systems, which poorly reflect the crowded in vivo scenario. Cell interior is highly crowded with soluble and insoluble macromolecules that alter macromolecular properties. Macromolecular crowding is known to enhance the rate and/or extent of protein aggregation. However, most of the understandings were derived from studies with α-rich or predominantly α-proteins. Indeed, α-proteins fold faster than β-proteins and conversion of α-helices to cross β-sheets are responsible for aggregate/amyloid formation. Therefore, it is important to investigate how macromolecular crowding affects the aggregation propensity of β-rich proteins. In this study, we investigated the effect of synthetic macromolecular crowders on bovine carbonic anhydrase (BCA, a β-rich protein) aggregation. In contrast to the effect of macromolecular crowding on α-rich proteins, BCA aggregation was observed to be reduced due to decrease in the population of aggregation-prone intermediates as a consequence of increased native state stability. In addition, the extent of aggregation was found to depend on the nature of the crowder under consideration. Combining the published data on α-proteins and this study, we conclude that macromolecular crowding can have opposite consequences on protein aggregation process depending on the fold type of the protein.

Evaluation of a carbonic anhydrase mimic for industrial carbon capture

Zinc(II) cyclen, a small molecule mimic of the enzyme carbonic anhydrase, was evaluated under rigorous conditions resembling those in an industrial carbon capture process: high pH (>12), nearly saturated salt concentrations (45% K2CO3) and elevated temperatures (100-130 °C). We found that the catalytic activity of zinc cyclen increased with increasing temperature and pH and was retained after exposure to a 45% w/w K2CO3 solution at 130 °C for 6 days. However, high bicarbonate concentrations markedly reduced the activity of the catalyst. Our results establish a benchmark level of stability and provide qualitative insights for the design of improved small-molecule carbon capture catalysts.

Carbonic anhydrase promotes the absorption rate of CO2 in post-combustion processes

The rate of carbon dioxide (CO2) absorption by monoethanol amine (MEA), diethanol amine (DEA), N-methyl-2,2'-iminodiethanol (MDEA), and 2-amino-2-methyl 1-propanol (AMP) solutions was found to be enhanced by the addition of bovine carbonic anhydrase (CA), has been investigated using a vapor-liquid equilibrium (VLE) device. The enthalpy (-ΔHabs) of CO2 absorption and the absorption capacities of aqueous amines were measured in the presence and/or absence of CA enzyme via differential reaction calorimeter (DRC). The reaction temperature (ΔT) under adiabatic conditions was determined based on the DRC analysis. Bicarbonate and carbamate species formation mechanisms were elucidated by (1)H and (13)C NMR spectral analysis. The overall CO2 absorption rate (flux) and rate constant (kapp) followed the order MEA > DEA > AMP > MDEA in the absence or presence of CA. Hydration of CO2 by MDEA in the presence of CA directly produced bicarbonate, whereas AMP produced unstable carbamate intermediate, then underwent hydrolytic reaction and converted to bicarbonate. The MDEA > AMP > DEA > MEA reverse ordering of the enhanced CO2 flux and kapp in the presence of CA was due to bicarbonate formation by the tertiary and sterically hindered amines. Thus, CA increased the rate of CO2 absorption by MDEA by a factor of 3 relative to the rate of absorption by MDEA alone. The thermal effects suggested that CA yielded a higher activity at 40 °C.

Exploring local flexibility/rigidity in psychrophilic and mesophilic carbonic anhydrases

Molecular flexibility and rigidity are required to determine the function and specificity of protein molecules. Some psychrophilic enzymes demonstrate a higher catalytic efficiency at low temperatures, compared to the efficiency demonstrated by their meso/thermophilic homologous. The emerging picture suggests that such enzymes have an improved flexibility of the structural catalytic components, whereas other protein regions far from functional sites may be even more rigid than those of their mesophilic counterparts. To gain a deeper insight in the analysis of the activity-flexibility/rigidity relationship in protein structure, psychrophilic carbonic anhydrase of the Antarctic teleost Chionodraco hamatus has been compared with carbonic anhydrase II of Bos taurus through fluorescence studies, three-dimensional modeling, and activity analyses. Data demonstrated that the cold-adapted enzyme exhibits an increased catalytic efficiency at low and moderate temperatures and, more interestingly, a local flexibility in the region that controls the correct folding of the catalytic architecture, as well as a rigidity in the hydrophobic core. The opposite result was observed in the mesophilic counterpart. These results suggest a clear relationship between the activity and the presence of flexible and rigid protein substructures that may be useful in rational molecular and drug design of a class of enzymes playing a key role in pathologic processes.

Endogenous alkaline transients boost postsynaptic NMDA receptor responses in hippocampal CA1 pyramidal neurons

In hippocampus, activation of the Schaffer collaterals generates an extracellular alkaline transient both in vitro and in vivo. This pH change may provide relief of the H+ block of NMDA receptors (NMDARs) and thereby increase excitability. To test this hypothesis, we augmented extracellular buffering in mouse hippocampal slices by adding 2 microM bovine type II carbonic anhydrase to the superfusate. With addition of enzyme, the alkaline transient elicited by a 10 pulse, 100 Hz stimulus train was reduced by 33%. At a holding potential (V(H)) of -30 mV, the enzyme decreased the half-time of decay and charge transfer of EPSCs by 32 and 39%, respectively, but had no effect at a V(H) of -80 mV. In current clamp, a 10 pulse, 100 Hz stimulus train gave rise to an NMDAR-dependent afterdepolarization (ADP). Exogenous enzyme curtailed the ADP half-width and voltage integral by 20 and 25%, respectively. Similar reduction of the ADP was noted with a brief 12 Hz stimulus train. The effect persisted in the presence of GABAergic antagonists or the L-type Ca2+ channel blocker methoxyverapamil hydrochloride but was absent in the presence of the carbonic anhydrase inhibitor benzolamide or when the exogenous enzyme was heat inactivated. The effects of the enzyme in voltage and current clamp were noted in 0 Mg2+ media but were abolished when (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine maleate was included in the patch pipette. These results provide strong evidence that endogenous alkaline transients are sufficiently large in the vicinity of the synapse to augment NMDAR responses.

Why are proteins charged? Networks of charge-charge interactions in proteins measured by charge ladders and capillary electrophoresis

Almost all proteins contain charged amino acids. While the function in catalysis or binding of individual charges in the active site can often be identified, it is less clear how to assign function to charges beyond this region. Are they necessary for solubility? For reasons other than solubility? Can manipulating these charges change the properties of proteins? A combination of capillary electrophoresis (CE) and protein charge ladders makes it possible to study the roles of charged residues on the surface of proteins outside the active site. This method involves chemical modification of those residues to generate a large number of derivatives of the protein that differ in charge. CE separates those derivatives into groups with the same number of modified charged groups. By studying the influence of charge on the properties of proteins using charge ladders, it is possible to estimate the net charge and hydrodynamic radius and to infer the role of charged residues in ligand binding and protein folding.

Dynamics of a partially stretched protein molecule studied using an atomic force microscope

The dynamics of a single protein molecule subjected to forced mechanical unfolding was investigated in a millisecond time domain using a custom-made atomic force microscope (AFM) apparatus, which allows simultaneous measurements of an average tensile force applied to a single molecule and its mechanical response with respect to an external oscillation. Our target protein was genetically engineered bovine carbonic anhydrase II (BCA) which is a monomeric globular protein, and it has been shown that the as-expressed BCA from Escherichia coli contains two conformational isomers, one with enzymatic activity (type I) and the other without (type II). An interesting feature observed from the dynamic measurements was that when the type I BCA conformer was extended, it often exhibited a clear out-of-phase response against an external oscillation. The type II BCA conformer, however, always exhibited an in-phase response to the external oscillation. This relationship between different types of BCA and their dynamical behaviors was evidently observed around the discontinuous transition point from type I to II.

A model for the unusual kinetics of thermal denaturation of rubredoxin

The thermal denaturation of the simple, redox-active iron protein rubredoxin is characterized by a slow, irreversible decay of the characteristic red color of the iron center at elevated temperatures in the presence of oxygen at pH 7.8. The denaturation rate is essentially constant and the time period for complete bleaching is nearly independent of protein concentration. These two characteristics of the kinetics can be fit by a simple self-catalyzed kinetics model consisting of the combination of a first-order decay and catalysis by some product of that decay, i.e., dP/dt = k1[A] + (k2[P][A])/(K(m) + [A]), where A is native rubredoxin, P, is unspecified product, k1 is a first-order rate constant, and k2 and K(m) are the catalytic constants. In order for the second term to be of this simple form over the full course of a decay, the model must include the condition that the reaction is effectively irreversible. This model has properties which suggest other biological roles in regulation (changes in k1 or k2 can dramatically modulate the kinetics), in timing (titer-independent fixed reaction time), and in self-activation reactions. At one extreme (k1 >> k2) the kinetics becomes exponential, but at the other extreme (k2 >> k1) they show a dramatic and rapid terminal increase after a lag period. Some obvious possible roles in the kinetics of programmed cell death, prion disease, and protease autoactivation are discussed.