Enzymatic activity under pressure
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Introduction Enzymes are the main class of nature’s catalysts. Within an organism, all chemical reactions proceed at rather constant temperatures, well below 50°C, and mostly at ambient pressure, therefore necessitating the need for catalysis. Although the structure and activity of enzymes have been optimized through evolution, some acceleration of enzymatic catalyzed reactions can still be achieved at elevated temperatures. This is a principle applicable to all chemical reactions, where increased thermal energy is favorable to overcome the activation energy of a reaction. However, heating of enzymes is limited by the temperature of unfolding of an enzyme, where denaturation (loss of activity) occurs. On the other hand, the activity of some enzymes can also be enhanced by elevated pressures.1–5 In this case, the transition state, which is associated with a maximum of energy halfway between the reactants and the product, is favored under higher pressures because it has a lower volume than the reactants. The reaction path is then characterized by a negative activation volume, ΔV≠ (≠ refers to the transition state of the chemical reaction), and the activation Gibbs energy, ΔG≠, is lowered by dΔG≠ = ΔV≠ dp, where dp is the pressure increase (Figure 1). On a molecular level, pressure effects include changes in the hydration and interaction of molecules. Hydrogen bonds are often strengthened, whereas ionic and hydrophobic interactions
can be broken in aqueous solution under high pressures.6–8 In this way, reaction paths are selected by pressure, where the transition state or the products are characterized by smaller volumes.9–11 Moreover, pressure can induce a higher conformational flexibility of the reactants, which is usually favorable for enhanced reaction rates. Thus, the application and the investigation of high pressure in biosciences and biotechnology is promising.12 α-chymotrypsin (α-CT) is a well-known enzyme used as a model enzyme in many studies.13–17 α-CT has a molar mass of 25,000 g mol–1 and a positive net charge in neutral solution. This charge facilitates adsorption of α-CT on negatively charged surfaces. α-CT adsorption has been observed on a wide variety of surfaces.18–21 α-CT hydrolyzes peptide bonds, ideally those formed by aromatic amino acids (Trp, Tyr, Phe). The enzymatic activity of α-CT can be approximately described by the Michaelis–Menten mechanism, where the substrate molecule binds reversibly to the enzyme in a first step.14 The conversion of the substrate to the product is then achieved in the bound state in an irreversible second step. For α-CT, a third step of product release must be added: k1 ⇀ α -CT + S ↽ α -CT • S k−1 k2 → P2 − α -CT + P1 k3 → α -CT + P1 + P2 , + H2 O
Claus Czeslik, TU Dortmund University, Germany; [email protected] Trung Quan Luong, TU Dortmund University, Germany; [email protected] Roland Winter, TU Dortmund University, Germany; [email protected] doi:10.1557/mrs.2017.211
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• VOLUME 42 • OCTOBER 2017 • www.mrs.org/bulletin 20
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