Supuran / De Simone Carbonic Anhydrases as Biocatalysts

From Theory to Medical and Industrial Applications
1. Auflage 2015
ISBN: 978-0-444-63263-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 398 Seiten

Carbonic anhydrases (CAs, EC 4.2.1.1) are ubiquitous metalloenzymes, present throughout most living organisms and encoded by five evolutionarily unrelated gene families. The Carbonic Anhydrases as Biocatalysts: From Theory to Medical and Industrial Applications presents information on the growing interest in the study of this enzyme family and their applications to both medicine and biotechnology. - Offers comprehensive coverage of the carbonic anhydrases enzyme family and their properties as biocatalysts - Includes current applications of carbonic anhydrases in biotechnology on the basis of their catalytic efficiency, including new technologies for CO2 capture processes - Identifies new targets for drug design studies - Provides a selectivity profile for the different carbonic anhydrases and their related biomedical applications

Autoren/Hrsg.

Supuran, Claudiu T.

De Simone, Giuseppina

Leseproben

Chapter 1 Carbonic Anhydrases: An Overview
Claudiu T. Supuran*,** Giuseppina De Simone***
*    Università degli Studi di Firenze, Polo Scientifico, Laboratorio di Chimica Bioinorganica, Sesto Fiorentino, Florence, Italy
**    NEUROFARBA Department, Sezione di Scienze Farmaceutiche, Sesto Fiorentino, Florence, Italy
***    Istituto di Biostrutture e Bioimmagini-CNR, Napoli, Italy Abstract
Carbonic anhydrases (CAs, EC 4.2.1.1) are metalloenzymes that catalyze the carbon dioxide hydration to bicarbonate and protons, with five genetically distinct classes known to date, the a-, ß-, ?-, d-, and ?-CAs. These enzymes are involved in a multitude of physiologic processes in organisms all over the phylogenetic tree. Their biochemical features are known in detail for all classes, together with their distribution and physiologic roles in various organisms. a-CAs, but not the other classes, possess catalytic versatility, being efficient esterases with a variety of substrates (carboxylic acid esters, phosphate esters, sulfate esters, etc.). Some ß-CAs evolved as catalysts for the hydration of COS and CS2. In this chapter, an overview of all classes of this enzyme family is provided. In particular, we summarize the main catalytic, structural, inhibition, and activation studies so far reported for a-, ß-, ?-, d-, and ?-CAs and how these data have been utilized for the most important medical and biotechnological applications of these enzymes. Keywords
Carbonic anhydrases drug design CO2 capture processes carbonic anhydrase inhibitors Contents 1.1 Carbonic anhydrase families 3 1.2 Catalytic features 5 1.3 CA inhibition and activation 7 1.4 Biomedical applications of the CAs 10 1.5 Biotechnological applications of the CAs 11 References 11 1.1. Carbonic anhydrase families
CO2 is one of the simple molecules that were highly abundant in the primeval earth atmosphere being a very stable form of carbon, the central element for life on this planet. This gas may react with water leading to H2CO3 that is an unstable compound, being spontaneously transformed into bicarbonate and protons. However, this reaction is particularly slow at pH values of 7.5 or lower, which is usually the physiologic pH value in many tissues and organisms. CO2 hydration becomes, on the other hand, very effective at higher pH values, being instantaneous at pH values over 12 (1,2). As CO2 is such an important molecule in all life processes, and because it is generated in high amounts in most organisms (3–7), catalysts evolved for its transformation into bicarbonate. These catalysts are the enzymes known as carbonic anhydrases (CAs, EC 4.2.1.1). They are metalloenzymes that catalyze the reversible hydration of CO2 to bicarbonate and protons (reaction 1, Figure 1.1) (1–4,8–14). However, a range of other hydrolytic processes such as COS and CS2 hydration (reactions 2 and 3), cyanamide hydration (reaction 4), ester hydrolysis (reactions 5–7), etc. (1–5,12–14), are catalyzed by some members of this enzyme superfamily (15,16). The way in which these processes are achieved is very simple, as an activated “metal hydroxide” acts as nucleophile in all the catalytic processes mediated by the CAs (see later in the text). Figure 1.1 Several hydrolytic reactions catalyzed by members of the CA superfamily, CO2 hydration (all classes), COS hydration (ß-CAs), CS2 hydration (ß-CAs), cyanamide hydration to urea (a-CAs), and ester hydrolysis (a-CAs) (1,2,15,16). Organisms in all life kingdoms (Bacteria, Archaea, and Eukarya) need CAs for being able to manage the high amounts of CO2 formed in metabolic reactions (12–14,17–26). The gas is “processed,” being hydrated to bicarbonate and protons, which generates water-soluble products from a lipophilic gas (5–12). By an interesting process of convergent evolution, organisms on earth have developed at least five distinct families of such enzymes, the a-, ß-, ?-, d-, and ?-CAs (1–5,10–13), not only to face the high amounts of CO2 formed in the metabolic processes but also to manage the possible acid–base disequilibria connected to this, considering the fact that the products formed in reaction 1 are an ion with strong buffering activity (bicarbonate) as well as an acid (H+ ions). Indeed, pH regulation is a highly important process in all life forms, since many biochemical reactions are tightly regulated by pH (1–5,10–13). a-CAs are present mainly in vertebrates, fungi, protozoa, corals, algae, and cytoplasm of green plants, but also in some bacteria (1–3,7,8,24). ß-CAs have been found in bacteria, algae and chloroplasts of both monocotyledons and dicotyledons, as well as many fungi and Archaea (1,2,4,12,14,17–23,25,26). The ?-CAs have been described in Archaea, bacteria, and plants (1,9,12), the d-CAs in the marine phytoplankton, being present in haptophytes, dinoflagellates, diatoms, and chlorophytic prasinophytes, contributing to the CO2 fixation by sea organisms (10,13), whereas ?-CAs seem to be present only in marine diatoms (11,13). In all these organisms, the CAs play crucial physiologic roles connected with pH and CO2 homeostasis, respiration and transport of CO2/bicarbonate, electrolyte secretion in many tissues/organs, biosynthetic reactions (e.g., gluconeogenesis, lipogenesis, and ureagenesis in which bicarbonate, not CO2, acts as a substrate for the carboxylation reaction), bone resorption, calcification, and tumorigenicity, all of them thoroughly studied in vertebrates (1–3,27–33). In algae, plants, and cyanobacteria, CAs play an important role in photosynthesis, by concentrating CO2/bicarbonate nearby the RUBISCO enzyme complex, and in several other biosynthetic reactions (5,7,10–13). In diatoms, d- and ?-CAs also play a crucial role in CO2 fixation but probably also in the SiO2 cycle (13). 1.2. Catalytic features
All CAs are metalloenzymes, and the metal ion is critical for catalysis, as the apoenzyme is devoid of activity (1,2,4,15,16). The five CA families differ in their preference for metal ions used within the active site for performing the catalysis: Zn(II) ions are used by all five classes mentioned above, but the ?-CAs are probably Fe(II) enzymes (being active also with bound Zn(II) or Co(II) ions) (1,2,12), whereas the ?-class uses Cd(II) or Zn(II) to perform the physiologic catalytic reaction (11,13). The metal ion coordination is shown schematically in Figure 1.2. Figure 1.2 Metal ion coordination in the various CA families: (A) by three His residues and a water molecule/hydroxide ion in the a-, ?-, and d-CAs. In some members of the ?-class, a bipyramidal coordination of the Zn2+ ion by three His residues and two water molecules is also observed (34). (B) By one His and two Cys residues, together with the water molecule/hydroxide ion, in the ß- and ?-CAs (the last one when containing zinc and not cadmium at the active site). (C) By one His, one Asp, and two Cys residues in the closed active site of the ß-CAs (20). In all CA classes, the catalytic reaction (O2+H2O?HCO3-+H+) follows a two-step catalytic mechanism, in which a metal hydroxide species of the enzyme (E-M2+-OH-) is the catalytically active species. Indeed, in the first step of the reaction, this species acts as a strong nucleophile (at neutral pH) on the CO2 molecule bound in a hydrophobic pocket nearby with consequent formation of HCO3-, which is then displaced from the active site by a water molecule (1,2) (see Eq. 1.1). The second step regenerates the metal hydroxide species through a proton transfer reaction from the M2+-bound water molecule to an exogenous proton acceptor or to an active site residue, represented by B in Eq. 1.2. M2+–OH-+CO2?EM2+–HCO3-?EM2+–H2O+HCO3- (1.1) M2+–H2O+B?EM2+–OH-+BH+ (1.2) In many enzymes, generation of the metal hydroxide species from the metal-coordinated water one is the rate-determining step of the catalytic turnover, which for some a- and ?-CAs achieves...