What is an example of an oligomeric enzyme

Enzymes

Enzyme [from Greek en = in, zymē = fermentation agent, sourdough; Adj. enzymatic], Biocatalysts i.e.S., outdated designation Ferments,Biopolymers, mostly proteins, which are involved as catalysts (catalysis) in almost all chemical conversions in the organisms by providing the necessary for the course of every chemical reaction Activation energy (Height of the energy barrier) and thus initiate chemical reactions under the conditions prevailing in living cells (e.g. body temperature, aqueous solutions, normal pressure) that otherwise only take place at noticeable speeds under non-physiological conditions. Compared to the uncatalyzed reaction, the catalyzed reaction can be increased by a factor of 103 – 106 be accelerated. A classic example is the conversion of air-nitrogen (N2) to ammonia (NH3) in the Haber-Bosch process (temperature of 250 OC and pressure of approx. 300 bar!) And nitrogen-fixing bacteria (free living or in symbiosis e.g. in root nodules of soybeans; temperature of 20 OC and 1 bar). Since metabolic processes (metabolism) are generally composed of numerous individual reactions, each of which is catalyzed by a certain enzyme specific for each individual reaction, enzymes are of fundamental importance for the course of the entire cell metabolism.
Biosynthesis, structure and Compartmentalization: As with all proteins, the enzyme proteins are synthesized starting from the corresponding genes (one-gene-one-enzyme hypothesis) via mRNA (messenger RNA) on the ribosomes (translation). Continuously formed enzymes are called constitutive enzymes called, the enzymes formed only under certain conditions or when needed are called adaptive enzymes. The regulation of the new synthesis of enzyme protein takes place mainly at the level of transcription (Enzyme induction and Enzyme repression;Gene regulation), but some well-studied examples (e.g. in the case of single-stranded RNA phages) of regulatory mechanisms at the level of translation are known. - The vast majority of the known enzymes are proteins, which consist of 1 (monomeric enzymes) or more (oligomeric or. multimeric enzymes) Polypeptide chains can exist. There are also so-called. Ribozymes, Biocatalysts based on RNA (ribonucleic acids). Monomeric enzyme proteins are e.g. the enzymes of the digestive tract (intestines, digestion) and the enzymes contained in the blood (blood proteins), which are excreted by the producing cells into the extracellular space (extracellular enzymes). Multimeric enzymes tend to be cellular enzymes. They can either consist of several identical peptide subunits, often as dimers (α2), Tetramers (α4) or octamers (α8), or be composed of several different peptide subunits. For example, aspartate transcarbamylase is made up of 2 different subunits (α, β), while ribulose-1,5-diphosphate carboxylase is made up of 2 × 8 different subunits (α8, β8) composed. In addition to the protein content, the so-called. Apoenzyme (also called enzyme protein), non-proteinogenic groups, so-called. Coenzymes such as vitamins and nucleotides that are either loosely bound to the apoenzyme, i.e. non-covalently (chemical bonding), or temporarily or firmly, i.e. covalently bound (prosthetic group). The term Cofactors also includes metals known as trace elements such as iron, copper, magnesium, manganese or zinc, which in ionized and complex-bound form serve as electron acceptors. Apoenzyme and coenzyme together result in the alone effective Holoenzyme. As Enzyme systems groups of enzymes are grouped together, through which related, multi-stage reaction sequences are catalyzed (see Fig.), e.g. the enzyme systems of glycolysis (color table), the structure of individual amino acids (Fig.), the structure of nucleotides, DNA synthesis (deoxyribonucleic acids ), DNA repair. Enzyme systems can also, as in the case of the fatty acid synthetase complex, by combining several enzymes to form a larger association than Multi-enzyme complexes are present. As a rule, enzyme systems are located in certain compartments (compartmentalization) of the cell, e.g. the enzymes of glycolysis in the cytoplasm, the enzymes of the citric acid cycle in the mitochondria, the enzymes of the Calvin cycle in the chloroplasts, the enzymes of DNA synthesis and - Repair in the cell nucleus. Within the individual compartments a distinction is made between soluble enzymes and enzyme systems that are freely moving in the corresponding plasma (cytoplasm, nuclear plasma, matrix of mitochondria, stroma of chloroplasts, etc.) and those that are anchored in the corresponding membranes, such as those in the mitochondrial membrane localized enzyme systems of the respiratory chain and respiratory chain phosphorylation or the systems of photosynthesis and photophosphorylation anchored in the thylakoid membrane of the chloroplasts. The membrane-bound enzymes show high specificity with regard to their orientation to the inside or outside of the membrane in question. However, they can - but always remain on the same side of the membrane bilayer - diffuse two-dimensionally within the membrane surface and interact with other membrane-bound enzymes (e.g. exchange substrates), so that they are not to be understood in the strict sense as molecules anchored to certain points on the membrane (membrane, Membrane proteins). Frequently, enzymes that catalyze the same reaction but have more or less large differences in protein structure and / or kinetic properties (see below) are found in different individuals of the same species, in different organs of an individual or even in different compartments of a cell observed. Multiple enzymes of this type will be Isoenzymes called.
Mechanism of action: As catalysts, enzymes always increase the and Reverse reactions (reaction rate, rate constant), which lead to a chemical equilibrium, so that under the action of enzymes only the rate of equilibrium is increased, but the position of the equilibrium does not change. The chemical compound converted under the action of an enzyme is called Substrate designated. This will be temporary during the implementation on active center (Fig.) Of the corresponding enzyme with the formation of the so-called. Enzyme-substrate complexes bound. Substrate and active center of an enzyme are complementary to each other (complementarity, key-lock principle), which is why each enzyme from the large number of molecules occurring in the cell binds the appropriate substrate and only this (or in the reverse reaction, the reaction product of the same) and can implement. This selectivity with respect to the substrates to be converted is called Substrate specificity designated; it goes as far as the distinction between stereoisomers. The sequential connection of several enzyme-catalyzed reactions of a substrate to form a product creates steady state equilibria (dynamic equilibrium), with the help of which biological systems succeed in also thermodynamically (thermodynamics; energy conservation, enthalpy, entropy) unfavorable reactions proceeding almost completely in the required direction to let. The concentration of the intermediate products in the reaction cascades is negligible. A frequent reaction condition is the binding of high-energy nucleotides (high-energy compounds) such as ATP (adenosine triphosphate) or GTP (guanosine-5'-triphosphate) and their hydrolysis during the chemical conversion of the substrate (thermodynamic coupling of ATP hydrolysis and substrate conversion). At the branch points of metabolic pathways, individual compounds (e.g. C in the following scheme) can be converted into various products:





The enzymes ECD and EC, E have overlapping, i.e. partly identical substrate specificities (they bind or convert compound C together, but differ in the binding of products D and E in the reverse reactions). However, they show different Effect specificity, since enzyme ECD only the conversion C ⇌ D, enzyme EC, E only the conversion C ⇌ E is catalyzed. While the substrate specificity is primarily based on the interaction between the substrate and the active center of the enzyme, the interaction between the substrate and the coenzyme is also important for the specificity of action, especially since the coenzymes, together with the conversion of the substrate molecules, often undergo cyclic reactions (e.g. redox reactions with the coenzymes respiratory chain enzymes). That is why the term Cosubstrate often used as a synonym for the term coenzyme. When the enzyme-substrate complex is formed (see Fig.), The substrate enters into non-covalent interactions, hydrogen bonds and possibly also covalent bonds with the active center. The enzyme changes the electron distribution in the substrate in such a way that the so-called. Transitional state (transition state) is stabilized. It is the state with the highest energy on the way from the substrate to the product. Depending on the structure of the enzyme, certain chemical bonds are selectively influenced. The enzyme-product complex, in turn, is very unstable due to the specific properties of the enzyme. During dissociation, it breaks down into the product and the enzyme, which is then ready for a new reaction cycle. For some enzyme-catalyzed reactions, the individual steps occurring in the active center could be analyzed down to atomic details. As an example, the effect of chymotrypsin in the cleavage of a peptide bond (peptides) is shown in the illustration (see illustration). The amino acid residues His-57, Asp-102 and Ser-195, which are located far away in the primary structure of chymotrypsin, but which, due to the tertiary structure, are in spatial proximity, exert a concerted action (charge relay system). This example therefore also illustrates the special importance of the folding of protein primary structures to secondary and tertiary structures for the activity of enzymes, since only because of these folds can the amino acid residues forming the active center come into spatial proximity.
Classification: Enzymes can be classified according to their occurrence in nature (animal, vegetable, microbial enzymes), according to their metabolic function (e.g. digestive, blood clotting enzymes), according to their functional groups (e.g. serine, SH enzymes) or according to their physical properties. According to that of the International Enzyme Nomenclature Commission (engl. enzyme commission;EC) developed so-called. E.C. nomenclature the enzymes are divided into 6 main classes based on their specificities of action (see table), which are further subdivided into enzyme groups, enzyme subgroups and series based on the coenzymes involved and the substrate groups. The designation of individual enzymes occurs through a combination of substrate (possibly also coenzyme), specificity of action and the general ending for enzymes -ase. For example, an enzyme that converts alcohol (ethanol) as a substrate to hydrogen for NAD+ (Nicotinamide adenine dinucleotide), i.e. catalyzes a dehydrogenation reaction (dehydration; effect specificity), called NAD-dependent alcohol dehydrogenase.
Enzyme kinetics: In addition to the structural properties (amino acid sequence and number of peptide chains, folding to secondary and tertiary structures, relative molecular mass, etc.) and the substrate or effect specificities, the kinetic parameters of the enzyme-controlled reactions and the inhibition or stimulation by certain substances are used to characterize individual Enzymes used. If, with a given constant amount of enzyme, the speed (amount of the converted substrate per unit of time) of the reaction in question is measured as a function of the substrate concentration, one often observes the curve shown in the figure (see figure), from which the maximum speed (vMax) and the substrate concentration KM.at which half saturation of the enzyme has been reached, read off (rate equation). The relationship derived from this is named after its discoverers L. Michaelis and M. Menten Michaelis-Menten equation designated, KM.as Michaelis constant. KM.Values ​​(see table) are a measure of the bond strength between enzyme and substrate. Low KM.-Values ​​indicate high binding strength and vice versa. For example, the very low concentration of 4 Â · 10 is enough–7mol / l tRNA (transfer RNA) is already sufficient to transfer half of the arginine tRNA synthetase into the enzyme-substrate complex (high binding strength), while the binding of ATP to the same enzyme results in the much higher concentration of 3 Â · 10–4 mol / l required (low binding strength). As some of the examples listed in the table show, several molecules are often converted in enzyme-controlled reactions (e.g. arginine, tRNA and ATP in arginine-tRNA synthetase), for each of which has its own but neighboring binding sites or binding affinities (expressed by the KM.Values) exist in the active center. At allosteric enzymes shows the dependence of the reaction rate on the substrate concentration [S] a characteristic sigmoid curve (allostery, fig.). As a rule, however, the characteristics of the enzymatic reaction are not determined on the basis of the simple representation v versus [S], but in the double reciprocal representation of the Lineweaver-Burk diagram (1/v versus 1 / [S]). Another kinetic parameter for characterizing enzymes is that Change number. It indicates the number of substrate molecules that can be converted by an enzyme molecule per minute. The turnover rate of many enzymes is between 103and 104. Carbonic anhydrase (36 Â · 106), Catalase (5 Â · 106) and acetylcholine esterase (2 Â · 106). - The Enzyme activity depends within certain limits on the external test conditions, such as temperature, pH value, salt concentration, concentration of divalent cations (especially Mg2+) and SH reagents (Activators). The optimal conditions with regard to these components as well as with regard to temperature and pH value can therefore differ considerably from enzyme to enzyme, which is on the one hand of practical importance for the standardized measurement of enzyme activities, but on the other hand also often reflects the conditions under which the enzymes in question in the Cell are active. The internationally customary unit of enzyme activity was originally the Enzyme unit (1 U) defines, i.e. the amount of enzyme that converts 1 μmol substrate per minute under standard conditions. This definition is still widely used today, although the enzyme commission of the "International Union of Pure and Applied Chemistry" (IUPAC) redefined the unit in 1972. According to this recommendation, which is valid today, the unit of enzyme activity is that Catal (Symbol kat), the amount of enzyme that can convert 1 mol of substrate per second. However, since this unit is very large, the units μkat, nkat and pkat are used for common enzyme quantities. The following applies for the conversion between the units: 1 kat = 6 Â · 107 U or 1 U = 16.67 nkat. Also of practical importance are: the specific activity of enzymes, defined as enzyme units per mg protein or, since 1972, as catal per kg protein (kat / kg), the molar activity of enzymes, which is identical to the turnover number, and which concentration of enzymes as enzyme units per ml or since 1972 as catal per liter. - The high reaction rates of many enzymatically catalyzed reactions require special techniques in the experimental enzyme kinetics, e.g. for the addition and mixing of the reactants as well as the determination of the concentration of intermediate and end products, which can e.g. It is also possible to slow down catalytic processes by cooling processes and thereby stabilize short-lived enzyme-substrate complexes for analysis (cryoenzymology).
Enzyme inhibition and Enzyme activation (see Fig.): Enzymes are very sensitive to external physical and chemical factors. Changes in temperature, pH value and redox potential as well as fluctuations in the concentration of substrates, products and other molecules have a great influence on the action of enzymes. The inhibition of enzyme activities by inhibitors can be irreversible or reversible. The irreversible inhibition (Enzyme poisoning) takes place through so-called. Enzyme blockers (enzyme poisons). Many enzymes containing heavy metals, including the cytochromes involved in the respiratory chain, are irreversibly blocked by cyanide ions (cyanides). Enzymes with SH groups in the active center (so-called SH enzymes) are inhibited by reacting the SH groups with iodoacetamide or N-ethylmaleimide (NEM), enzymes with serine residues (serine) in the active center (so-called serine enzymes, e.g.Chymotrypsin) lose their activity through phosphorylation of the serine hydroxyl group with diisopropyl fluorophosphate. By reacting with these and other inhibitors and by the markings of active centers associated with the inactivation, on the other hand, conclusions could be drawn about the localization of the latter and the fine mechanism of enzymatic reactions. - In contrast to the high specificity of the enzyme blockers mentioned, the inactivation of enzymes under the influence of denaturation conditions (denaturation; temperature above 50 ° C, extreme pH values, exposure to detergents) is mostly unspecific, ie without selectivity for certain enzymes or certain functional groups thereof. The enzymes of thermophilic organisms that are active even at higher temperatures, the digestive enzymes that work even at the strongly acidic pH of the mammalian stomach and the enzymes that can often only be isolated from membranes with the help of detergents are partially excluded from inactivation by these denaturing conditions. - The reversible inhibition of enzyme activities by so-called Enzyme inhibitors can take place according to various mechanisms (see Fig.). According to the principle of competitive inhibition this is done by molecules which, due to their structural similarity to substrates, bind in the active center and thus block access for the substrate molecules, but which, due to their different structure, cannot be converted. In the uncompetitive inhibition the inhibitor does not compete with the substrate for binding to the active site, since the inhibitor can only bind to the enzyme-substrate complex. The non-competitive inhibition works on the principle of allosteric inhibition (Allostery), whereby so-called Effectors evoked conformational changes of the enzyme modulate the catalytic activity. As a special case of competitive inhibition is the Product inhibition to be understood, since products as substrates of the respective reverse reactions also have an affinity for the active center and can therefore always displace the substrate of the forward reaction from this (end product inhibition). In addition to allosteric inhibition, reversible inhibition is also often observed in allosterically regulated enzymes activation through allosterically acting effectors. - Another variant of enzyme regulation consists in the post-translational modification of an enzyme (post-translational protein modification) by other enzymes (e.g. splitting off of propeptides, phosphorylation of amino acid side chains). It is often used to activate entire enzyme cascades. The autocatalytic activation is found in some proteases, e.g. in the digestive system. The protein digestive enzymes trypsin and chymotrypsin are synthesized in the pancreas as inactive forms (zymogens) with an extended peptide chain. The so-called propeptide is split off autocatalytically in the intestinal lumen, with the activity of the proteases increasing dramatically. - The total activity of the enzymes of a reaction type is controlled by the regulation of gene expression under the control of external signals, e.g. from hormones or rare substrates.
Insulation, technical and medical application: A number of standard methods are available today for the enrichment or pure preparation of enzymes from cell material, including in particular column chromatography, differential centrifugation, fractional precipitation and ultrafiltration, so that the properties of the enzymes known today are mainly used in pure or enriched enzyme preparations outside the cell ( in vitro) can be examined. The elaboration and application of methods for the isolation of enzymes as well as for the investigation of their structural and functional properties is the subject of Enzymology, one of the main areas of biochemistry. With the help of genetic engineering processes, enzymes that are difficult to isolate or that only occur in small quantities, as well as more complicated enzyme systems, can now be made accessible. Enzymes used on an industrial scale (see table) are mostly obtained with the help of fermentation with microorganisms and corresponding mutants (biotechnology, table). - The advantages of enzyme catalysis, such as particularly gentle conditions, high specificity of action for complicated, chemically often very expensive reactions as well as high yield and purity of the products, are increasingly being used industrially. Because of the mild reaction conditions and the mostly low by-product formation, the Enzyme technology counted among the generally more environmentally friendly, “gentle” technologies. Areas of application are pharmaceutical, food, beverage, textile, paper and detergent industries. Preferred substrates are polysaccharides and proteins. Enzymes immobilized on the surfaces of solid substances (e.g. ion exchangers) are of great technical importance, as they enable frequent reuse or easy separation from the products. In the case of many enzymes, immobilization by means of cross-linking leads to what are known as Enzyme crystals (Protein crystallization) to increased chemical-mechanical stability and faster conversion. An alternative to these carrier-fixed enzymes is the use of enzyme membrane reactors (membrane reactors) in biotechnology. - In medicine is the enlightenment Enzyme pattern (Fig.) Of organs of high diagnostic value. The analysis of enzyme samples in serum or urine is used, among other things, to diagnose and control the therapy of various organ damage (e.g. heart attack, hepatitis, pancreatitis), circulatory and cancer diseases, prenatal diagnosis of hereditary diseases and the establishment of family relationships. Certain enzymes are used to Substitution therapy for digestive disorders, blood coagulation and fibrinolysis as well as for the treatment of burns, wounds, transplants, heart, circulatory and cancer diseases. Are of importance Enzyme immunassays (e.g. ELISA), which enable a quick determination of the concentration of e.g. hormones, immunoglobulins, antigens or drugs. So-called. Enzyme electrodes in biosensors (e.g. with glucose oxidase) are used to record measured variables in electrochemical diagnostic / control devices. As Enzyme defects or Enzymopathies (genetic enzyme defects) Genetically determined and therefore hereditary metabolic abnormalities are referred to, which lead to the failure of certain enzymes - caused by mutations in the relevant genes - to block the corresponding metabolic reactions (e.g. in albinism, alkaptonuria and phenylketonuria). - Significant contributions to the research of enzymes were made by, among others (see table): S. Altman, W. Arber, P. Berg, G. Bredig, D.J. Cram, E.H. Fischer, J.B. van Helmont, W.N. Lipscomb, D. Nathans, J.H. Northrop, H.O. Smith, J.B. Sumner, W.H. Stein, H.A.T. Theorell, O.H. Warburg, H.T.F. Wieland, R. Willstätter. - Abzyme, biochemistry (history of), biophysics, CK / GOT quotient, De-Ritis quotient, enzymatic analysis, enzyme diagnostics, enzymoblot, enzyme reactor, organ-specific enzymes, proenzymes.

H.K./M.B.

Lit .:Colowick, S.P., Kaplan, N.O. (eds.): Methods in enzymology. New York 1955 ff. Enzyme nomenclature. Amsterdam - New York 1973. Lehninger, A.L., Nelson, D.L., Cox, M.M .: Principles of Biochemistry. Heidelberg 21996. Michal, G.. (Ed.): Biochemical Pathways. Biochemistry Atlas. Heidelberg, Berlin 1999. Ruttloff, H.. (Ed.): Industrial enzymes. Hamburg 1994. Schellenberger, A.. (Ed.): Enzyme catalysis. Introduction to the chemistry, biochemistry and technology of enzymes. Jena 1989. Stryer, L .: Biochemistry. Heidelberg 41996. Theil, F .: Enzymes in Organic Synthesis. Heidelberg 1997. Voet, D., Voet, J.G .: Biochemistry. Weinheim 1992. Zollner, H .: Handbook of Enzyme Inhibitors. 2 vols. Weinheim 21992.



Enzymes

Types of spatial arrangement of enzymes in the cell:
Fig. Above: free in the plasma (Example glycolysis); the product of one enzyme reaction is the substrate for the next reaction. A – E are diffusing intermediates.
Center: Multi-enzyme complex (Example of fatty acid synthesis); the distance of a reaction chain is reduced.
Below: membrane-bound (Example of the respiratory chain). For the researcher, membrane-bound enzyme systems are difficult to study because the enzymes cannot be isolated individually from the membrane.



Enzymes

Scheme of the Enzyme Catalysis:
Enzyme (E) and substrate (S) form one Enzyme-substrate complex. The substrate transformation takes place on it (formation of the product P). The reactants then disintegrate again. All reactants and intermediates are in equilibrium with one another.



Enzymes

Enzymatic hydrolysis of a peptide bond by chymotrypsin via the intermediate stages a – h