Classification of enzymes

Enzymes are classified according to what they do rather than by what they are. There are six major classes of enzymes which have to be recognised according to the recommendations of the International Union of Biochemistry (1964). There is, however, by no means a universally accepted scheme of enzyme nomenclature, but there is an international coding system whereby each enzyme 

Enzymes are classified into six major groups according to their mode of action. 

The oxidoreductases catalyse the transfer of hydrogen, oxygen or electrons from one molecule to another. For example, lactate is oxidised to pyruvate in the presence of lactate dehydrogenase. In the process, two electrons and two hydrogen atoms are removed from the alcohol group, and are transferred to NAD to form NADff(+ff ), leaving a ketone. 

The transferases are a large group of enzymes that catalyse the transfer of groups such as acetyl, amino and phosphate from one molecule to another. For example, in the formation of citrate from oxalacetate during the release of energy in the body, addition of an acetyl group takes place in the presence of citrate synthetase  

The hydrolases catalyse hydrolytic cleavage. Typical are the hydrolyses associated with fat and protein digestion, which are essential for the normal functioning of the organism. A fat may be broken down to glycerides (acylglycerols) and fatty acids under the influence of a lipase  

Similarly, peptidases split proteins by hydrolysis of the peptide linkages between the constituent amino acids. 

Question On what basis are enzymes given their particular names  

All enzymes are named according to a classification system designed by the Enzyme Commission (EC) of the International Union of Pure and Applied Chemistry (IUPAC) and based on the type of reaction they catalyze. Each enzyme type has a specific, four-integer EC number and a complex, but unambiguous, name that obviates confusion about enzymes catalyzing similar but not identical reactions. In practice, many enzymes are known by a common name, which is usually derived from the name of its principal, specific reactant, with the suffix -ase added. Some common names do not even have -ase appended, but these tend to be enzymes studied and named before systematic classification of enzymes was undertaken. 

Examples of typical enzyme names are arginase, which acts on arginine, and urease, which acts on urea (Chap. 15). Two atypical common names are pepsin, a digestive tract proteolytic enzyme (EC number 3.4.23.1), and, more exotically, rhodanese (thiosulfate cyanide sulfurtransferase, EC 2.8.1.1), which is in mammalian liver and kidney and catalyzes the removal of cyanide and thiosulfate from the body. In the latter case, it is understandable why the old name has remained in common use. 

The first integer in the EC number designates to which of the six major classes an enzyme belongs (see Table 8.1 for details). 

The second integer in an EC enzyme number indicates a sub-class for hydrolases, this second integer indicates the type of bond acted upon by the enzyme (Table 8.2). 

The roughly 3000 enzymes currently known are grouped into six main classes according to the type of the reaction catalyzed. At present only a limited number are used for analytical purposes. 

Oxidoreductases catalyze oxidation and reduction reactions by transfer of hydrogen or electrons. The following are of analytical importance  

Transferases transfer C-, N-, P-, or S-containing groups (alkyl, acyl, aldehyde, amino, phosphate, glycosyl) from one substrate to another. Transaminases, transketolases, transaldolases and transmethylases belong to this class  

Hydrolases catalyze cleavages or the reverse fragment condensation. According to the type of bond cleaved, a distinction is made between peptidases, esterases, glycosidases, phosphatases, etc. examples cholesterol esterase (EC 3.1.1.13.) 

Lyases nonhydrolytically remove groups from their substrates under formation of double bonds, or add groups to double bonds. Only few enzymes of this class are used in analysis. 

An enzyme is often named by adding the suffix -ase to the name of the substrate it works on in nature. For example, the first enzyme discovered was urease, which catalyzes the hydrolysis of urea producing ammonia and carbon dioxide. However, some enzymes have been given uninformative names such as catalase, which catalyzes hydrogen peroxide to water and oxygen. The lUBMB has adopted standards for enzyme nomenclature according to the type of reactions they catalyze and the substrates acted upon. 

Six types of reactions catalyzed by enzymes have been identified, and accordingly the enzymes have been classified into six main groups. These are oxidoreduc-tases, transferases, hydrolases, lyases, isomerases, and ligases. These six groups and their respective main functions are summarized in Table 1.3. Of the six groups in 

Catalyze oxidoreduction reactions by adding/ removing hydrogen bonds Transfer of amino, fatty acid, methyl or phosphate functional groups from one molecule to another 

Catalyze the hydrolysis of carbohydrates, Upids, proteins, or phosphoric acids esters by breaking single bond and add water across bond 

Catalyze the breaking/forming of chemical bonds by means other than hydrolysis Catalyze the sttuctural rearrangement of isomers Catalyze reactions by binding two chemical groups to form one molecule with the need of ATP energy 

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Systematic Classification of Enzymes According to the Enzyme Commission ... 

Carboxylesterases Esterases that hydrolyze organic compounds with carboxylester bonds. Carboxylesterases that are inhibited by organophosphates (OPs) belong to the category EC 3.1.1.1 in the lUB classification of enzymes. 

Classification of Enzymes. A systematic classification and nomenclature has been established by the Commission on Enzymes of the International Union of Biochemistry (6), which divides enzymes into six general groups ... 

Table 1 Classification of enzymes respective enzymes and in vitro production of typical polymers catalyzed by...

Krantz, A. A classification of enzyme inhibitors. Bioorg. Med. Chem. Lett. 1992, 2, 1327-1334. 

The first Enzyme Commission, in its 1961 report, devised a system for classification of enzymes that also serves as a basis for assigning code numbers to them. Every enzyme has got a 4-digit number EC A.B.C.D. where EC stands for Enzyme Commission with the following properties encoded ... 

Table 1. International classification of enzymes with examples of bioenzymatic sensors for each enzyme main class.13... 

Table 1 summarizes the international classification of enzymes. Classes EC1 and EC3 are the most widely used for the development of optical biosensors. Sometimes different enzymes and transducing schemes can be applied to the analysis of a single analyte and the best combination should be selected depending on the application. 

Both are entities in well-defined languages that represent specific abstractions SMILES, the valence model of a molecule ECN, a classification of enzyme functionality. 

USMLE Road Map Biochemistry Table 3-2. Classification of enzymes. 

For biotechnological purposes enzymes are used as biocatalysts to accelerate a desired reaction to its end point. As catalysts enzymes can catalyze a reaction in both directions. Unfortunately the name of an enzyme given by the Enzyme Commission for the classification of enzymes, and biochemistiy text books, stresses the function as a catalyst in one direction only Hydrolases, a group of enzymes of considerable importance in... 

Enzyme Nomenclature 1978. Recommendations of the Nomenclature Committee of the International Union of Biochemistry on the Nomenclature and Classification of Enzymes, Academic Press, New York, 1979. 

Examples of the six major classes of the international classification of enzymes (THF is tetrahydrofolate). 

Table 2.1 Partial Outline of the Systematic Classification of Enzymes...

Consider the classification of enzymes in your textbook (Sect. 22.1) and reaction (1) of this experiment. How would you classify urease ... 

Enzymes are proteinaceous catalysts peculiar to living matter. Hundreds have been obtained in purified and crystalline form. Their catalytic efficiency is extremely high—one mole of a pure enzyme may catalyze the transformation of as many as 10,000 to 1,000,000 moles of substrate per minute. While some enzymes are highly specific for only one substrate, others can attack many related substrates. Avery broad classification of enzymes would include hydrolytic enzymes (esterases, proteases), phosphorylases, oxidoreductive enzymes (dehydrogenases, oxidases, peroxidases), transferring enzymes, decarboxylases and others. 

Enzymatic transition state theory permits classification of enzymes for potential inhibition susceptibility. The simple rule suggests that the higher the catalytic rate enhancement, the tighter the binding the transition state analogue should be. Hence, slow enzymes demonstrating a small kca(/kchem is expected to bind weakly to TS analogues. 

In 1963 W. W. Cleland published a classification of enzyme-catalyzed reactions based on the number of substrates and products in the reaction. This classification is as follows ... 

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