Учебно-методический комплекс дисциплины " Basis of biochemistry " Для специальности

Enzymes Definition

Enzymes are large biomolecules that are responsible for many chemical reactions that are necessary to sustain life. Enzyme is a protein molecule and are biological catalysts. Enzymes increase the rate of the reaction. Enzymes are specific, they function with only one reactant to produce specific products. Enzymes have a three-dimensional structure and they utilize organic molecules like biotin and inorganic molecules like metal ions (magnesium ions) for assistance in catalysis. 

Substrate is the reactant in an enzyme catalyzed reaction. The portion of the molecule that is responsible for catalytic action of enzyme is the active site. 

Characteristics of Enzymes

Characteristics of enzymes are as follows:

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  Enzymes possess great catalytic power. 
  
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  Enzymes are highy specific.
  
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  Enzymes show varying degree of specificities.
  
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  Absolute specificity where the enzymes react specifically with only one substrate. 
  
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  Stereo specificity is where the enzymes can detect the different optical isomers and react to only one type of isomer. 
  
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  Reaction specific enzymes, these enzymes as the name suggests reacts to specific reactions only. 
  
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  Group specific enzymes are those that catalyze a group of substances that contain specific substances.
  
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  The enzyme activity can be controlled but the activity of the catalysts can not be controlled.
  
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  All enzymes are proteins.
  
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  Like the proteins, enzymes can be coagulated by alcohol, heat, concentrated acids and alkaline reagents. 
  
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  At higher temperatures the rate of the reaction is faster. 
  
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  The rate of the reaction invovlving an enzyme is high at the optimum temperature.
  
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  Enzymes have an optimum pH range within which the enzymes function is at its peak. 
  
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  If the substrate shows deviations larger than the optimum temperature or pH, required by the enzyme to work, the enzymes do not function such conditions.
  
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  Increase in the concentration of the reactants, and substrate the rate of the reaction increase until the enzyme will become saturated with the substrate; increase in the amount of enzyme, increases the rate of the reaction.
  
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  Inorganic substances known as activators increase the activity of the enzyme.
  
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  Inhibitors are substances that decrease the activity of the enzyme or inactivate it.
  
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  Competitive inhibitors are substances that reversibly bind to the active site of the enzyme, hence blocking the substrate from binding to the enzyme. 
  
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  Incompetitive inhibitors are substances that bind to any site of the enzyme other than the active site, making the enzyme less active or inactive.
  
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  Irreversible inhibitors are substances that from bonds with enzymes making them inactive.
  

Enzyme Classification

The current system of nomenclature of enzymes uses the name of the substrate or the type of the reaction involved, and ends with "-ase". Example:'Maltase'- substrate is maltose. 'Hydrolases'- reaction type is hydrolysis reaction. 

Classification of enzymes

Enzymes are classified based on the reactions they catalyze into 6 groups: Oxidoreductases, transferases, hydrolases, lyases, isomearses, ligases.

Oxidoreductases - Oxidoreductase are the enzymes that catalyze oxidation-reduction reactions. These emzymes are important as these reactions are responsible for the production of heat and energy.

Transferases - Transferases are the enzymes that catalyze reactions where transfer of functional group between two substrates takes place.

Hydrolases - Hydrolases are also known as hydrolytic enzymes, they catalyze the hydrolysis reactions of carbohydrates, proteins and esters.

Lyases - Lyases are enzymes that catlayze the reaction invvolving the removal of groups from substrates by processes other than hydrolysis by the formation of double bonds. 

Isomerases - Isomerases are enzymes that catalyze the reactions where interconversion of cis-trans isomers is involved.

Ligases - Ligases are also known as synthases, these are the enzymes that catalyze the reactions where coupling of two compounds is involved with the breaking of pyrophosphate bonds.

Structure of Enzymes

Enzymes are proteins, like the proteins the enzymes contain chains of amino acids linked together. The characteristic of an enzyme is determined by the sequence of amino acid arrangement. When the bonds between the amino acid are weak, they may be broken by conditions of high temperatures or high levels of acids. When these bonds are broken, the enzymes become nonfunctional. The enzymes that take part in the chemical reaction do not undergo permanent changes and hence they remain unchanged to the end of the reaction.

 Enzymes are highly selective, they catalyze specific reactions only. Enzymes have a part of a molecule where it just has the shape where only certain kind of substrate can bind to it, this site of activity is known as the 'active site'. The molecules that react and bind to the enzyme is known as the 'substrate'. 

Most of the enzymes consists of the protein and the non protein part called the 'cofactor'. The proteins in the enzymes are usually globular proteins. The protein part of the enzymes are known 'apoenzyme', while the non-protein part is known as the cofactor. Together the apoenzyme and cofactors are known as the 'holoenzyme'.

Cofactors may be of three types: prosthetic groups, activators and coenzymes. 

Prosthetic groups are organic groups that are permanently bound to the enzyme. Example: Heme groups of cytochromes and bitotin group of acetyl-CoA carboxylase.

Activators are cations- they are positively charged metal ions. Example: Fe - cytochrome oxidase, CU - catalase, Zn - alcohol dehydrogenase, Mg - glucose - 6 - phosphate, etc.

Coenzymes are organic molecules, usually vitamins or made from vitamins. they are not bound permanently to the enzyme, but they combine with the enzyme-substrate complex temporarily.  Example: FAD - Flavin Adenine Dinucleotide, FMN - Flavin Mono Nucleotide, NAD - Nicotinamide Adenine Dinucleotide, NADP - Nicotinamide Adenine Dinucleotide.