Enzyme Structure Part L

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Sequential reaction results in the formation of a ternary complex. This means that both of the substrates involved in the reaction bind with an enzyme to form the product figure 1. Sequential reaction is further divided into two types: the random and compulsory order mechanisms. As the name suggests, in a 'random' mechanism, either substrate can bind first and any product can leave first.

In contrast to the random order mechanism, in the compulsory order mechanism the order of binding of the substrate and order of release of the product is specific; this is also called the Theorell—Chance mechanism figure 1. In a non-sequential reaction, also called the 'ping-pong' mechanism, formation of ternary complex does not take place. In these types of reactions, when the first substrate binds with enzyme its product is released, and then the second substrate binds and its product is released. Such a reaction is called a double placement reaction.

Thus only a single substrate binds at a time; this may be due to the presence of a single binding site on the enzyme. Major differences between the sequential and non-sequential reactions are that the formation of a ternary complex takes place only in the sequential reaction, and that in the sequential reaction both substrates bind to the enzyme and release products, while in the non-sequential mechanism the substrates bind and release their products one after the other figure 1. Another type of sequential mechanism is the systematic mechanism, which involves the addition of substrates and formation of products in a specific order.

Several protein enzymes use general acid—base catalysis as a way to increase reaction rates [ 26 ]. The amino acid histidine is optimized for this function because it has a pK a where K a is the acid dissociation constant near physiological pH [ 26 ]. When the substrate has been bound at the catalytic site, the charged functional groups of the side chains of neighboring aminoacyl residues may contribute in catalysis by behaving as acidic or basic catalysts.


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There are two extensive groups of acid—base catalysis by enzymes: general and specific acid or base catalysis. In contrast to specific catalysis, general acid or general base catalysis are the reactions whose rates are very reactive to all acids proton donors or bases proton acceptors present in the solution.

To examine whether a given enzyme-catalyzed reaction is a general or specific acid or base catalysis, the rate of reaction is determined under two sets of circumstances:. The major role of these metal ions is investigated using techniques such as x-ray crystallography, magnetic resonance imaging MRI and electron spin resonance ESR.

A metalloprotein is a protein that contains a metal ion co-factor. Metallozymes contain a certain amount of functional metal ion that is retained during the course of purification [ 27 ]. A metal-activated enzyme binds with metals less firmly, but needs to be activated by addition of metals. Four types of complexes are possible for the tertiary complexes of the catalytic site Enz , a metal ion M and substrate S that exhibit stoichiometry:. All of these complexes are possible for metal-activated enzymes. Metallozymes cannot form the EnzSM complex substrate—bridge complexes , as the purified enzyme exists as Enz—M.

Three generalization can be made:. The metal ions participate in each of the four mechanisms by which the enzymes are known to accelerate the rates of chemical reaction:. Metal ions are electrophiles attracted to electrons and share an electron pair forming a sigma bond.

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Enzyme Structure, Part L, Volume 131

They may also be considered as super acids as they exist in neutral solutions, frequently having a positive charge which is greater than their quantity. Two metal ions, iron and manganese are used in the form of haemprotein. Metal ions have the potential to accept electrons via sigma or pi bonds to successively activate electrophiles or nucleophiles.

By means of donating electrons, metals can activate nucleophiles or act as nucleophiles themselves. The co-ordination sphere of a metal may bring together the enzyme and substrate or form chelate-producing distortion in either the enzyme or substrate [ 28 ]. A metal ion may also mask a nucleophile and thus avoid an otherwise probable side reaction. Metals can also function as three-dimensional templates for the co-ordination of basic groups on the enzyme or substrate.

Enzyme inhibition decreases the activity of an enzyme without significantly disrupting its three-dimensional macromolecular structure. Inhibition is therefore distinct from denaturation and is the result of a specific action by a reagent directed or transmitted to the active site region. When low molecular weight compounds interfere with the activity of enzymes by partially reducing or completely inhibiting the enzyme activity either reversibly or irreversibly, it is known as enzyme inhibition.

The compounds responsible for such inhibition are called enzyme inhibitors. To protect the enzyme catalytic site from any change, a ligand binds with a critical side chain in the enzyme. Chemical modification can be performed to test the inhibitor for any drug value. Studies of enzymes can yield much information about the following:. The pharmacological action of drugs is mainly based on enzyme inhibition, e.

In the majority of cases the enzyme inhibited is known. The development of nerve gases, insecticides and herbicides is based on enzyme inhibition studies. There are two major types of enzyme inhibition: reversible and irreversible. Reversible inhibitors efficiently bind to enzymes by forming weak non-covalent interactions, e. Reversible inhibitors do not form any strong chemical bonds or reactions with the enzyme, they are formed quickly and can easily be removed, in contrast to irreversible inhibitors.

Reversible inhibition includes competitive inhibition, uncompetitive inhibition and noncompetitive inhibition.


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Irreversible inhibition includes group specific inhibition reacts only to a certain chemical group , reactive substrate analogs affinity label and inhibitors that are structurally similar to the substrate and will bind to the active site, and mechanism-based inhibitors enzymes transform the inhibitor into a reactive form within the active site. Currently, enzymes are often utilized for a broad range of applications such as: washing powders e.

Details on the applications of individual enzymes are provided in table 1.

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Industrially produced enzymes from plant sources and their applications. Enzymes have been used widely in diagnostic applications varying from immunoassays to biosensors. Enzyme immunoassay methods hold great promise for application under a wide variety of conditions. Under laboratory conditions they can be as sensitive as radio-immunoassays, but they can also be adapted as simple field screening procedures [ 29 , 30 ].

The examination of enzyme quantity in the extracellular body fluids blood plasma and serum, urine, digestive juices, amniotic fluid and cerebrospinal fluid are vital aids to the clinical diagnosis and management of disease. Most enzyme-catalyzed reactions occur within living cells, however, when an energy imbalance occurs in the cells because of exposure to infective agents, bacterial toxins, etc, enzymes 'leak' through the membranes into the circulatory system.

This causes their fluid level to be raised above the normal cell level. Estimation of the type, extent and duration of these raised enzyme activities can then furnish information on the identity of the damaged cell and indicate the extent of injury. Enzyme assays can make an important contribution to the diagnosis of diseases, as a minute change in enzyme concentration can easily be measured.

Determination of the changes in enzyme level thus offers a greater degree of organ and disease differentiation in comparison to other possible clinico-chemical parameters, e. Currently, the diagnostic specificity of enzyme tests is such that they are limited primarily to confirming diagnosis, offering data to be weighed alonside other clinical reports, owing to lack of disease specific enzymes. The diseases of the liver and gastrointestinal tract were among the first to which serum enzyme tests were applied.

They have proved to be most effective owing to the large size of the organs and the wide range and abundance of enzymes [ 32 — 36 ]. Several enzymes employed in the diagnosis of liver diseases along with their respective levels are listed in table 1.

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Liver diseases and enzymes used in diagnosis [ 32 — 36 ]. According to previous reports, no single enzyme has yet been reported to cure myocardial damage. The discovery of serum glutamine oxalacetic acid transaminase determination GOT in was considered a significant step forward in the diagnosis of acute myocardial infarction.

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The rise in enzyme levels is fairly moderate, AST and CPK increase by four to ten times their respective normal levels and LD 1 is approximately five-fold higher than normal. An enzyme known as hyaluronidase hyaluronate hydrolysis has been reported to cure heart attack [ 38 ].

1.1. Introduction

The activity of many enzymes including aldolase, malic dehydrogenase, isomerase and ICD may increase following myocardial infarction [ 38 ]. Skeletal muscle disorders include diseases of the muscle fibers myopathies or of the muscle nerves neurogenic disorders [ 40 ].

In the case of neurogenic diseases and hereditary diseases, CPK is occasionally raised 2—3 fold [ 40 ]. Damage to the muscle may be due to extensive muscular exercise, drugs, physical trauma, inflammatory diseases, microbial infection or metabolic dysfunction, or it may be genetically predisposed. In muscular disorders the level of CPK is elevated in serum with the highest frequency and is assayed in the diagnosis of these disorders.

An additional useful assayed enzyme is acetyl cholinesterase AChE , which is significant in regulating certain nerve impulses [ 41 ]. Various pesticides affect this enzyme, so farm labors are frequently tested to be sure that they have not received accidental exposure to significant agricultural toxins.

How Enzymes Work

There are number of enzymes that are characteristically used in the clinical laboratory to diagnose diseases. There are highly specific markers for enzymes active in the pancreas, red blood cells, liver, heart, brain, prostate gland and many of the endocrine glands [ 41 ]. Enzymes have two significant features that differentiate them from all other types of drugs. First, enzymes frequently bind and act on their targeted sites with high affinity and specificity.

Second, enzymes are catalytic and convert numerous target molecules to the desired products. These two important features make enzymes specific and potent drugs that can achieve therapeutic biochemistry in the body that small molecules cannot. These features have resulted in the development of many enzyme-based drugs for a wide range of disorders [ 42 ]. Currently, numerous enzymes are used as therapeutic agents, owing to the following features:.

Enzymes as therapeutic agents also have some serious disadvantages which restrict their application. Their bulky structure, due to their large molecular weight, excludes them from the intracellular domain. Owing to their high proteinaceous nature they are highly antigenic and are rapidly cleared from blood plasma. Extensive purification from pyrogens and toxins is essential for parenteral enzymes, which increases the cost. In traditional medicine, proteolytic enzymes derived from plant extracts have been used for a long time In addition to proteolytic enzymes from natural resources such as plants, 'modern' enzyme therapy includes pancreatic enzymes.

Therapeutically, the use of proteolytic enzymes is partly based on scientific reports and is partly empirical [ 43 ]. Clinical evidence of the use of proteolytic enzymes in cancer studies has typically been obtained with an enzyme preparation comprising a combination of papain, trypsin and chymotrypsin. Earlier reports proved that enzyme therapy can reduce the adverse effects caused by radiotherapy and chemotherapy.

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