The cyclooxygenase isozymes

3.1 Isozymes of Cyclooxygenase

Cyclooxygenase has assorted isozymes. The chief isozymes are COX-1 and COX-2, nevertheless there is now grounds of a 3rd form- COX-3.

COX, originally known as prostaglandin H synthase is responsible for the oxidization of arachadonic acid to prostaglandin G2 and prostaglandin H2. It catalyses the reaction in which the arachadonic acid substrate and two molecules of O2 are converted to prostaglandin G2 and so in the perioxidase reaction Prostaglandin G2 is reduced to PGH2 by a 2 negatron decrease.

The COX isozymes are heme incorporating enzymes that are homodimers. Each monomer contains three chief spheres ; A membrane binding sphere, a N-terminal cuticular growing factor sphere and a C-terminal catalytic sphere. Cyclooxygenase-1 is made up of 602 aminic acids while COX-2 is comprised of 604.1

The catalytic reaction in COX takes topographic point in a hydrophobic channel in the nucleus of the enzyme while the peroxidise reaction takes topographic point in the haem incorporating part near the surface of the enzyme. The membrane adhering sphere consists of four alpha spirals with one spiral that fuses with the catalytic sphere. These spirals congregate around an gap and through these gaps fatty acids and NSAIDS are considered to come in the active site. The COX-1 isozyme is considered a constituent enzyme. It is present in high volumes in most cells and tissues i.e. nephritic collection tubules, monocytes, endothelium etc. However COX-2 is barely noticeable in most cells, it is an inducible enzyme so it becomes more abundant in cells or tissues when macrophages are activated or by any other redness go-betweens e.g. TNF-a ( tumor necrosis factor-alpha ) or IL-1 ( interleukin-1 ) .1

Both COX-1 and COX-2 isozymes are attatched to the endoplasmic Reticulum and atomic envelope. The COX isozymes need to be N-linked glycosylated to enable them to be folded and attatched to the endoplasmic Reticulum and atomic envelope. The COX isozymes have really similar constructions for their binding site, catalytic mechanisms and produce the same biosynthetic products.1

Table?

COX-3

COX-3 a 3rd isozyme was discovered in 2002 by Simmons and colleagues. They conducted a survey on Canis familiariss and this resulted in them detecting a fresh COX-1 splicing discrepancy termed COX-3 that was sensitive to acetaminophen ( paracetamol ) . It was suspected for a piece that acetaminophen worked by suppressing a different specific isozyme due to the fact that it did non straight suppress COX-1 and COX-2 really efficaciously at curative concentrations but it generated prostanoids in neural systems. 1

The Simmons and colleague group showed that Datril was the existent mark for COX-3, and that it acted individually from COX-1 and COX-2. 1

Canine COX-3 is a membrane edge protein dwelling of 613 aminic acids with a molecular weight of ~65 kDa. It has a high look in cells and tissues like COX-1 proposing it may be a constituent enzyme. However the inquiry that needs to be asked is if generalizations can genuinely be made on the presence of COX-3 in worlds based on Canine surveies, so future experiments need to be designed to find whether a human COX-3 exists that acts independently from COX-1 and COX-2 in vivo. 1

NSAIDs are known to suppress COX in order for them to exhibit their anti-inflammatory actions, a structural NSAID binding survey was carried out.

The COX-1 active site contains a long hydrophobic channel that extends from the membrane adhering sphere to the nucleus of the COX monomer. The tip of the COX active site houses Tyr385 that is located near the haem Fe. Ser530 is positioned merely below Tyr385 and that is the site for aspirin acetylation. Glu524 and Arg120 are positioned at the oral cavity of the COX-1 channel. A typical NSAID such as fluobriprofen, its carboxylate mediety is normally directed towards the oral cavity of the COX-1 channel in order for it to be positioned in the most ideal topographic point that will let it to interact with the two polar residues Glu524 and Arg120. From these surveies a better penetration into the binding profiles of NSAIDs.

Non selective NSAIDs can adhere in three different ways:

• Reversibly ( e.g. Ibuprofen )
• Fast, low affinity reversible binding followed by a higher affinity, clip dependent easy reversible binding ( e.g. fluobriprofen )
• Rapid, reversible binding followed by a covalent alteration of the enzyme ( e.g. Aspirin ) 1

Arg120, Glu524, Tyr355 and His90 form a web of H bonds at the entryway of the COX channel moving like a gate to the binding site. NSAIDs by and large bind between the upper part of the COX channel near Tyr 385 and Arg 120 which is at the oral cavity of the COX channel. 1

Through the usage of H bonding and electrostatic interactions, the carboxyl mediety of acidic NSAIDs like fluoribiprofen interact with Arg120 in both COX isozymes. The important differences in the construction of the binding sites for both COX isozymes has been manipulated to enable the design of selective COX-2 inhibitors.

In the COX-2 active site there is an excess accessible pocket due to the presence of a smaller valine amino acid residue at place 523 and a valine permutation at place 434, unlike COX-1, this difference increases the overall volume at the COX-2 active site by about 20 % . 1 refer to Joyces diagram. This means that due to cut down steric and ionic crowding at the oral cavity of the channel by Arg120, not acidic selective COX-2 inhibitors can demo an enhanced and specific binding to the COX-2 enzyme. Another structural difference exists at the amino acid residue 513 where COX-1 has a histidine residue and COX-2 has a arginine mediety. 1 These little differences provides flexibleness in the substrates that can be utilised in the COX-2 active site. 1

Problems Associated With Non Selective Non Steroidal Anti-Inflammatory Drugs

NSAIDs are one group of drugs that are on a regular basis used by the universe ‘s population to alleviate hurting, cut down redness and lower temperature. They are COX inhibitors and act to suppress the catalysation of arachadonic acid to PGH2. COX-1 is constitutively present in most cells while COX-2 is induced by chemical go-betweens of redness and activated macrophages.

Naproxen

Ibuprofen

Figure 1. Some representative illustrations of non selective COX inhibitors.

COX-1 and COX-2 as mentioned above have 2 specific functions. The first function gives PGG2 and the other function is in the peroxidise reaction that gives PGH2. Both COX-1 and COX-2 inhibitors work by suppressing the 1st and chief function i.e. suppressing the transition of arachadonic acid to PGG2. COX-1 and COX-2 possesses hydrophobic channels within their nucleus. The classical NSAIDs exhibit their effects by barricading these enzymes halfway down the COX channel near Tyr385 and the Arg120 which is at the oral cavity of the COX channel by H bonding to the Arg120 residue. This consequences in the prohibition of any fatty acid substrates from come ining the catalytic sphere of the COX enzyme.

Mechanism

In COX-1, these drugs tend to suppress the enzyme rapidly yet by and large the suppression is frequently reversible, nevertheless in COX-2 the suppression is clip dependent and frequently consequences in irreversible suppression.

As mentioned before, the COX-1 and COX-2 isozyme differ somewhat. In the COX-2 active site there is an excess accessible side pocket due to the presence of a smaller valine amino acid residue at place 523 alternatively of isoleucin as in COX-1. This is of import for understanding why some Nonsteroidal anti-inflammatory are selective for the COX-2 isozyme.

There are a figure of side effects associated with traditional NSAID therapy. NSAIDs can do nephritic failure, liver damage/disorders, sterile meningitis, skin reactions and bone marrow perturbations which can interfere with bone break healing. However amongst them all GI ( GI ) toxicities is amongst the most common. These are believed to originate from the suppression of COX-1 in the stomachic mucous membrane.

In worlds and other species it has been shown that COX-1 non COX-2 is constitutively expressed throughout the GI piece of land. COX-1 is responsible for the synthesis of prostaglandins like PGE2 and PGI2 which are responsible for protecting the GI mucous membrane by cut downing acerb secernment in the tummy by the parietal cells, increasing blood flow in the mucous membrane and exciting the release of syrupy mucose. This leads to conditions of ulcers, indigestion, diarrhea, sickness and emesis and can even take to stomachic hemorrhage in some instances.

This has led to the development of COX-2 selective inhibitors.

Structure

These drugs are effectual anti-inflammatories and reflect good analgetic effects nevertheless they have considerable less stomachic harm due to the fact they selectively inhibit COX-2 with minimum action on COX-1.

Unfortunately the usage of COX-2 selective drugs has been associated with increased incidence of myocardial infarction and shot.

Nephritic effects

Prostaglandins particularly PGE2 and PGI2 are involved in modulating nephritic blood flow and vascular tone. Recent surveies have shown that COX-2 is constitutively expressed in the sunspot densa, epithelia cells run alonging the go uping cringle of henle and medullary interstitial cells of the nephritic papillae, while COX-1 is constitutively expressed in the collection canals, cringle of henle and in the vasculature. The COX-2 enzyme is associated with normal nephritic map and suppression of COX-2 consequences in NSAID-induced Na keeping while suppression of COX-1 consequences in a disease in glomerular filtration rate.

This once and for all tells us that both COX-1 and COX-2 are involved in the physiology of the kidneys. However curative doses in patients with normal nephritic map are at small hazard of nephritic complications. It is largely newborns and the aged who are more susceptible every bit good as patients with bosom, liver or kidney disease.

Mechanism of action of COX inhibitors in the intervention of human diseases- Anti-inflammatory

Inflammation is characterised by dolour, inflammation, calor and tubor, it ‘s one of the organic structures ways of reacting to harmful stimulations, pathogens, hurt or disease. These normally initiate an inflammatory response either ague or chronic.

An addition in microvascular permeableness occurs selectively in post-capillary venulas. The endothelial cells undergo conformational alteration taking to vascular escape through spreads between the next endothelial cells. Potent go-betweens that addition microvascular permeableness include histamine, bradykinin, substance P, thrombocyte triping factor, leukotriene C4 and leukotriene D4. These go-betweens act via specific receptors on endothelial cells. At site of hurt scavenger cells are attracted and travel into the affected tissue along with plasma. The plasma causes the associated swelling observed in redness and the scavenger cells engulf dead cells and bacteriums. As the go-betweens released Begin to vanish no more scavenger cells are attracted and the tissue bit by bit begins to mend and return to normal.

Something about arthritis and redness diseases.

A figure of anti-inflammatory drugs are available worldwide and are widely used to alleviate hurting and other symptoms associated with soft tissue redness. A immense bulk of these drugs act via the suppression of COX. Prostanoic acids are produced via the metamorphosis of fatty acids through the COX tract. These agents are said to be really of import go-betweens in symptoms associated with redness such as hydrops ( swelling ) and hurting. The COX enzyme plays an of import function in the synthesis of prostanoic acids from arachadonic acids.

There are 2 COX isozymes in the organic structure. COX-1 mediates cellular procedures and produces prostanoic acids, while COX-2 is mediated by proinflammatory cytokines. Cox inhibitors such as NSAIDs exhibit their effects by suppressing the first measure in the biosynthetic tract of change overing arachadonic acid to PGG2 hence forestalling the synthesis of PGH2.