Structural Biochemistry/Cyclooxygenase enzyme
COX-2 (cyclooxygenase) is an enzyme in our body that is vital in the formation of important biological mediators called prostanoids. Prostanoids is a class of signaling molecules that consists of prostaglandins, thromboxanes, and prostacyclins. Prostanoids are what is responsible for inflammation that occurs in our body.
Function of COX
The COX enzyme has two active sites which are the heme group and the cyclooxygenase site. The heme group has the ability to perform peroxidase activity which is responsible for reducing PGG2 to PGH2. The other active site, cyclooxygenase site, is where the arachidonic acid is converted to hydroperoxy endoperoxide prostaglandin, denoted PGG2. COX functions by converting arachidonic acid to prostaglandins, which is the precursor of series-2 prostanoids. A tyrosine radical is produced by the peroxidase active site, which then abstracts an H atom from the arachidonic acid to create an arachidonic acid radical. Then, two oxygen molecules react with the arachidonic acid radical to yield PGG2. There are three isoenzymes of cyclooxygenase (COX) that are known today. These three isoenzymes are COX-1, COX-2, and COX-3. COX-3 is a splice variant of enzyme COX-1 which means that COX-3 has a similar genetic code to COX-1. COX-3 retains intron one from COX-1 enzyme and has a frame shift mutation which is what causes its variation from the COX-1. For this reason, COX-3 is often referred to as COX-1b or COX-1 variant. According to each type of tissues, the amounts of COX-1 and COX-2 enzymes expressed within the tissue varies. Both enzymes have similar functions and so behave in similar manners. However, selective inhibition may result in a difference in side-effects. COX-1 is found in most mammalian cells and is considered to be a constitutive enzyme, which means that this enzyme is not controlled by repression or induction. COX-1 is produced constitutively by the cell under all physiological conditions. Contrary to the COX-1, COX-2 is an inducible enzyme which means that it is abundant in activated cells and macrophages. COX-2 is also abundant in sites of inflammation. COX-2 is undetectable in normal cells and it is has been recently shown that COX-2 is up-regulated in many carcinomas. This implies that COX-2 may have an active role in the formation of tumors.
The significant variation between COX-1 and COX-2 is that COX-1 has the amino acid isoleucine in position 523 while COX-2 has the amino acid valine instead. Valine is a smaller amino acid than isoleucine which is why COX-2 has the ability to access hydrophobic side pocket in the enzyme. COX-1 is unable to access this side pocket due to steric hindrance from the larger isoleucine. This special characteristic of COX-2 enables drug molecules such as DuP-697 to inhibit COX-2 by binding to this site. This discovery allowed for the production of drugs that are said to be selective inhibitors of COX-2 because they disable COX-2 without interrupting COX-1 activity.
The main COX inhibitors are called non-steroidal anti-inflammatory drugs, often denoted as NSAIDs. NSAIDs are unselective of which COX enzyme it inhibits, thus inhibiting all COX enzymes. This has both favorable and unfavorable side-effects. By inhibiting COX-2, NSAIDs have the effect of reducing inflammation and antipyretic, antithrombotic, and analgesic effects. However, because NSAIDs also inhibit COX-1 activity, it may cause negative side effects such as gastric irritation. COX-1 is responsible for producing mucous that is necessary in protecting the gastrointestinal tract. By inhibiting COX-1 activity, the production of the mucous is also inhibited, which may have an adverse effect on gastrointestinal tract.
Research into newer drugs has led to the discovery of drugs that selectively inhibit COX-2 without having much of an effect COX-1. COX-2 is usually specified to inflamed tissues, which is why there is a lesser risk of gastric ulceration associated with COX-2 inhibition. These selective inhibitors are common ingredients in arthritis medication such as Celebrex. Recent studies have been showing a correlation between the inhibition of COX-2 and higher risk of cardiovascular disease such as myocardial infarction. Vioxx was another brand that contained selective COX-2 inhibitors in its drugs but was removed from the market due to the recent discoveries that COX-2 inhibition may lead to increased risk of strokes and myocardial infarction.
COX-2 and Parkinson’s Disease
Neurologists have been studying COX-2 activity and its possible effects on Parkinson’s disease. Researchers believe that COX-2 inhibitors may preserve neurons, which is important to Parkinson’s disease because this disease is characterized by the death of neurons. The correlation between Parkinson’s disease and COX-2 is that these enzymes are responsible for inflammation in damaged tissues in the brain. Researchers have been noticing that inflammation has a critical role in neurodegenerative disease such as Parkinson’s disease and Alzheimer’s. Many believe that inhibiting COX-2 enzymes may be beneficial in stopping Parkinson’s disease and reduce the risks of Alzheimer’s.
Studies have been conducted by faculty members in Columbia University involving postmortem brains of patients who have been diagnosed with Parkinson's Disease. The research discovered that there was high levels of COX-2 enzymes in the dopamine neurons of these patients compared to those of without the disease. It was also discovered that dopamine neurons had suffered the most damage from Parkinson’s disease. Studies were also conducted on mice in order to test the importance of COX-2 in diseases similar to Parkinson’s disease. This further confirmed that diseases similar to Parkinson’s disease lead to high levels of COX-2 in dopamine neurons. When COX-2 enzymatic activity was diminished by using a selective inhibitor, the mice’s dopamine neurons were able to survive. Although COX-2 enzymes may be the responsible for the depletion of neurons, it is still unclear as to what causes the actual inflammation that is commonly associated with Parkinson’s disease. When COX-2 enzymes were removed, there were a larger number of dopamine neurons that survived yet inflammation was not reduced. From this information, it can be deduced that the COX-2 enzyme does not kill neurons through inflammation. An alternative theory on why COX-2 enzymes damage neurons is that COX-2 oxidized other molecules in the dopamine neuron which then react with other molecules, thus damaging other components in the cell. Eventually, this reactivity and excessive damage will lead to the death of dopamine neurons.