Figure-1- Effect of enzyme inhibitor. Km value has increased, Vmax is unchanged
Figure-2- Double reciprocal curve showing the effect of inhibitor.
A 20 year- old man was brought to the emergency with vomiting, sweating, drooling and a decreased heart rate. History reveals that he was in a corn field when it was sprayed by a crop duster. The pesticide causing his symptoms is an organo phosphate that covalently binds to acetyl cholinesterase and inactivates the enzyme. The enzyme kinetics affected by the inhibitor are shown in the figures. What is the mechanism of inhibition of this enzyme ?
Case discussion– It is a case of Organophosphate poisoning. Organophosphate poisoning results from exposure to organophosphates (OPs), which cause the inhibition of acetyl cholinesterase (AChE), leading to the accumulation of acetylcholine (ACh) in the body.
Acetylcholine is a chemical neurotransmitter found widely in the body. It triggers the stimulation of post-synaptic nerves, muscles, and exocrine glands.
Acetylcholinesterase (generally referred to as cholinesterase) is an enzyme that rapidly breaks down the neurotransmitter, acetylcholine, so that it does not over-stimulate post-synaptic nerves, muscles, and exocrine glands (Figure-3).
Figure-3- The normal mechanism of action of Acetyl cholinesterase. Serine is present at the active site of the enzyme. Acetylcholine, attaches to the enzyme, Acetyl cholinesterase and is broken down to Acetic acid and Choline.
Acetylcholinesterase inhibitor (generally referred to as cholinesterase inhibitor) is a chemical that binds to the enzyme, cholinesterase, and prevents it from breaking down the neurotransmitter, acetylcholine. With toxic doses, the result is that excessive levels of the acetylcholine build up in the synapses and neuromuscular junctions and glands.
Thus, the primary manifestations of acute cholinesterase inhibitor toxicity are those of cholinergic (neurotransmitter) hyperactivity. There are also other delayed and chronic pathological effects of inhibitors of the cholinesterase enzyme which are less well understood.
Organophosphate (OP) compounds are a diverse group of chemicals used in both domestic and industrial settings. Examples of organophosphates include insecticide, nerve gases, ophthalmic agents and herbicides etc
OPs are one of the most common causes of poisoning worldwide .There are around 1 million OP poisonings per year with several hundred thousand resulting in fatalities annually.
Exposure to organophosphates (OPs) is also possible via intentional or unintentional contamination of food sources. Although no clinical effects of chronic, low-level organophosphates (OPs) exposure from a food source have been shown.
The mechanism of action of the inhibitor shown in the graphs and the mechanism stated in the case study do not correspond with each other.
As per the graphs- The Km value has increased , while Vmax is constant. This is possible only in competitive enzyme inhibition.
The effects of competitive inhibitors can be overcome by raising the concentration of the substrate. Most frequently, in competitive inhibition the inhibitor, (I), binds to the substrate-binding portion of the active site and blocks access by the substrate. The structures of most classic competitive inhibitors therefore tend to resemble the structures of a substrate, and thus are termed substrate analogs. In effect, a competitive inhibitor acts by decreasing the number of free enzyme molecules available to bind substrate, ie, to form ES, and thus eventually to form product.
In noncompetitive inhibition, binding of the inhibitor does not affect binding of substrate. Formation of both EI and EIS complexes is therefore possible. However, while the enzyme-inhibitor complex can still bind substrate, its efficiency at transforming substrate to product, reflected by Vmax, is decreased. Noncompetitive inhibitors bind enzymes at sites distinct from the substrate-binding site and generally bear little or no structural resemblance to the substrate.
As per the details provided in the case- The Organophosphate covalently binds to enzyme to bring about its inactivation. Since these covalent changes are relatively stable, the enzyme gets “poisoned” by the irreversible inhibitor.
This is not possible in competitive enzyme inhibition.
Organophosphates inhibit AChE, causing OP poisoning by phosphorylating the serine hydroxyl residue on AChE, which inactivates AChE. AChE is critical for nerve function, so the irreversible blockage of this enzyme, causes acetylcholine accumulation, resulting in muscle overstimulation. (Figure-4)
Figure-4- Organophosphorus compounds bind to the active site causing its phosphorylation to form a phosphorylated inactive enzyme, that fails to catalyze the degradation of Acetyl choline. The phosphorylation occurs by loss of an organophosphate leaving group and establishment of a covalent bond with AChE.
Thus it is not a competitive enzyme inhibition, it is non competitive enzyme inhibition, though the inhibitor is bound to the active site but it has brought about the inactivation of the enzyme.
In non competitive enzyme inhibition, Km remains unchanged but Vmax is deceased, thus the inhibition of Acetyl cholinesterase by Organophosphates is non competitive enzyme inhibition. The comparison of the two types of inhibitions can be seen in the figures-
Figure-5- In competitive enzyme inhibition, Vmax is unchanged, Km is increased, while reverse occurs in non competitive enzyme inhibition, Km remains constant, but Vmax is decreased.
Figure-6- Double reciprocal curve (Line weaver burk plot) showing the comparison between competitive and non competitive enzyme inhibition.