3.10. Enzyme Inhibiton and Drug design
Introduction: The drug is most commonly a small organic molecule that generates biochemical and physiological effect on patient’s body when it enters. Drug does not impart any new function on any system, cell or organ. It only alters the pace of ongoing cellular activity must by interacting with the biomolecules such as a protein present in cell which in turn results in a therapeutic effect to the patient. To generate a specific response drug must interact specifically with a specific biomolecules i.e. drug must interact with a specific target to show its potency. Therefore, to have a potent drug candidate for a specific disease, one must have to design a drug rationally. Now, the cause of disease is the malfunctioning of certain biomolecules like, gene, protein or enzymes or because of foreign organism (microbes). Finding the diseases related target, one can administer drug. Thus, Drug design, (rational drug design or structure based drug design), is the innovative process of finding new medications based on the knowledge of the biological target. Thus, in the most basic sense, drug design involves design of small molecules that are complementary in shape and charge to the biomolecular target to which they interact and therefore will bind to it. That a drug can only show its effect when it interacts with its target can be proved by the example of knockout mice. A lack of effect of a drug in mice lacking a particular target can provide strong support that the effects of the drug are mediated by that target.

3.10.1. What Is Drug Targets?
Most drugs work because by binding to the target in the cell, they can either block the physiological function of the target, or mimics its effect. If a drug causes the target to respond in the same way as the naturally occurring substance, then the drug is referred to as an agonist. Antagonists are drugs that interact selectively with the target but do not lead to an observed effect. Instead they reduce the action of an agonist at the target site involved. So what is a drug target?
A “target” of a drug may be defined as a biomolecular structure that undergoes a specific interaction with chemicals (drugs) because they are administered to treat or diagnose a disease. The interaction has a connection with the clinical effect(s). Almost all the biomolecules acting as a drug target have a specific three-dimensional structure which allows only substances that fit precisely to attach to it. According to this definition we are restricting the target as a static bio-molecular architecture. However, Life process, including disease state is a dynamic because we do not yet directly observe the interactions of drugs and targets and only what we notice is the biochemical response they produce.
For most drugs, many targets were identified. Consequently, for drug administration we must have to rely on the existing data that showed some connection between the interaction of the drug with the biochemical structure of the target and the beneficial clinical effect(s). A chemical with a certain reactivity or binding property is used as a drug because of its clinical effects. However it would be challenging to prove that a certain molecular interaction triggers the drug effect(s). That a drug can only show its effect when it interacts with its target can be proved by the example of knockout mice. A lack of effect of a drug in mice lacking a particular target can provide strong support that the effects of the drug are mediated by that target. All drugs somehow interfere with signal transduction, receptor signaling and biochemical equilibrium. Also several drugs interact with more than one target. So, there will be simultaneous changes in several biochemical signals, and there will be feedback reactions of the pathways disturbed, drug action is a dynamic process. All the discussion about the target led to conclude that a clinically relevant 'target' might consist not of a single biochemical entity, but the simultaneous interference of a number of pathways, enzymes and so on. Only this will give a net clinical effect that might be considered beneficial.
Greater knowledge of how drugs interact with the body (mechanisms of action, drug–target interactions) has led to a reduction of established drug doses and inspired the development of newer, highly specific drug substances with a known mechanism of action. Considering system biology, dynamic nature of drug target and drug response we should say at molecular level that the nature of target is in-between “macro”- and “micro”-target. Therefore a drug target can best be defined as a key bio-moleculear entity of a cell involved in a particular metabolic or signaling pathway that is specific to a disease condition or pathology, or to the infectivity or survival of a microbial pathogen.