Difference between negative allosteric regulation and non-competitive inhibition
There Are Two Similar but Distinct Types of Noncompetitive Binding.
Starting from a pharmacological perspective, there are 2 definitions of &quot;noncompetitive&quot; binding that have similar macroscopic effects but differ slightly in their molecular mechanisms. Depending on which definition you use, noncompetitive ligands can bind either orthosterically or allosterically. Aspirin at cyclooxygenase and alanine at pyruvate kinase have both been referred to as &quot;noncompetitive&quot; (see below), despite aspirin binding orthosterically and alanine binding allosterically. Both types of inhibition involve depression of the maximum response, efficacy or enzyme activity. In other words, they reduce $y_{\mathrm{max}}$, $E_{\mathrm{max}}$ or $V_{\mathrm{max}}$. This is described in (Lippincott. (2015). Illustrated Reviews Pharmacology. 6th ed. p. 34). Similarly, (Goodman and Gilman. (2011). The Pharmacological Basis of Therapeutics. 12th ed. p. 46) gives an operational definition of noncompetitive antagonism as that which depresses the maximal response to the agonist but the molecular mechanism of action &quot;really cannot be inferred unequivocally from the effect&quot;. The two types of noncompetitive molecular mechanisms are (1) irreversible antagonism and (2) allosteric antagonism. Lippincott claims that both of these mechanisms are &quot;noncompetitive&quot; antagonism. Textbooks trump Wikipedia on credibility so I'm afraid to say that Wikipedia may have been leading people astray for years by saying that noncompetitive ligands can only bind allosterically. Due to this confusion, I think it's better to avoid the term noncompetitive entirely and opt for &quot;insurmountable&quot; inhibitor if the mechanism is unknown, or &quot;irreversible/allosteric inhibitor&quot; if the mechanism is.
Type 1: Irreversible Antagonism
Goodman and Gilmans' definition in the previous paragraph notes that the effect is for &quot;a slowly dissociating antagonist&quot; (i.e. an irreversible antagonist). This is mirrored in (Rang and Dale. (2015). Pharmacology. 8th ed. p. 11) and (Katzung. Basic and Clinical Pharmacology 11th ed. p. 31), which define noncompetitive antagonism as irreversible antagonism. Rang and Dale also makes the point that the term is ambiguous with other meanings of noncompetitive. Rang and Dale claim that aspirin is a noncompetitive antagonist at cyclooxygenase. It may be an irreversible antagonist but it's certainly not an allosteric one, as it modifies the orthosteric binding site. Another &quot;noncompetitive&quot; drug in this class (according to Rang and Dale) is phenelzine, which irreversibly binds MAO.
Type 2: Allosteric Antagonism
Not to be confused with negative allosteric modulators, (Goodman and Gilman. p. 46), (Rang and Dale. p. 17), (Lippincott. p. 34) and (Katzung, p. 32) all give a second definition of noncompetitive antagonism as antagonism at an allosteric site. This antagonism may be reversible or not but as the agonist can't displace the antagonist, it is insurmountable in all cases. Rang and Dale give the example of ketamine at the NMDA receptor, which depresses the maximal response by binding in the channel (an allosteric site) but doesn't bind irreversibly. For ligands relevant to enzymes, alanine and ATP allosterically (yet reversibly) bind pyruvate kinase. So this would cause insurmountable inhibition of pyruvate kinase activity but not irreversible inhibition. Various authors, such as (Mustafa and Hochachka, 1970) refer to this as &quot;noncompetitive&quot; inhibition.
Interestingly, I can't find any exogenous ligand that is both reversible and allosteric. It seems that allosteric antagonists are more relevant to receptors and transporters than enzymes. To make it even more confusing, Katzung claims that benzodiazepines (BDZ) act noncompetitively, since they bind allosterically. But this doesn't fit with any other definition of noncompetitive binding since BDZ is a positive modulator. Also interesting is that hexamethonium at the nAChR fits the definition of &quot;noncompetitive&quot;, regardless of which one you use. It is an irreversible blocker at an allosteric site.
Marangoni, on p. 70 of Enzyme Kinetics: A Modern Approach, also points out the difficulty in distinguishing between irreversible orthosteric and reversible allosteric inhibition. Using an irreversible inhibitor is equivalent to removing some enzyme from the system, so $V_{\mathrm{max}}$ drops. But at the same time, an allosteric inhibitor causes insurmountable inhibition (reduces $V_{\mathrm{max}}$). Neither type of inhibition alters the affinity of the substrate for the active site of the enzyme (apart from the irreversibly inhibited enzymes with no affinity) so $K_m$ is left unchanged. The figure below, from (Jahangirvand et al., 2016), shows the antagonism of catalase by cimetidine on a Lineweaver Burke Plot. A glance at this graph tells us that $V_{\mathrm{max}}$ is depressed, while $K_m$ is unchanged, so we could (arguably) call this noncompetitive inhibition. But this would not tell us the molecular mechanism of the inhibition.