As I remember according the classical enzymology there are four types of inhibition:
Competitive, Non-competitive, Uncompetitive and allosteric Inhibition .
There is an enzyme with Ca2+ as a co-factor.
So, if we add some chelator like EDTA, what type of inhibition would we have?
I think that it's non-competitive but I'm not sure.
I would not classify the effect of the chelator on a Ca2+-dependent enzyme as any of those. In each case, the inhibitor is a substance that binds to a specific site on the enzyme in a 1:1 stoichiometry with the individual binding sites and is in dynamic equilibrium between free and bound states.
EDTA does not do this. It prevents the activator or co-substrate from binding. Since the Ca2+-EDTA complex is extremely tight (at neutral or alkaline pH), the free Ca2+ concentration will be essentially equal to the original Ca2+ concentration minus the EDTA concentration, until it reaches zero. The enzyme activity will follow the free Ca2+ concentration when titrating with EDTA in the same relationship as it would without EDTA present.
There are other ways of inhibiting enzymes that don't conform to the 4 types of inhibitors described by Michaelis-Menten kinetics. For example, you can inhibit an enzyme that has a cysteine in the active site by treating it with a chemical that reacts covalently with the thiol group, such as N-ethylmaleimide or iodoacetamide. Similarly, many enzymes with serine nucleophiles in the active site, such as serine proteases, can be inhibited covalently by reagents such as PMSF or DFP. You might think of these as competitive inhibition, since they block substrate binding, but they are not in equilibrium, so they do not conform to the assumptions of M-M kinetics.
Another way to inhibit enzymes is by adding a high concentration of a chaotropic agent that causes mild or complete denaturation, such as guanidine, urea or SDS. Still another way is by compounds that behave as aggregators, forming tiny droplets to which the enzyme binds in such a way that it is inhibited. The stoichiometry of inhibition by these mechanisms is much greater than 1:1, so they also do not conform to M-M kinetics.
The effect of a Ca2+ chelator like EDTA on an enzyme that requires Ca2+ as a cofactor can be precisely determined using classical enzymology and biochemistry. Chelators, such as EDTA, bind and sequester metal ions, including Ca2+, thereby reducing their availability for the enzyme. This action results in the inhibition of enzyme activity, as the enzyme loses its necessary cofactor for catalytic function.
Enzyme inhibitions are classified as competitive, non-competitive, uncompetitive, and allosteric. The effect of Ca2+ chelators does not fit exactly into these traditional categories, as the primary mechanism is not based on a direct interaction with the enzyme itself but on the removal of an essential cofactor. However, the inhibition induced by Ca2+ chelators could be most closely associated with non-competitive inhibition. This is because the chelator does not compete with the substrate for the enzyme's active site but indirectly affects enzymatic activity by removing an essential cofactor. Non-competitive inhibition occurs when an inhibitor binds to a site on the enzyme that is different from the substrate binding site. This alters the enzyme's conformation or functional capacity, regardless of the presence of the substrate.
Ca2+ chelators, for example, affect the structural integrity or catalytic capacity of the enzyme by removing Ca2+. This leads to a reduction in enzymatic activity without direct competition with the substrate. Therefore, although the specific action of Ca2+ chelation does not fit perfectly into classical categories, the effect of this inhibition most closely resembles non-competitive inhibition. The enzyme activity is reduced by a mechanism that does not involve direct competition for the active site.
To summarise, the inhibition exerted by Ca2+ chelators can be considered a form of non-competitive inhibition, in that it reduces enzyme activity indirectly, by removing an essential cofactor, without directly competing with the substrate for binding to the active site. This insight highlights the significance of metal cofactors for enzymatic activity and offers perspectives for understanding and developing therapeutic approaches based on the modulation of these cofactor-dependent processes.
Phil Käding Thank You. As I know Ca2+ plays a role as conformation stabilizer in mostly. Its interaction with substrate isn't clear. Anyway thanks a lot.
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