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The effect of competitive inhibition is reduced at high substrate concentration.
So end-product inhibition being non-competitive means the metabolic process proceeds with greater efficiency. Less wastage of resources as inhibition occurs at all substrate concentrations.
A COMPETITIVE inhibitor of an enzyme, as the name tells you, COMPETES with the substrate for active sites on the enzyme. Therefore, the higher the substrate conc-n, the lesser the efficacy of the inhibitor.
Conversely, a non-competitive inhibitor, also called an irreversible inhibitor (as you cannot reduce its activity by increasing the concentration of substrate, due to its irreversible binding to the active site), is, (to repeat) not competing with the substrate for active sites.
It is easier to understand with an example:-
The antichoninesterase drugs used to treat myasthenia gravis, act by competitively blocking the decomposition of acetylcholine (ACh) by cholinesterase [into acetic acid and choline]; therefore, they can be safely used to treat the disease in which auto-antibodies against the ACh receptor destroy these receptors [in some patients, there are also anti-MUSK antibodies (Abs against MUscle Specific Kinase)], in case of overdose, there is a chance of recovery using drugs that inhibit the action of ACh (on muscarinic and nicotinic receptors).
However, in the case of the organophosphates (used as insecticides), the anticholinesterase action is non-competitive and irreversible, so can be fatal (ideally to insects ONLY!). Some snake venoms have a similar action.