An enzyme inhibitor is something which inhibits enzyme activity. If I add to the reaction another substrate together with the primary one, I slow down the rate of my first substrate transition into products, but I do not really inhibit the enzyme. So I doubt whether it is correct to call this second compound a competitive inhibitor.
I think it is more appropriate to call this a secondary substrate or a competitive substrate. An inhibitor is traditionally defined as a ligand which blocks catalytic activity of the enzyme, but is not chemically modified by the enzyme. If this secondary substrate is converted to a product by the enzyme, it shouldn't really be considered an inhibitor.
The technical term for the situation you describe is an "alternate substrate." The enzyme is not inhibited, but the consumption of the first substrate is reduced due to competition for enzyme active sites by the alternate substrate.
The enzyme don't recognize the active sites for the completion of target reaction due to the presence two substrates at same time. So the secondary substrate don't called as competitive inhibitior. I think it's called competitive reaction between two substrates which alter the target reaction pathway.
Dear colleagues!! Thank you very much for the explanation. I was also thinking in that direction. But I am not enzyme specialist, so better ask.
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