Positive, sigmoidal cooperativity is known to occur to a monomeric single-site enzyme with slow conformational fluctuations in its unbound (E) states when the enzyme undergoes steady-state catalytic turnover (mnemonic enzymes, hysteretic enzyme). We show that positive cooperativity occurs even when the E state is a single thermodynamic state, provided that the fluctuation amplitude of the state is sufficiently greater than that of the enzyme-substrate complex ES and the fluctuation times being comparable. This can occur even without mean structural change between E and ES. Slow conformational fluctuations are widely observed in enzymes. Our result suggests that enzymes with substrate association that reduces conformational fluctuations while maintaining fluctuation temporality can exhibit sigmoidal binding inside of living cells. Implications of this result on drug-target interactions are discussed.