An enzyme (E) catalyzes the biochemical reaction $A \rightarrow B$ with $k_{c a t}$ equal to $500 s^{-1}$. If the initial reaction velocity $\left(V_{0}\right)$ is $10 \mu M . s^{-1}$ at the total enzyme concentration $\left[E_{t}\right]$ of $30 n M$ and substrate concentration $[\mathrm{A}]$ of $40 \mu M$, the value of $K_{m}($ in $\mu M)$ is ______________.