I understand that $V_\mathrm{max} = k_3[\ce{E}]_0$ in ordinary Michaelis–Menten (MM) kinetics. According to the lecture notes provided by my university (I don't believe they are available online), when the amount of enzyme is treated as a dynamic variable the reaction may be described by
$$\frac{\mathrm d[\ce{P}]}{\mathrm dt} = \frac{V'_\mathrm{max}[\ce{S}][\ce{E}]}{K_\mathrm{M}+[\ce{S}]}.$$
What does $V'_\mathrm{max}$ represent mathematically in this case? And how can such an equation be derived?
I am also told that this equation assumes that the concentration of the enzyme changes slowly compared to the change in $\ce{P}.$ Why?