Where to find compounds IC50 on an enzyme?
To my knowledge, there are no data bases accumulating  half maximal inhibitory concentration values on a enzyme ($\mathrm{IC_{50}}$). According to Wikipedia:


  Half maximal inhibitory concentration ($\mathrm{IC_{50}}$) is a measure of the potency of a substance in inhibiting a specific biological or biochemical function (i.e. an enzyme, cell, cell receptor, microorganism, etc.).


$\mathrm{IC_{50}}$ is commonly used as a measure of antagonist drug potency in pharmacological research. According to the FDA, $\mathrm{IC_{50}}$ represents the concentration of a drug that is required for 50% inhibition in vitro. It is comparable to an $\mathrm{EC_{50}}$ for agonist drugs.

The major reason for not to accumulate $\mathrm{IC_{50}}$ values is their abundance. For example, for given specific enzyme, there may be vast number of inhibitors present. All of these inhibitors have their specific $\mathrm{IC_{50}}$ value. Thus, if you need to find a $\mathrm{IC_{50}}$ value of specific inhibitor on a particular protein (or enzyme) or a receptor or other biological system, you should search using both names of the inhibitor and biological system as key words. Thus, you would find any literature, which may have $\mathrm{IC_{50}}$ values for sought combination.

For example, Galectin-3 is  a protein that is a member of the lectin family (carbohydrate-binding proteins). Galectin-3 is approximately $\pu{30 kDa}$, and contains a carbohydrate-recognition-binding domain with about 130 amino acids that enable the specific binding of $\beta$-galactosides, such as N-acetyllactosamine (14; the parent substrate; $\mathrm{IC_{50}} = \pu{158 \mu M}$ under the conditions; see insert red box). Several derivatives of N-acetyllactosamine (15 - 18) have displayed inhibitory activities with different $\mathrm{IC_{50}}$ values ranging from $13$ to $\pu{31 \mu M}$ (See the plot below):



On the other hand one inhibitor can be active against more than one biological systems with different $\mathrm{IC_{50}}$ values.



Relevant References:


P. Sörme, B. Kahl-Knutsson, U. Wellmar, B.-G. Magnusson, H. Leffler, U. J. Nilsson, “[12] Design and Synthesis of Galectin Inhibitors,” In Methods in Enzymology, Volume 363 – Recognition of Carbohydrates in Biological Systems, Part B: Special Applications;   Y. C. Lee and R. C. Lee, Eds.; Academic Press: Amsterdam, The Netherlands, 2003, pp. 157–169.
P. Sörme, Y. Qian, Per‐G. Nyholm,  H. Leffler, U. J. Nilsson, “Low micromolar inhibitors of galectin-3 based on 3'-derivatization of N-acetyllactosamine,” ChemBio Chem 2002, 3(2-3), 183–189 (https://doi.org/10.1002/1439-7633(20020301)3:2/3&lt;183::AID-CBIC183>3.0.CO;2-%23).
J. Hynes, Jr., S.  K. Nair, “Chapter Nine - Advances in the Discovery of Small-Molecule IRAK4 Inhibitors,” Annual Reports in Medicinal Chemistry 2014, 49(1), 117–133 (https://doi.org/10.1016/B978-0-12-800167-7.00009-2).