Why does a Hydrogen Bond with NH in histidine make the molecule more basic?

Question

In a catalytic triad, glutamate forms a strong H-bond with the H of NH in histidine. This apparently makes the histidine residue a stronger base so it can extract protons better but I can't figure out why? I use the thought experiment that a stronger base has more electron density to "grab" protons with but I can't figure out how the sp2 hybridized N in histidine gets more electron density.

Thanks for your help.

Answer 1

The imidazole ring(the histidine side group) has two Nitrogens the Nitrogen which has two bonds to carbon and a hydrogen is acidic because it can donate its lone pair around the imidazole ring. The other Nitrogen which has a double bond to a carbon and a single bond to a carbon is basic as its lone pair is not in the plane. Imidazole can exist in a tautomeric form with the basic Nitrogen switching to the acidic form and the acidic Nitrogen switching to the basic form. But as the oxygen of the glutamate side chain is hydrogen bonded to the the hydrogen of the acidic Nitrogen, The nitrogen will be less able to transform into the basic form, making the other Nitrogen more basic