Aspartate and glutamate both have carboxylic acid groups at the terminus of their side chains. This allows them to adopt protonated and deprotonated states, conveying a charge. This charging ability can be exploited in enzyme-catalysed reactions, allowing bonds to be made and broken in the substrate.
The environmental pH alters the ability of aspartate and glutamate to adopt a specific polarity, with an acidic environment increasing the probability their side chains will be protonated, and an alkaline environment making it more likely the carboxyl group will be deprotonated. This can impact the function of an enzyme or protein.