When a protein molecule approaches a charged surface, its protonation state can undergo dramatic changes due to the imposed electric potential. This has a large impact on adsorption strengths that may be enhanced by several kT. Using mesoscopic simulation techniques as well as analytical theories, we have investigated this regulation mechanism and demonstrate how it is influenced by salt concentration and solution pH. Using hisactophilin as a test case, we show how the binding to a lipid membrane is governed by small changes in pH and that this is intimately coupled to the charge regulation mechanism.