Why does the research at Holmesburg Prison raise ethical concerns, despite the fact that informed consent was obtained?

The Tuskegee victims were not informed — in fact they were deliberately misinformed — about the nature of the study in which they were participants. The inmates at Holmesburg Prison were informed and signed consent forms. A basic guideline for human subject research, specified in both the Nuremberg Code and the Belmont Report is the requirement of informed consent.

What would have constituted informed consent in the case of the Tuskegee Study?

If such informed consent had been obtained from the subjects, would this remove all questions about whether the study was ethical?

Why does the research at Holmesburg Prison raise ethical concerns, despite the fact that informed consent was obtained?