So we have a partial positive charge here because the oxygen contains a partial negative charge. And so by the same analogy here when the nitrogen pulls away the H it gives the electrons to the sulfur and so the electron density on the sulfur increases and it becomes a better nucleophile. Here when the negative charge the oxygen pulls away the H, it makes the oxygen a better nucleophile because we essentially transform water into a hydroxide and the hydroxide is a much stronger nucleophile. At the same time, what happens is the other residue, the spartic acid, uses its H to basically interact with the oxygen of the Carbonal. So the oxygen here contains a partial negative charge and the H here contains a partial positive charge. And so, as they interact here, what begins to take place is we basically make the carbon of the carbonyl a better electrophile.