Enzymes catalyze reactions using various physical and chemical mechanisms to lower the activation energy. These mechanisms may increase the reaction rate by positioning the enzyme and substrate appropriately, stabilizing the transition state, and helping to form or break bonds. Most enzymes adopt an induced fit when they bind their substrates. This conformational change alters the relative positions at the amino acids to increase interactions with the substrate. In reactions with multiple substrates, enzymes often position the molecules to resemble the transition state to aid their transformation. Many enzymes require metal ions for catalysis. Metal ions attract oppositely charged groups on substrates and orient them for the reaction. Ions can also change their oxidation state to stabilize charges on reaction intermediates. In some enzymes, amino acids accept or donate protons to locally change the pH or react directly with the substrate. This can strengthen or weaken bonds to increase reaction rates.