Some amino acids have R groups that carry an electric charge in the conditions inside cells. An ionic bond in a protein is the attraction between a positively charged R group and a negatively charged R group. It is sometimes called a salt bridge.• Amino acids such as lysine and arginine have an extra amino group in their R group. At normal pH it gains an H+ and becomes –NH3+.
• Amino acids such as aspartic acid and glutamic acid have an extra carboxyl group in their R group. At normal pH it loses an H+ and becomes –COO−.–NH3+ ··· −OOC– When the chain folds so that two oppositely charged R groups are close together, they attract each other and help to hold the tertiary structure. Ionic bonds between R groups on different polypeptides can also help to hold the quaternary structure.
Strength. Ionic bonds between R groups are stronger than individual hydrogen bonds but weaker than disulfide bridges. They only form where the charges are close together, and they are weakened in watery surroundings.Effect of pH. Ionic bonds are very sensitive to pH. If the solution becomes more acidic, extra H+ ions add to –COO− groups, making them uncharged –COOH. If it becomes more alkaline, –NH3+ loses H+ and becomes uncharged. Either way the attraction is lost, the protein's shape changes, and an enzyme's active site may no longer fit its substrate: the protein is denatured.Worked example: lowering the pH to 2 turns –COO− into –COOH, so ionic bonds involving aspartic acid break.
• Amino acids such as aspartic acid and glutamic acid have an extra carboxyl group in their R group. At normal pH it loses an H+ and becomes –COO−.
Strength. Ionic bonds between R groups are stronger than individual hydrogen bonds but weaker than disulfide bridges. They only form where the charges are close together, and they are weakened in watery surroundings.Effect of pH. Ionic bonds are very sensitive to pH. If the solution becomes more acidic, extra H+ ions add to –COO− groups, making them uncharged –COOH. If it becomes more alkaline, –NH3+ loses H+ and becomes uncharged. Either way the attraction is lost, the protein's shape changes, and an enzyme's active site may no longer fit its substrate: the protein is denatured.Worked example: lowering the pH to 2 turns –COO− into –COOH, so ionic bonds involving aspartic acid break.