Buffer systems in the body maintain proper pH balance in order for life processes to proceed. Proteins found in the blood plasma are often used as buffers. What makes a protein able to function as a buffer?

The protein’s molecular shape must enable it to bind with other proteins in the plasma.
The protein must be capable of both accepting and releasing protons.
The protein must maintain its ability to release various amino acids, thus changing its molecular shape.
The protein must take on a conformation change according to the activity of the associated enzyme in regulating the blood’s pH.