What is the molecular weight of alpha-2 macroglobulin?

• A. 720,000 daltons
• B. 180,000 daltons
• C. 1,800,000 daltons
• D. 72,000 daltons

Answer: (A)

Alpha-2 macroglobulin is a very large plasma protease inhibitor that circulates as a tetramer. Each subunit is about 180 kDa, so four subunits together give roughly 720 kDa. That places its molecular weight at about 720,000 daltons, which is why the correct value is 720,000. In contrast, a single subunit would be around 180 kDa, and much larger or smaller numbers don’t reflect its four-subunit assembly. This large size also explains why it’s noticeably bigger than proteins like albumin (66–67 kDa) or IgG (150 kDa).