Alpha-2-Macroglobulin is the largest major nonimmunoglobulin protein in plasma. The alpha-2-macroglobulin molecule is synthesized mainly in liver, but also locally by macrophages, fibroblasts, and adrenocortical cells.
Alpha 2 macroglobulin acts as an antiprotease and is able to inactivate an enormous variety of proteinases. It functions as an inhibitor of fibrinolysis by inhibiting plasmin and kallikrein. It functions as an inhibitor of coagulation by inhibiting thrombin. Alpha-2-macroglobulin may act as a carrier protein because it also binds to numerous growth factors and cytokines, such as platelet-derived growth factor, basic fibroblast growth factor, TGF-β, insulin, and IL-1β.
No specific deficiency with associated disease has been recognized, and no disease state is attributed to low concentrations of alpha-2-macroglobulin.
The concentration of alpha-2-macroglobulin rises 10-fold or more in the nephrotic syndrome when other lower molecular weight proteins are lost in the urine. The loss of alpha-2-macroglobulin into urine is prevented by its large size. The net result is that alpha-2-macroglobulin reaches serum levels equal to or greater than those of albumin in the nephrotic syndrome, which has the effect of maintaining oncotic pressure.
Human alpha-2-macroglobulin is composed of four identical subunits bound together by -S-S- bonds. In addition to tetrameric forms of alpha-2-macroglobulin, dimeric, and more recently monomeric aM protease inhibitors have been identified.
Each monomer of Human alpha-2-macroglobulin is composed of many functional domains, including macroglobulin domains, a thiol ester-containing domain and a receptor-binding domain.
The alpha-macroglobulin (aM) family of proteins includes protease inhibitors, typified by the humantetrameric alpha-2-macroglobulin (a2M); they belong to the MEROPS proteinase inhibitor family I39, clan IL. These protease inhibitors share several defining properties, which include (i) the ability to inhibitproteases from all catalytic classes, (ii) the presence of a 'bait region' and a thiol ester, (iii) a similar protease inhibitory mechanism and (iv) the inactivation of the inhibitory capacity by reaction of the thiol ester with small primary amines. aM protease inhibitors inhibit by steric hindrance. The mechanism involves protease cleavage of the bait region, a segment of the aM that is particularly susceptible to proteolytic cleavage, which initiates a conformational change such that the aM collapses about the protease. In the resulting aM-protease complex, the active site of the protease is sterically shielded, thus substantially decreasing access to proteinsubstrates. Two additional events occur as a consequence of bait region cleavage, namely (i) the h-cysteinyl-g-glutamyl thiol ester becomes highly reactive and (ii) a major conformational change exposes a conserved COOH-terminal receptorbinding domain  (RBD). RBD exposure allows the aM protease complex to bind to clearance receptors and be removed from circulation. Tetrameric, dimeric, and, more recently, monomeric aM protease inhibitors have been identified.
Alpha-2-macroglobulin has in its structure a 35 amino acid "bait" region. Proteinases binding and cleaving the bait region become bound to α2M. The proteinase-α2M complex is recognised by macrophage receptors and cleared from the system.
Fibrinolysis (simplified). Blue arrows denote stimulation, and red arrows inhibition.
Alpha-2-macroglobulin levels are increased in nephrotic syndrome, a condition wherein the kidneys start to leak out some of the smaller blood proteins. Because of its size, alpha-2-macroglobulin is retained in the bloodstream. Increased production of all proteins means alpha-2-macroglobulin concentration increases. This increase has little adverse effect on the health, but is used as a diagnostic clue. Longstanding chronic renal failure can lead to amyloid by alpha-2-macroglobulin (see main article: amyloid).
^Andersen GR, Koch TJ, Dolmer K, Sottrup-Jensen L, Nyborg J (October 1995). "Low resolution X-ray structure of human methylamine-treated alpha 2-macroglobulin". J. Biol. Chem.270 (42): 25133–41. doi:10.1074/jbc.270.42.25133. PMID7559647.
^Sottrup-Jensen L, Stepanik TM, Kristensen T, Wierzbicki DM, Jones CM, Lønblad PB et al. (1984). "Primary structure of human alpha 2-macroglobulin. V. The complete structure.". J Biol Chem259 (13): 8318–27. PMID6203908.
^Sottrup-Jensen L (July 1989). "Alpha-macroglobulins: structure, shape, and mechanism of proteinase complex formation". J. Biol. Chem.264 (20): 11539–42. PMID2473064.
^Enghild JJ, Salvesen G, Thøgersen IB, Pizzo SV (July 1989). "Proteinase binding and inhibition by the monomeric alpha-macroglobulin rat alpha 1-inhibitor-3". J. Biol. Chem.264 (19): 11428–35. PMID2472396.
^Enghild JJ, Thøgersen IB, Roche PA, Pizzo SV (February 1989). "A conserved region in alpha-macroglobulins participates in binding to the mammalian alpha-macroglobulin receptor". Biochemistry28 (3): 1406–12. doi:10.1021/bi00429a069. PMID2469470.
^Van Leuven F, Cassiman JJ, Van den Berghe H (December 1986). "Human pregnancy zone protein and alpha 2-macroglobulin. High-affinity binding of complexes to the same receptor on fibroblasts and characterization by monoclonal antibodies". J. Biol. Chem.261 (35): 16622–5. PMID2430968.