ABSTRACT
Structurally, adiponectin belongs to the C1q/TNF-α superfamily. Th e protein has a signal peptide, a variable N-terminal region, a collagen domain and a C-terminal globular domain homologous to the immune complement C1q (Fig. 1). Despite little protein sequence identity, the crystal structure of adiponectin, with a three-fold symmetry, strikingly resembles the trimeric structure of TNF-α (Fig. 2) (Shapiro and Scherer 1998). Most adiponectin in plasma exists as fulllength protein. A small amount of the cleaved globular head can also be detected, but the function of this form is unknown (Fruebis et al. 2001). Further, adiponectin can form homomultimers through disulfi de bond formation. Th e three most abundant multimers of adiponectin are the trimer, hexamer and higher molecular weight (HMW) oligomer of 18 subunits (Tsao et al. 2002). Each of the adiponectin isoforms is stable and does not interchange in vivo; their proportions in serum change with metabolic and disease states (Pajvani et al. 2004).
