Analysis of the glucagon receptor first extracellular loop by the substituted cysteine accessibility method

► SCAM of the first glucagon receptor extracellular loop (197-224). ► TM2 continues to position 202, TM3 from 215 ► Most side chains important for active glucagon conformation. ► S217 and D218 contact glucagon, V221 very close to the binding site. ► Receptor activation increases the accessibility of L198C mutant cysteine.