le asing ▁fol ate . ▁ ▁Str ucture ▁▁ ▁The ▁three ▁domains ▁of ▁the ▁ext rac ell ular ▁portion ▁of ▁GC PI I — the ▁prote ase , ▁ap ical ▁and ▁C - term inal ▁domains — coll abor ate ▁in ▁substr ate ▁recognition . ▁The ▁prote ase ▁domain ▁is ▁a ▁central ▁seven - str and ed ▁mixed ▁ β - sheet . ▁The ▁ β - sheet ▁is ▁fl ank ed ▁by ▁ 1 0 ▁α - hel ices . ▁The ▁ap ical ▁domain ▁is ▁located ▁between ▁the ▁first ▁and ▁the ▁second ▁str ands ▁of ▁the ▁central ▁ β - sheet ▁of ▁the ▁prote ase ▁domain . ▁The ▁ap ical ▁domain ▁creates ▁a ▁pocket ▁that ▁facil it ates ▁substr ate ▁binding . ▁The ▁C - term inal ▁domain ▁is ▁an ▁Up - Down - Up - Down ▁four - hel ix ▁bundle . ▁ ▁The ▁central ▁pocket ▁is ▁approximately ▁ 2 ▁nan om eters ▁in ▁depth ▁and ▁opens ▁from ▁the ▁ext rac ell ular ▁space ▁to ▁the ▁active ▁site . ▁ ▁This ▁active ▁site ▁contains ▁two ▁z inc ▁ ions . ▁During ▁in hib ition , ▁each ▁acts ▁as ▁a ▁lig and ▁to ▁an ▁o xygen ▁in ▁ 2 - P MP A ▁or ▁ph osph ate . ▁There ▁is ▁also ▁one ▁calci um ▁ion ▁coordin ated ▁in ▁GC PI I , ▁far ▁from ▁the ▁active ▁site . ▁It ▁has ▁been ▁proposed ▁that ▁calci um ▁holds ▁together ▁the ▁prote ase ▁and ▁ap ical ▁domains . ▁In ▁addition , ▁human ▁GC PI I