Chain A ( P477 V478 R480 V481 A482 L509 C510 Q511 T512 D513 E514 S515 E517 I519 I520 S521 V550 V551 S552 Y553 R554 E555 P628 S661 K662 E663 G664 V665 C667 E668 E669 N670 W791 I812 R815 K816 )