HK consists of 644 amino acid residues, which are separated into six different domains. Domains 1, 2, and 3 are called the “heavy chain” with Domains 2 and 3 having cysteine protease activity.
The protein HK has 644 amino acids organized into six distinct structural units.  Domains 1, 2, and 3 constitute the "heavy chain" and exhibit cysteine protease activity in domains 2 and 3.