The serpin SQN-5 is a dual mechanistic-class inhibitor of serine and cysteine proteinases.
SQN-5 is a mouse serpin that is highly similar to the human serpins SCCA1 (SERPINB3) and SCCA2 (SERPINB4). Previous studies characterizing the biochemical activity of SQN-5 showed that this serpin, like SCCA2, inhibited the chymotrypsin-like enzymes mast cell chymase and cathepsin G. Using an expanded panel of papain-like cysteine proteinases, we now show that SQN-5, like SCCA1, inhibited cathepsins K, L, S, and V but not cathepsin B or H. These interactions were characterized by stoichiometries of inhibition that were nearly 1:1 and second-order rate constants of >10(4) M(-1) s(-1). Reactive site loop (RSL) cleavage analysis showed that SQN-5 employed different reactive centers to neutralize the serine and cysteine proteinases. To our knowledge, this is the first serpin that serves as a dual inhibitor of both chymotrypsin-like serine and the papain-like cysteine proteinases by employing an RSL-dependent inhibitory mechanism. The ability of serpins to inhibit both serine and/or papain-like cysteine proteinases may not be a recent event in mammalian evolution. Phylogenetic studies suggested that the SCCA and SQN genes evolved from a common ancestor approximately 250-280 million years ago. When the fact that mammals and birds diverged approximately 310 million years ago is considered, an ancestral SCCA/SQN-like serpin with dual inhibitory activity may be present in many mammalian genomes.
Al-Khunaizi
May
M
Department of Pediatrics, The Harvard Medical School, Children's Hospital, 300 Longwood Avenue, Enders 970, Boston, Massachusetts 02115, USA.
Luke
Cliff J
CJ
Askew
Yuko S
YS
Pak
Stephen C
SC
Askew
David J
DJ
Cataltepe
Sule
S
Miller
David
D
Mills
David R
DR
Tsu
Christopher
C
Brömme
Dieter
D
Irving
James A
JA
Whisstock
James C
JC
Silverman
Gary A
GA
D000595:Amino Acid Sequence; D000818:Animals; D000951:Antigens, Neoplasm / Q000737:chemistry / Q000378:metabolism; D001667:Binding, Competitive; D002403:Cathepsins / Q000378:metabolism; D003546:Cysteine Endopeptidases / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D051379:Mice; D008969:Molecular Sequence Data; D010802:Phylogeny; D011480:Protease Inhibitors / Q000737:chemistry / Q000145:classification / Q000494:pharmacology*; D017386:Sequence Homology, Amino Acid; D012697:Serine Endopeptidases / Q000187:drug effects / Q000378:metabolism*; D015843:Serpins / Q000737:chemistry / Q000145:classification / Q000494:pharmacology*
D000951:Antigens, Neoplasm; D011480:Protease Inhibitors; D015843:Serpins; C051072:squamous cell carcinoma-related antigen; D002403:Cathepsins; D012697:Serine Endopeptidases; D003546:Cysteine Endopeptidases; C117627:CTSV protein, human