REVIEW ARTICLE
Insights into and speculations about snake venom
metalloproteinase (SVMP) synthesis, folding and disulfide
bond formation and their contribution to venom
complexity
Jay W. Fox
1
and Solange M. T. Serrano
2
1 Department of Microbiology, University of Virginia, Charlottesville, VA, USA
2 Laborato
´
rio Especial de Toxinologia Aplicada-CAT ⁄ CEPID, Instituto Butantan, Sao Paulo, Brazil
Introduction
Since the first discovery of zinc-dependent proteinases
in viperid snake venom, investigators have intensively
studied the structure and function of these proteinases
in order to understand their role in envenomation
pathologies [1]. With the advent of the first complete
sequence determination of these proteinases, it was
thought that they belonged to the matrix metallo-
proteinase family of proteinases [2]. However, it soon
became obvious that they in fact comprised a novel
family of metalloproteinases, the M12 family, to which
the ‘a disintegrin and metalloproteinase’ (ADAM)
proteins also belong [3]. As studies progressed, the
snake venom metalloproteinases (SVMPs), as this
group of proteinases is now named, were further cate-
gorized into the PI, PIIa and PIIb, PIIIa and PIIIb,
and PIV classes [4,5]. The criterion for this differential
classification essentially was based on the presence or

plexity, however, is derived not only from the venom genomic structure
but also from transcriptome generation and translation and, perhaps most
importantly, post-translation modification of the nascent venom proteome.
In this review, we examine the snake venom metalloproteinases, some of
the predominant components in viperid venoms, with regard to possible
synthesis and post-translational mechanisms that contribute to venom com-
plexity. The aim of this review is to highlight the state of our knowledge
on snake venom metalloproteinase post-translational processing and to
suggest testable hypotheses regarding the cellular mechanisms associated
with snake venom metalloproteinase complexity in venoms.
Abbreviations
ER, endoplasmic reticulum; MMP, matrix metalloproteinase.
3016 FEBS Journal 275 (2008) 3016–3030 ª 2008 The Authors Journal compilation ª 2008 FEBS
PIV class into the PIII class pending the finding of a
transcript in venom glands representing a PIV struc-
ture containing lectin-like domains. In Fig. 1, we show
a modified classification scheme that reflects the nas-
cent P structural classes and well as the observed
products following post-translational modification and
processing. Functionally, the SVMPs display a wide
array of biological activities, many of which are toxic,
and this scope of activities reflects the multitude of
products derived from the four SVMP classes [5].
Over the past several years, as the result of a variety
of proteomic and transcriptomic investigations of
snake venom and venom glands respectively, numerous
databases have been generated that illuminate the com-
plexity of snake venoms, particularly the viperid
venoms. Hundreds of proteins comprise viperid venom
proteomes, and it is estimated that most viperid

venom.
J. W. Fox and S. M. T. Serrano Snake venom metalloproteinases and venom complexity
FEBS Journal 275 (2008) 3016–3030 ª 2008 The Authors Journal compilation ª 2008 FEBS 3017
proteins are synthesized in the cytoplasm of secretory
cells in the gland and transferred to the rough endo-
plasmic reticulum, and then the Golgi apparatus, and
finally transported via secretory granules to the lumen
of the venom gland [10]. On the basis of cDNA studies,
all venom proteins have a signal sequence that probably
targets the nascent protein to a signal recognition parti-
cle on the endoplasmic reticulum (ER). During trans-
port into the ER, the signal sequence is removed.
Presumably, like most secretory proteins, it is in the
rough ER that the nascent venom proteins fold, and
undergo disulfide bond oxidation ⁄ formation, glycosyla-
tion, and, in the specialized instances of some venom
proteins, multimerization, such as is the case with
dimeric disintegrins [PIId and PIIe, dimeric PIII class
SVMPs (PIIIc)] and the multimeric PIIId class SVMPs
(formerly PIV). As in other eukaryotic cells, in the
venom gland secretory cells, incorrectly folded proteins
would not be expected to enter into the Golgi network.
Proteolytic processing of the proforms of venom pro-
teins probably occurs, as with most latent protein forms
in the trans-Golgi network and nascent secretory vesi-
cles, and is completed by the time that the vesicles are
released into the venom gland lumen. In only a few
instances have sequences associated with the prodomain
of SVMPs been detected in the venom [11] (Serrano,
unpublished results), suggesting that, in fact, the bulk

the oxidative extracellular world; hence, most have
evolved to contain a number of disulfide bonds that
stabilize the particular molecular scaffold that is criti-
cal for toxic action [14] as well as participate in the
generation of multimeric venom proteins. On the basis
of studies of venoms using two-dimensional PAGE
under reducing and nonreducing conditions, Serrano
et al. [15] clearly demonstrated that disulfide bond
formation among venom proteins, leading to multimeric
structures, has a profound effect on venom complexity.
Reducing gels of Crotalus atrox and Bothrops jararaca
venom, as compared to nonreducing gels, indicated
that apparent venom complexity is reduced by 60%
due to disulfide bond crosslinking of venom proteins.
The biological synthesis of toxins, particularly the
SVMPs, must entail a somewhat complex phenomenon
that probably involves a variety of chaperones and
other proteins to help ensure appropriate folding and
disulfide bond pairing, as shown by the fact that heat-
shock proteins, protein disulfide isomerases and pept-
idyl-prolyl cis ⁄ trans isomerases have been identified in
snake venoms and venom glands [6,16]. The need for
ancillary proteins for appropriate post-translational
modifications is further substantiated by the fact that
it has proven to be relatively difficult for investigators
to produce recombinant SVMPs or, for that matter,
their various domains in heterologous in vitro
expression systems.
Cysteinyl residues in PI SVMPs range in number
from four, to six, to seven, with two to three disulfide

post-translational proteolytic processing that contrib-
utes to the SVMP-derived venom complexity, and will
be further discussed below. The proteinase domains
are observed to have five, six or seven cysteinyl resi-
dues, most having three disulfide bonds paired in a
similar way to that observed for the PI SVMPs. As
was the case with PI adamalysin, the unpaired, free
cysteinyl residue, such as in the PIIa trigramin, is
located in the N-terminal portion of the domain and
does not seem to affect folding or disulfide bond pair-
ing (Fig. 3). This may explain why the proteinase
domains, when processed from the nascent PII struc-
ture, are stable like the PI SVMPs. An interesting
exception to the typical PII SVMP processing scheme
that gives rise to a proteinase and a disintegrin in
venom is jerdonitin, a PIIb proteinase isolated from
the venom of Trimeresurus jerdonii. In jerdonitin, the
processing of these domains does not appear to occur
[28]. The structural analysis of jerdonitin reveals two
additional cysteinyl residues, one located in the spacer
region and one in the disintegrin domain, which could
promote a more compact structure that may preclude
the proteolytic processing between the two domains
(Fig. 1). Here again, we observe that the generation of
an additional SVMP structure based on variations in
post-translational proteolytic processing contributes to
SVMP-based venom complexity.
Some disintegrins, as processed from the nascent PII
SVMPs, contain four, six (four intrachain and two
interchain), six, seven or eight disulfide bonds [29]. The

bond pairing to form a heterodimeric disintegrin
occurs, it presumably happens between the nascent dis-
integrin gene product and a PIIe SVMP. This suggests
some structural or mechanistic advantage to having
one disintegrin domain residing in the structural envi-
ronment of the unprocessed PIIe for the formation of
the heterodimer later in the post-translational process.
The PIII SVMPs are the most intriguing of the
SVMP categories in terms of their contribution to
venom complexity and function. One of the first
sequences determined for a PIII SVMP was atroly-
sin A from C. atrox. The mature protein was observed
to be composed of three domains: a metalloproteinase
domain, a disintegrin-like domain, and a cysteine-rich
domain [31]. The metalloproteinase domains of the
PIII SVMP class have either six or seven cysteinyl resi-
dues with three disulfide bonds. The disulfide bond
pattern of the PIII proteinase domains appears to be
different from that observed in the PI or PII protein-
ase domains (Fig. 4). Most of the PIIIs have an odd
(seventh) cysteinyl residue in the proteinase domain.
The positions of the seventh cysteinyl residues in the
PIIIs appear to segregate to either the region near the
cysteinyl cluster of residues 350, 352 and 357 or
between cysteinyl residues 374 and 390 (Fig. 4).
Recently, crystal structures for three PIII SVMPs
have been determined: VAP1, a dimeric PIIIc, catro-
collastatin ⁄ VAP2B, a PIIIb, both from the venom of
C. atrox, and RVV-X, a PIIId from Vipera russelli
[32–34]. Many important observations regarding PIII

residues of both jararhagin and catrocollastatin are
located at position 379 in the loop between cystei-
nyls 374 and 390 (Fig. 4; VAP1 numbering). Two pro-
teins, which to date have not been observed to be
processed in the venom gland, atrolysin A and VAP1,
have their unpaired cysteinyl residues located at posi-
tions 360 and 365 respectively, N-terminal to the
unpaired cysteinyl residues observed in jararhagin and
catrocollastatin. Jarahagin and catrocollastatin are
known to undergo post-translational processing to
yield a disintegrin-like ⁄ cysteine-rich domain product in
the venom. Furthermore, it is interesting to note that
the PII atrolysin E, which is readily processed to yield
a stable proteinase domain and free disintegrin in the
venom, also has an unpaired cysteinyl residue located
at position 379, the same position as the unpaired
Fig. 3. Continued.
J. W. Fox and S. M. T. Serrano Snake venom metalloproteinases and venom complexity
FEBS Journal 275 (2008) 3016–3030 ª 2008 The Authors Journal compilation ª 2008 FEBS 3023
cysteinyl residue in the PIIIbs jararhagin and catrocol-
lastatin. This leads one to speculate that the presence
of an unpaired cysteinyl residue at that region of the
proteinase domain may be important for subsequent
proteolytic processing of the disintegrin-like ⁄ cysteine-
rich domains from the PIII SVMP proteinase domain.
Fig. 4. Sequence alignment of SVMP catalytic domains. Disulfide bonds of adamalysin II ( ) [17] and VAP1 (—) [32] revealed by
crystal structure analysis are depicted. Cyteine residues are numbered according to VAP1 [32]. Cysteine residues not involved in bonding in
adamalysin II and catrocollastatin ⁄ VAP2B are shown in bold; Cys365 of VAP1 is underlined.
Fig. 5. Disulfide bonds of catrocollastatin ⁄
VAP2B disintegrin-like (underlined) and

Another possible reason for these differences, although
perhaps less likely, is that the cellular mechanisms of
post-translation proteolytic processing of the PIIas and
PIIIbs are different.
Several years ago, we became intrigued by the obser-
vation that both the processed and unprocessed forms
of PIIIs could be found in venoms. For example, both
full-length jararhagin and catrocollastatin and their
processed disintegrin-like ⁄ cysteine-rich domains
(jararhagin C and catrocollastatin C) products could
be isolated from their respective crude venoms, but not
the product proteinase domain [37,42]. This led us to
ask the question as to why both forms were found in
the venom; why are not 100% of those particular
PIIIb SVMPs completely processed rather than only a
fraction of them? We proceeded to investigate this by
attempting to manipulate jararhagin in vitro to
undergo autolysis. What we observed during the
course of those experiments was the following:
(a) under most conditions, jararhagin as isolated from
the venom is stable against autolysis, as would be
expected, given that it is present in the venom; and
(b) only under conditions such alkaline pH, low
calcium or the presence of reducing agents did some
low fraction of jararhagin undergo autolysis to pro-
duce a disintegrin-like ⁄ cysteine-rich domain. Interest-
ingly, the N-terminus of the jararhagin C produced
in vitro was different from that observed in the natu-
rally occurring jararhagin C, suggesting several possi-
bilities. Perhaps the jararhagin C found in venom may

disulfide bond pattern determined for catrocolla-
statin C could not occur without significant structural
rearrangement ⁄ folding. The different disulfide bond
patterns observed between VAP2b ⁄ catrocollastatin and
catrocollastatin C may be explained by disulfide bond
shuffling during experimental determination of cyste-
inyl pairs for catrocollastatin C (an explanation that
we feel to be unlikely). Another possibility is that
during proteolytic processing of the nascent catrocol-
lastatin, disulfide bond rearrangement occurred. Alter-
natively, the two different disulfide bond patterns
representing folding isomers were in place before post-
translational proteolytic processing and only one of
those isomeric forms (the catrocollastatin C form) was
processed. We hypothesize that it is most likely that
the former case is valid. In summary, we propose the
following scenario for PIIIb post-translational
proteolytic processing: In the case of catrocollastatin
and jararhagin, where a certain population of the pro-
teins has been seen to be processed, there are folding
isomers, perhaps promoted by the presence and rela-
tive positions of the unpaired cysteinyl residues in the
proteinase domain. One of the folding isomers is read-
ily processed to give rise to an unstable proteinase
domain and a biologically active disintegrin-like ⁄ cyste-
ine-rich domain. The other folding isomer is refractory
J. W. Fox and S. M. T. Serrano Snake venom metalloproteinases and venom complexity
FEBS Journal 275 (2008) 3016–3030 ª 2008 The Authors Journal compilation ª 2008 FEBS 3025
to proteolytic processing in the ER ⁄ Golgi system, and
is found in the venom in the form of the canonical PI-

SVMP catrocollastatin ⁄ VAP2b, the spacer region, or
linker as per their terminology, appears to be accessi-
ble to the solvent and contributes to the flexibility
observed between the proteinases and the disintegrin-
like domains [33]. However, in the case of this specific
molecule, processing did not occur, suggesting that
simple solvent accessibility of the spacer is not suffi-
cient for processing. Overall, spacer accessibility is
probably associated with the structure of the adjacent
proteinase and disintegrin-like domains and the pack-
ing of those two domains together in the structure.
Comparison of the sequences in the spacer domain
among the P classes shows significant sequence con-
servation and leads to no obvious conclusions as to
why some spacer regions are cleaved and some are
not, based on simple peptide bond specificities of
processing proteinases. Thus, at this point we can
only suggest that the overall accessibility of the
spacer to processing enzymes during post-transla-
tional processing is necessary, but not sufficient, for
proteolytic processing.
The PII SVMP class precursor gives rise to a mini-
mum of five product classes in the venom (Fig. 1). In
fact, this precursor class is probably responsible for a
significant amount of the venom complexity associated
with the SVMPs, in that the PII precursor class gives
rise to a proteinase and a disintegrin. As is observed
with the PII SVMP class, the PIII class also signifi-
cantly contributes to venom complexity. In Fig. 1, it
can be seen that there are essentially four protein

grin flavoridin and VAP1. The disulfide bond patterns
of the disintegrins and disintegrin-like domains are
different in the N-terminal region, and reflect the
fact that the lack of the first cysteinyl residue (Fig. 6;
residue 406, VAP1 numbering) in the disintegrins
gives rise to the necessity for a different disulfide bond
pattern in this region.
The C-terminal region of the PIII class has been
termed ‘cysteine-rich’, due to the abundance of cyste-
inyl residues (13 residues) in this domain. Interestingly,
neither the disintegrin-like nor the cysteine-rich
domain from the PIII class has been separately
Snake venom metalloproteinases and venom complexity J. W. Fox and S. M. T. Serrano
3026 FEBS Journal 275 (2008) 3016–3030 ª 2008 The Authors Journal compilation ª 2008 FEBS
isolated from venom; they have only been found as a
single biologically active protein composed of these
domains [35,37]. This suggests that, structurally, these
domains are very closely associated, which may
preclude further processing. Over the past several
years, evidence has been mounting which indicates that
the functionality attributed to the disintegrin-like ⁄
cysteine-rich proteins found in venom resides in the
cysteine-rich domain region [44–49]. The biological
activities attributed to this domain result from its
ability to interact with other proteins, such as FACIT
collagens, von Willebrand factor and integrins. Fur-
thermore, in the case of the PIIId factor X activating
proteinase, RVV-X (formerly classified as a PIV class
member), the hypervariable region in the cysteine-rich
domain has been suggested to manifest the functional-

In this review, we have attempted to present, based on
the literature, the current understanding of the struc-
tural features that contribute to the complexity
observed in many snake venoms. The significance of
this resides in the concept that the pathophysiological
effects observed following envenomation can be attrib-
uted to the combination of individual activities of dis-
crete toxins in the venom as well as, in some cases,
synergistic effects among the toxins, and therefore
understanding venom complexity and the source of
such complexity represents an important challenge in
the field. Although recent transcriptome and proteome
studies on venoms have provided us with tremendous
insights into venom complexity, very little is empiri-
cally known about how the biosynthetic process of
nascent venom protein production contributes to the
complexity. It is our hope that this review will provide
a solid foundation on which future investigations can
be conducted to illuminate the biosynthetic processes
that give rise to venom complexity and, as such,
Fig. 6. Sequence alignment of disintegrins (blue) trigramin (CAA35910), flavoridin (1FVLA), contortrostatin (Q9IAB0) and bitistatin (P17497),
and disintegrin-like (blue) ⁄ cysteine-rich domains (red) of the SVMPs atrolysin A (AAA03326), jararhagin (CAA48323), VAP1 (2EROA), Bjus-
suMP_I (ABD73129) and kaouthiagin (P82942)] by the program
CLUSTALW. Cysteine residues are highlighted in gray. Disintegrin and disinte-
grin-like sequences are shown in italic bold type. Disulfide bonds of VAP1 [32] and flavoridin [54,55] are depicted (———). Common disulfide
bonds between VAP1 and flavoridin are shown in red. Cysteine residues are numbered according to the VAP1 sequence [32].
J. W. Fox and S. M. T. Serrano Snake venom metalloproteinases and venom complexity
FEBS Journal 275 (2008) 3016–3030 ª 2008 The Authors Journal compilation ª 2008 FEBS 3027
lead to a better understanding of venom action and
possibly therapeutic intervention.

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