Acrosin

Template:Short description Template:Infobox enzymeAcrosin is a digestive enzyme that acts as a protease. In humans, acrosin is encoded by the ACR gene.<ref name="pmid2298447">Template:Cite journal</ref><ref name="pmid12398221">Template:Cite journal</ref> Acrosin is released from the acrosome of spermatozoa as a consequence of the acrosome reaction. It aids in the penetration of the Zona Pellucida.

Enzyme MechanismEdit

Acrosin is a typical serine proteinase with trypsin-like specificity.<ref name=":0">Template:Cite journal</ref>

File:Acrosin Catalytic Mechanism.jpg

Acrosin catalytic mechanism

The reaction proceeds according to the usual serine protease mechanism. First, His-57 deprotonates Ser-195, allowing it to serve as a nucleophile. Deprotonated Ser-195 then reacts with the carbonyl carbon of a peptide, forming a tetrahedral intermediate. The tetrahedral intermediate then collapses, resulting in an H₂N-R₁ leaving group, which is protonated through His-57. Finally, His-57 deprotonates a water molecule, which can then serve as a nucleophile by similarly reacting with the carbonyl carbon. Collapse of the tetrahedral intermediate then results in a Ser-195 leaving group, which is protonated through His-57, resulting in all residues returned to their pre-catalytic state, and a carboxylic acid where there was previously a peptide bond.

Biological FunctionEdit

Acrosin is the major proteinase present in the acrosome of mature spermatozoa. It is stored in the acrosome in its precursor form, proacrosin. Upon stimulus, the acrosome releases its contents onto the zona pellucida. After this reaction occurs, the zymogen form of the protease is then processed into its active form, β-acrosin. The active enzyme then functions in the lysis of the zona pellucida, thus facilitating penetration of the sperm through the innermost glycoprotein layers of the ovum.<ref name=":0"/>

The importance of acrosin in the acrosome reaction has been contested. It has been found through genetic knockout experiments that mouse spermatozoa lacking β-acrosin (the active protease) still have the ability to penetrate the zona pellucida.<ref>T. Baba, S. Azuma, S. Kashiwabara, Y. Toyoda. Sperm from mice carrying a targeted mutation of the acrosin gene can penetrate the oocyte zona pellucida and effect fertilization" J. Biol. Chem. 1994; 269, pp. 31845–31849</ref> Thus, some argue for its role in assisting in the dispersal of acrosomal contents following the acrosome reaction, while others demonstrate evidence for its role as a secondary binding protein between the spermatozoa and zona pellucida.<ref>K. Yamagata, T. Baba, et al. Acrosin accelerates the dispersal of sperm acrosomal proteins during acrosome reaction" J. Biol. Chem. 1998; 273, pp. 10470–10474</ref><ref>R. Jones, C.R. Brown. Identification of a zona-binding protein from boar spermatozoa as proacrosin. Expl" Cell Res 1987; 171, pp. 505–508</ref><ref>R. Jones. Interaction of zona pellucida glycoproteins, sulphated carbohydrates and synthetic polymers with proacrosin, the putative egg-binding protein from mammalian spermatozoa" Development 1991; 111, pp. 1155–1163</ref> Under the secondary binding protein hypothesis, acrosin could serve a role in binding to molecules on the zona pellucida, tethering the spermatozoa to the egg. This "tethering" would ensure penetration due to the applied motile force of the spermatozoa.<ref>D.P. Green. The head shapes of some mammalian spermatozoa and their possible relationship to the shape of the penetration slit through the zona pellucida. J. Reprod. Fertil., 83 (1988), pp. 377–387</ref>

Acrosin regulation has been found to occur through protein C inhibitor (PCI). PCI is present in the male reproductive tract at 40x higher concentrations than in blood plasma.<ref name=":1">Template:Cite journal</ref> PCI has been demonstrated to inhibit the proteolytic activity of acrosin.<ref name=":1" /> Thus, PCI has been hypothesized to have a protective role: if acrosomal enzymes were released prematurely, or if the spermatozoa was degenerated within the male reproductive tract, the high concentrations of PCI would inhibit acrosin from inflicting proteolytic damage on nearby tissues.<ref>Template:Cite journal</ref>

StructureEdit

β-acrosin demonstrates a high degree of sequence identity (70-80%) between boar, bull, rat, guinea pig, mouse, and human isoforms.<ref name=":0" /> There exists a somewhat similar (27-35%) sequence identity between β-acrosin and other serine proteases such as trypsin and chymotrypsin.<ref name=":0" /> While most serine proteases are activated through one cleavage event, proacrosin requires processing at both the N and C-terminal domains. Proacrosin is first cleaved between Arg-22 and adjacent Valine to create a 22 residue light chain, and an active protease termed α-acrosin.<ref name=":0" /> This light chain remains associated with the heavy chain, cross-linked through two disulfide bonds to form a heterodimer. Following these N-terminal cleavage events, three cleavages at the C-terminal domain removes 70 residues, yielding β-acrosin.<ref name=":0" /> Acrosin has two sites which have been identified as possible N-glycosylation sites: Asn-2 and Asn-169.<ref name=":0" />

File:1FIW Active Site Surface.png

Acrosin active site surface contour, shown with competitive inhibitor benzamidine. Trp-215 "gatekeeper" residue is shown partially occluding the "top" (as pictured here) entrance to the binding cavity.

The catalytic triad consists of residues His-57, Asp-102, and Ser-195.<ref name=":0" /> These residues are found in a binding pocket that has been termed the "S1" pocket, consistent with the naming scheme that has been adopted for other proteases.<ref>I. Schechter, A. Berger. On the size of the active site in proteases. I. Papain. Biochim. Biophys. Res. Commun, 27 (1967), pp. 157–162.</ref> The S1 pocket regulates acrosin's specificity for Arg and Lys substrates, with a conserved Trp-215 serving as a "gatekeeper" residue for the binding site entrance.<ref name=":0" />

File:1FIW Active Site Residues.png

Acrosin active site residues, shown with competitive inhibitor benzamidine.

An important structural element of β-acrosin is a highly charged patch (formed through both amino acids and post-translational modifications) on its surface region, that has been termed the "anion binding exosite."<ref name=":0" /> This site consists of an area of excess positive charge, which has been hypothesized to be important in binding to the matrix of the zona pellucida, a heavily glycosylated and sulfated region with excess negative charge.<ref>Template:Cite journal</ref> This structural feature is consistent with the secondary binding protein hypothesis, as charge-charge interactions would stabilize a protein-zona pellucida "tethering" complex.<ref>S. Shimizu, M. Tsuji, J. Dean. In vitro biosynthesis of three sulphated glycoproteins of murine zonae pellucidae by oocytes grown in follicle culture" J. Biol. Chem. 1983; 258, pp. 5858–5863</ref> Further consistent with this structural hypothesis is the knowledge that suramin - a polysulfated drug (with substantial corresponding negative charge) has been found to inhibit sperm-zona pellucida binding.<ref>Template:Cite journal</ref>

Disease and Pharmaceutical RelevanceEdit

While one study which utilized mice models indicated that acrosin is not a necessary component of zona pellucida penetration, other studies in humans have shown an association between low acrosomal proteinase activity and infertility.<ref>Template:Cite journal</ref><ref>Tummon I.S.; Yuzpe A.A.; Daniel S.A.; Deutsch A. Total acrosin activity correlates with fertility potential after fertilization in vitro" Fertil Steril 1991 Nov;56(5):933-8.</ref> Other research groups have demonstrated a significant correlation between acrosin activity and sperm motility.<ref>Template:Cite journal</ref> In rabbit models, an intravaginal contraceptive device that secreted tetradecyl sodium sulfate, a known inhibitor of acrosin and hyaluronidases, had a complete contraceptive effect.<ref>Burck P.J., Zimmerman R.E. An intravaginal contraceptive device for the delivery of an acrosin and hyaluronidase inhibitor" Fertil Steril 1984 Feb;41(2):314-8.</ref> Although its exact mechanism of action is not entirely clear, acrosin could thus serve as a novel target for contraceptive agents. Acrosin may represent as a uniquely druggable target due to its location and high cellular specificity.<ref name=":2">Template:Cite journal</ref> Thus, developing inhibitors of acrosin could provide the basis for safe, reversible male contraceptives, or female contraceptives through the use of intravaginal contraceptive devices.<ref name=":2" />

Moreover, as serine proteases are important in the potentiation of HIV, research has found that an acrosin inhibitor, 4'-acetamidophenyl 4-guanidinobenzoate, possess the ability to inhibit HIV infection in virus-inoculated lymphocytes.<ref name=":3">Template:Cite journal</ref> This suggests the further role of acrosin inhibitors as potentially viable agents in the prevention of HIV transmission.<ref name=":3" />

ReferencesEdit

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External linksEdit

The MEROPS online database for peptidases and their inhibitors: S01.223
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