Acrosin

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Acrosin
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EC no. 3.4.21.10
CAS no. 9068-57-9
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Acrosin is a digestive enzyme that acts as a protease. In humans, acrosin is encoded by the ACR gene. [1] [2] Acrosin is released from the acrosome of spermatozoa as a consequence of the acrosome reaction. It aids in the penetration of the Zona Pellucida.

Contents

Enzyme Mechanism

Acrosin is a typical serine proteinase with trypsin-like specificity. [3]

Acrosin catalytic mechanism 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 Function

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. [3]

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. [4] 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. [5] [6] [7] 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. [8]

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. [9] PCI has been demonstrated to inhibit the proteolytic activity of acrosin. [9] 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. [10]

Structure

β-acrosin demonstrates a high degree of sequence identity (70-80%) between boar, bull, rat, guinea pig, mouse, and human isoforms. [3] There exists a somewhat similar (27-35%) sequence identity between β-acrosin and other serine proteases such as trypsin and chymotrypsin. [3] 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. [3] 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. [3] Acrosin has two sites which have been identified as possible N-glycosylation sites: Asn-2 and Asn-169. [3]

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. 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. [3] 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. [11] 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. [3]

Acrosin active site residues, shown with competitive inhibitor benzamidine. 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." [3] 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. [12] This structural feature is consistent with the secondary binding protein hypothesis, as charge-charge interactions would stabilize a protein-zona pellucida "tethering" complex. [13] 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. [14]

Disease and Pharmaceutical Relevance

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. [15] [16] Other research groups have demonstrated a significant correlation between acrosin activity and sperm motility. [17] In rabbit models, an intravaginal contraceptive device that secreted tetradecyl sodium sulfate, a known inhibitor of acrosin and hyaluronidases, had a complete contraceptive effect. [18] 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. [19] Thus, developing inhibitors of acrosin could provide the basis for safe, reversible male contraceptives, or female contraceptives through the use of intravaginal contraceptive devices. [19]

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. [20] This suggests the further role of acrosin inhibitors as potentially viable agents in the prevention of HIV transmission. [20]

Related Research Articles

<span class="mw-page-title-main">Spermatozoon</span> Motile sperm cell

A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote.

<span class="mw-page-title-main">Acrosome reaction</span> Sperm-meets-egg process

During fertilization, a sperm must first fuse with the plasma membrane and then penetrate the female egg cell to fertilize it. Fusing to the egg cell usually causes little problem, whereas penetrating through the egg's hard shell or extracellular matrix can be more difficult. Therefore, sperm cells go through a process known as the acrosome reaction, which is the reaction that occurs in the acrosome of the sperm as it approaches the egg.

<span class="mw-page-title-main">Zona pellucida</span> Glycoprotein layer surrounding the plasma membrane of mammalian oocytes

The zona pellucida is a specialized extracellular matrix that surrounds the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first appears in unilaminar primary oocytes. It is secreted by both the oocyte and the ovarian follicles. The zona pellucida is surrounded by the corona radiata. The corona is composed of cells that care for the egg when it is emitted from the ovary.

Capacitation is the penultimate step in the maturation of mammalian spermatozoa and is required to render them competent to fertilize an oocyte. This step is a biochemical event; the sperm move normally and look mature prior to capacitation. In vivo, capacitation occurs after ejaculation, when the spermatozoa leave the vagina and enter the upper female reproductive tract. The uterus aids in the steps of capacitation by secreting sterol-binding albumin, lipoproteins, and proteolytic and glycosidasic enzymes such as heparin.

Hyperactivation is a type of sperm motility. Hyperactivated sperm motility is characterised by a high amplitude, asymmetrical beating pattern of the sperm tail (flagellum). This type of motility may aid in sperm penetration of the zona pellucida, which encloses the ovum.

<span class="mw-page-title-main">Human fertilization</span> Union of a human egg and sperm

Human fertilization is the union of an egg and sperm, occurring primarily in the ampulla of the fallopian tube. The result of this union leads to the production of a fertilized egg called a zygote, initiating embryonic development. Scientists discovered the dynamics of human fertilization in the 19th century.

<span class="mw-page-title-main">Cortical reaction</span> Biological process that prevents polyspermy

The cortical reaction is a process initiated during fertilization that prevents polyspermy, the fusion of multiple sperm with one egg. In contrast to the fast block of polyspermy which immediately but temporarily blocks additional sperm from fertilizing the egg, the cortical reaction gradually establishes a permanent barrier to sperm entry and functions as the main part of the slow block of polyspermy in many animals.

Decapacitation factor (DF) is composed of sperm surface-associated proteins which modulate the fertilizing ability of spermatozoa. Decapacitation is a reversible process that converts fertile, capacitated sperm to less-fertile uncapacitated sperm. This activity is achieved by interaction between cholesterol, phospholipids and fibronectin-like substances and delivered via small vesicles in seminal plasma. DF prevents onset of capacitation. Many DFs are released in secretions from the epididymis and accessory organs of the male reproductive system. However, some DFs have been identified that are located on the acrosome of sperm. Normally, capacitation is initiated through the loss of DF before the spermatozoa can perform the acrosomal reaction. Physiologically decapacitation will inhibit the acrosomal reaction as DFs reassociate onto the sperm surface. For example, one way this can be achieved is through spermatozoal membrane stabilization by maintaining physiological cholesterol/phospholipid ratio.

The hamster zona-free ovum test, or hamster egg-penetration test, or sometimes just hamster test, is an in-vitro test used to study physiological profile of spermatozoa. The primary application of the test is to diagnose male infertility caused by sperm unable to penetrate the ova. The test has limited value, due to expense and a high false negative rate.

<span class="mw-page-title-main">Protein C inhibitor</span> Human protein

Protein C inhibitor is a serine protease inhibitor (serpin) that limits the activity of protein C.

<span class="mw-page-title-main">ZP3</span> Protein-coding gene in the species Homo sapiens

Zona pellucida sperm-binding protein 3, also known as zona pellucida glycoprotein 3 (Zp-3) or the sperm receptor, is a ZP module-containing protein that in humans is encoded by the ZP3 gene. ZP3 is the glycoprotein in the zona pellucida most important for inducting the acrosome reaction of sperm cells at the beginning of fertilization.

Immunocontraception is the use of an animal's immune system to prevent it from fertilizing offspring. Contraceptives of this type are not currently approved for human use.

<span class="mw-page-title-main">Glycodelin</span> Mammalian protein found in Homo sapiens

Glycodelin(GD) also known as human placental protein-14 (PP-14)progestogen-associated endometrial protein (PAEP) or pregnancy-associated endometrial alpha-2 globulin is a glycoprotein that inhibits cell immune function and plays an essential role in the pregnancy process. In humans is encoded by the PAEP gene.

<span class="mw-page-title-main">ZP2</span> Protein-coding gene in the species Homo sapiens

Zona pellucida sperm-binding protein 2 is a protein that in humans is encoded by the ZP2 gene.

<span class="mw-page-title-main">ZP4</span> Protein-coding gene in the species Homo sapiens

Zona pellucida sperm-binding protein 4, ZP-4 or avilesine, named after its discoverer Manuel Avilés Sánchez is a protein that in humans is encoded by the ZP4 gene.

<span class="mw-page-title-main">SPINK2</span> Protein-coding gene in the species Homo sapiens

Serine protease inhibitor Kazal-type 2 also known as acrosin-trypsin inhibitor is a protein that in humans is encoded by the SPINK2 gene.

<span class="mw-page-title-main">Cysteine-rich secretory protein</span>

Cysteine-rich secretory proteins, often abbreviated as CRISPs, are a group of glycoproteins. They are a subgroup of the CRISP, antigen 5 and Pr-1 (CAP) protein superfamily and also contain a domain related to the ShK toxins. They are substantially implicated in the functioning of the mammalian reproductive system. CRISPs are also found in a variety of snake venoms where they inhibit both smooth muscle contraction and cyclic nucleotide-gated ion channels.

<span class="mw-page-title-main">Cortical granule</span>

Cortical granules are regulatory secretory organelles found within oocytes and are most associated with polyspermy prevention after the event of fertilization. Cortical granules are found among all mammals, many vertebrates, and some invertebrates. Within the oocyte, cortical granules are located along the cortex, the region furthest from the cell's center. Following fertilization, a signaling pathway induces the cortical granules to fuse with the oocyte's cell membrane and release their contents into the oocyte's extracellular matrix. This exocytosis of cortical granules is known as the cortical reaction. In mammals, the oocyte's extracellular matrix includes a surrounding layer of perivitelline space, zona pellucida, and finally cumulus cells. Experimental evidence has demonstrated that the released contents of the cortical granules modify the oocyte's extracellular matrix, particularly the zona pellucida. This alteration of the zona pellucida components is known as the zona reaction. The cortical reaction does not occur in all mammals, suggesting the likelihood of other functional purposes for cortical granules. In addition to modifying the oocyte's extracellular matrix and establishing a block to polyspermy, the exocytosis of cortical granules may also contribute towards protection and support of the developing embryo during preimplantation. Once the cortical granules complete their functions, the oocyte does not replenish them.

<span class="mw-page-title-main">Acrosin binding protein</span> Protein-coding gene in humans

Acrosin binding protein is a protein that in humans is encoded by the ACRBP gene.

<span class="mw-page-title-main">Globozoospermia</span> Medical condition

Globozoospermia is a rare and severe form of monomorphic teratozoospermia. This means that the spermatozoa show the same abnormality, and over 85% of spermatozoa in sperm have this abnormality. Globozoospermia is responsible for less than 0.1% of male infertility. It is characterised by round-headed spermatozoa without acrosomes, an abnormal nuclear membrane and midpiece defects. Affected males therefore suffer from either reduced fertility or infertility. Studies suggest that globozoospermia can be either total or partial, however it is unclear whether these two forms are variations on the same syndrome, or actually different syndromes.

References

  1. ↑ Adham IM, Klemm U, Maier WM, Engel W (Jan 1990). "Molecular cloning of human preproacrosin cDNA". Human Genetics. 84 (2): 125–8. doi:10.1007/bf00208925. PMID   2298447. S2CID   39848902.
  2. ↑ Honda A, Siruntawineti J, Baba T (2002). "Role of acrosomal matrix proteases in sperm-zona pellucida interactions". Human Reproduction Update. 8 (5): 405–12. doi: 10.1093/humupd/8.5.405 . PMID   12398221.
  3. 1 2 3 4 5 6 7 8 9 10 Tranter, Rebecca; Read, Jon A.; Jones, Roy; Brady, R. Leo (2000-11-15). "Effector Sites in the Three-Dimensional Structure of Mammalian Sperm β-Acrosin". Structure. 8 (11): 1179–1188. doi: 10.1016/S0969-2126(00)00523-2 . ISSN   0969-2126. PMID   11080640.
  4. ↑ 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
  5. ↑ 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
  6. ↑ R. Jones, C.R. Brown. Identification of a zona-binding protein from boar spermatozoa as proacrosin. Expl" Cell Res 1987; 171 , pp. 505–508
  7. ↑ 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
  8. ↑ 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
  9. 1 2 Laurell, M; Christensson, A; Abrahamsson, PA; Stenflo, J; Lilja, H (1992). "Protein C inhibitor in human body fluids. Seminal plasma is rich in inhibitor antigen deriving from cells throughout the male reproductive system". J Clin Invest. 89 (4): 1094–101. doi:10.1172/JCI115689. PMC   442965 . PMID   1372913.
  10. ↑ Zheng, X; Geiger, M; Ecke, S (1994). "Inhibition of acrosin by protein C inhibitor and localization of protein C inhibitor to spermatozoa". Am. J. Physiol. 267 (2 Pt 1): C466–72. doi:10.1152/ajpcell.1994.267.2.C466. PMID   7521127.
  11. ↑ I. Schechter, A. Berger. On the size of the active site in proteases. I. Papain. Biochim. Biophys. Res. Commun, 27 (1967), pp. 157–162.
  12. ↑ Nakano M, Tobets T, et al. (1990). "Further fractionation of the glycoprotein families of porcine zona pellucida by anion exchange HPLC and some characterization of the separated fractions". J. Biochem. 107 (1): 144–150. doi:10.1093/oxfordjournals.jbchem.a122998. PMID   2110153.
  13. ↑ 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
  14. ↑ Jones R, Parry R, Leggio LL, Nickel P (1996). "Inhibition of sperm-zona binding by suramin, a potential "lead" compound for design of new anti-fertility agents". Mol. Hum. Rep. 2 (8): 597–605. doi: 10.1093/molehr/2.8.597 . PMID   9239672.
  15. ↑ Welker B, Bernstein GS, Diedrich K, Nakamura RM, Krebs D (Oct 1988). "Acrosomal proteinase activity of human spermatozoa and relation of results to semen quality". Hum Reprod. 3 (Suppl 2): 75–80. doi:10.1093/humrep/3.suppl_2.75. PMID   3068243.
  16. ↑ 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.
  17. ↑ Cui YH, Zhao RL, Wang Q, Zhang ZY (Sep 2000). "Determination of sperm acrosin activity for evaluation of male fertility". Asian J Androl. 2 (3): 229–232. PMID   11225983.
  18. ↑ 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.
  19. 1 2 Ning, Weiwei; Zhu, Ju; Zheng, Canhui; Liu, Xuefei; Song, Yunlong; Zhou, Youjun; Zhang, Xiaomeng; Zhang, Ling; Sheng, Chunquan (2013-04-01). "Fragment-Based Design of Novel Quinazolinon Derivatives as Human Acrosin Inhibitors". Chemical Biology & Drug Design. 81 (4): 437–441. doi:10.1111/cbdd.12106. ISSN   1747-0285. PMID   23331539. S2CID   21445655.
  20. 1 2 Bourinbaiar AS, Lee-Huang S (May 1995). "Acrosin inhibitor, 4'-acetamidophenyl 4-guanidinobenzoate, an experimental vaginal contraceptive with anti-HIV activity". Contraception. 51 (5): 319–22. doi: 10.1016/0010-7824(95)00094-q . PMID   7628208.

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