Proprotein convertases (PPCs) are a family of proteins that activate other proteins

January 27, 2024
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Many proteins are inactive when they are first synthesized, because they contain chains of amino acids that block their activity. Proprotein convertases remove those chains and activate the protein. The prototypical proprotein convertase is FURIN. Proprotein convertases have medical significance, because they are involved in many important biological processes, such as cholesterol synthesis. Compounds called proprotein convertase inhibitors can block their action, and block the target proteins from becoming active. Many proprotein convertases, especially furin and PACE4, are involved in pathological processes such as viral infection, inflammation, hypercholesterolemia, and cancer, and have been postulated as therapeutic targets for some of these diseases.

History

The phenomenon of prohormone conversion was discovered by Donald F. Steiner while examining the biosynthesis of insulin in 1967. At the same time, while conducting chemical sequencing of β-lipotrophic hormone (βLPH) with sheep pituitary glands Dr. Michel Chretien determined the sequence of another hormone, MELANOCYTE-STIMULATING HORMONE ( βMSH).

Donald F. Steiner is known for his work in diabetes research, protein processing, and hormone biology. In 1967, he published his discovery of proinsulin, precursor to the active hormone insulin. He and his colleagues discovered some of the enzymes that convert proinsulin into insulin, and also devised methods for measuring insulin and its precursors in human serum.

This was the chemical evidence, at the level of primary protein sequence that peptide hormones could be found within larger protein molecules. The identity of the responsible enzymes was not clear for decades. In 1984, David Julius, working in the laboratory of Jeremy Thorner, identified the product of the Kex2 gene as responsible for processing of the alpha factor mating pheromone.

David Jay Julius is an American physiologist and Nobel Prize laureate known for his work on molecular mechanisms of pain sensation and heat, including the characterization of the TRPV1 and TRPM8 receptors that detect capsaicin, menthol, and temperature. He is a professor at the University of California, San Francisco. He was awarded the 2021 Nobel Prize in Physiology or Medicine jointly with Ardem Patapoutian for their discoveries of receptors for temperature and touch. He attained his doctorate from University of California, Berkeley in 1984, under joint supervision of Jeremy Thorner and Randy Schekman, where he identified Kex2 as the founding member of furin-like proprotein convertases. In 1989, he completed his post-doctoral training with Richard Axel at Columbia University where he cloned and characterized the serotonin 1c receptor. While at Berkeley and Columbia, Julius became interested in how psilocybin mushrooms and lysergic acid diethylamide work, which led him to look more broadly into how things from nature interact with human receptors. In 1997, Julius’s lab cloned and characterized TRPV1 which is the receptor that detects capsaicin, the chemical in chili peppers that makes them “hot”. They found that TRPV1 also detects noxious heat (thermoception). TRPV1 is part of a large family of structurally related TRP (transient receptor potential) cation channels. Animals that lack TRPV1 (using genetic knockouts of the protein) lose sensitivity to noxious heat and capsaicin. Julius’s lab has also cloned and characterized TRPM8 (CMR1) and TRPA1, both members of the TRP superfamily. They demonstrated that TRPM8 detects menthol and cooler temperatures and TRPA1 detects mustard oil (allyl isothiocyanate). These observations suggested that TRP channels detect a range of temperatures and chemicals. David Julius’s lab has also made contributions to the study of nociception by discovering toxins that modulate these channels, describing unique adaptations of the channels in diverse species and solving the cryo-EM structures of numerous channels. From 2007–2020 Julius served as the editor of the peer-reviewed journal the Annual Review of Physiology.

Robert Fuller, working with Thorner, identified the partial sequence of the Kex2-homologous Furin gene in 1989. In 1990 human Kex2-homologous genes were cloned by the Steiner group, Nabil Seidah and co-workers, Wim J.M. van de Ven and co-workers, Yukio Ikehara and co-workers, Randal Kaufman and co-workers, Gary Thomas and co-workers, and Kazuhisa Nakayama and co-workers.

Nabil G. Seidah, is a Canadian Québécois scientist. Born in Egypt, he was educated at Cairo University, and subsequently at Georgetown University where he obtained his Ph.D. in 1973. He emigrated to Canada and has been working at the Clinical Research Institute of Montreal (IRCM) since 1974. He is the director of the laboratory of Biochemical Neuroendocrinology. He discovered and cloned seven (PC1, PC2, PC4, PC5, PC7, SKI-1 and PCSK9) of the nine known enzymes belonging to the convertase family. During this period, he also greatly contributed to demonstrating that the proteolysis by the proprotein convertases is a wide mechanism that also concerns “non-neuropeptide” proteins such as growth factors, α-integrins, receptors, enzymes, membrane-bound transcription factors, and bacterial and viral proteins. In 2003, he discovered PCSK9 and showed that point mutations in the PCSK9 gene cause dominant familial hypercholesterolemia, likely because of a gain of function related to the ability of PCSK9 to enhance the degradation of cell surface receptors, such as the low-density lipoprotein receptor (LDLR). He has since worked on the elucidation of the functions and mechanisms of action of PCSK9 and PCSK7 both in cells and in vivo, and is developing specific PCSK9 and PCSK7 inhibitors/silencers.

Kexin (EC 3.4.21.61) is a prohormone-processing protease, specifically a yeast serine peptidase, found in the budding yeast (S. cerevisiae). It catalyzes the cleavage of -Lys-Arg- and -Arg-Arg- bonds to process yeast alpha-factor pheromone and killer toxin precursors. The human homolog is PCSK4. It is a family of subtilisin-like peptidases. Even though there are a few prokaryote kexin-like peptidases, all kexins are eukaryotes. The enzyme is encoded by the yeast gene KEX2, and usually referred to in the scientific community as Kex2p. It shares structural similarities with the bacterial protease subtilisin. The first mammalian homologue of this protein to be identified was furin. In the mammal, kexin-like peptidases function in creating and regulating many differing proproteins.

The enzyme is also known as yeast KEX2 protease, proteinase yscF, prohormone-processing endoprotease, paired-basic endopeptidase, yeast cysteine proteinase F, paired-basic endopeptidase, andrenorphin-Gly-generating enzyme, endoproteinase Kex2p, gene KEX2 dibasic proteinase, Kex 2p proteinase, Kex2 endopeptidase, Kex2 endoprotease, Kex2 endoproteinase, Kex2 protease, proteinase Kex2p, Kex2-like precursor protein processing endoprotease, prohormone-processing KEX2 proteinase, prohormone-processing proteinase, proprotein convertase, protease KEX2, Kex2 proteinase, and Kex2-like endoproteinase.

Furin

One of the most well-known PPCs is furin. Furin is a serine endoprotease which cleaves protein precursors carboxyterminal of basic residues in motifs such as Arg–X–X–Arg and Lys/Arg–Arg. Cleavage usually results in activation of the proprotein but can also inactivate or modify the activity. Therefore, it is not surprising that it plays a major role in many physiological processes and pathologies, including cancer.

Some of its substrates are: proPARATHYROID HORMONE, TRANSFORMING GROWTH FACTOR BETA 1 precursor, proALBUMIN, pro-beta-secretase, membrane type-1 matrix metalloproteinase, beta subunit of pro-nerve growth factor and VON WILLEBRAND FACTOR. A furin-like pro-protein convertase has been implicated in the processing of RGMc (also called hemojuvelin). Both the Ganz and Rotwein groups demonstrated that furin-like proprotein convertases (PPC) are responsible for conversion of 50 kDa HJV to a 40 kDa protein with a truncated COOH-terminus, at a conserved polybasic RNRR site. This suggests a potential mechanism to generate the soluble forms of HJV/hemojuvelin (s-hemojuvelin) found in the blood of rodents and humans.

Hemojuvelin (HJV), also known as repulsive guidance molecule C (RGMc) or hemochromatosis type 2 protein (HFE2), is a membrane-bound and soluble protein in mammals that is responsible for the iron overload condition known as juvenile hemochromatosis in humans, a severe form of hemochromatosis. In humans, the hemojuvelin protein is encoded by the HFE2gene. Hemojuvelin is a member of the repulsive guidance molecule family of proteins. Both RGMa and RGMb are found in the nervous system, while hemojuvelin is found in skeletal muscle and the liver. Do a separate note for this and hepcidin.

Prohormone convertases

The two proprotein convertases that specialize in the processing of the precursors of peptide hormones and neuropeptides are also known in the field as “prohormone convertases”. Both “prohormone convertase” and “proprotein convertase” are interchangeably abbreviated as “PC”. PC1 (also known as PC3 and commonly referred to as PC1/3) and PC2 are the primary enzymes involved in the processing of the bioactive peptides precursors at paired basic residues. PC1/3 and PC2 do not directly produce most neuropeptides and peptide hormones, but instead generate intermediates that contain C-terminal extensions of lysine and/or arginine residues; these are subsequently removed by carboxypeptidase E.

Clinical significance

Current scientific evidence indicates that both up- and down-regulation of the expression of proprotein convertases are part of the multiple changes occurring in gynecological tumors. PCs activate crucial substrates implicated in the progression of gynecological cancers, including adhesion molecules, metalloproteinases, and viral proteins. Experimental evidences suggest that careful targeting of PCs in gynecological cancer may represent a feasible strategy to deter tumor progression. Variants of PCSK9 can reduce or increase circulating cholesterol. Furin plays a role in the activation of several different virus proteins, and inhibitors of furin have been explored as antiviral agents.

Biochemical structure

Kex2 was first purified and characterized by Charles Brenner and Robert Fuller in 1992.

The Kex2 crystal structure was solved by a group led by Dagmar Ringe, Robert Fuller and Gregory Petsko. That of Furin was determined by a group led by Manual Than and Wolfram Bode. The key features of Kex2 and Furin are a subtilisin-related catalytic domain, a specificity pocket that requires the amino acid amino terminal to the scissile bond to be arginine for rapid acylation, and a P-domain carboxy-terminal to the subtilisin domain, which is required for biosynthesis.

PCSK subtypes

To date there are 9 PCSKs with varying functions and tissue distributions. Often, due to similar times of discovery from different groups the same PCSKs have acquired multiple names. In an attempt to alleviate confusion, there is a trend towards using the PCSK prefix with the appropriate number suffix.

Current PCSK nomenclatureOther common names
PCSK1PC1, PC3 (new name: PC1/3)
PCSK2PC2
PCSK3Furin, Pace, PC1
PCSK4PC4
PCSK5PC5, PC6 (new name: PC5/6)
PCSK6PACE4
PCSK7PC7, PC8
PCSK8Site 1 Protease, S1P, SKI
PCSK9NARC-1

Proprotein convertase 1, also known as prohormone convertase, prohormone convertase 3, or neuroendocrine convertase 1 and often abbreviated as PC1/3 is an enzyme that in humans is encoded by the PCSK1 gene. PCSK1 and PCSK2 differentially cleave proopiomelanocortin and they act together to process proinsulin and proglucagon in pancreatic islets.

PC1/3 is an enzyme that performs the proteolytic cleavage of prohormones to their intermediate (or sometimes completely cleaved) forms. It is present only in neuroendocrine cells such as brain, pituitary and adrenal, and most often cleaves after a pair of basic residues within prohormones but can occasionally cleave after a single arginine. It binds to a protein known as proSAAS, which also represents its endogenous inhibitor. PC1 is synthesized as a 99 kDa proform quickly converted to an 87 kDa major active form, which itself is nearly completely cleaved to a 66 kDa active form within neuroendocrine cells.

Proprotein convertase 1 is the enzyme largely responsible for the first step in the biosynthesis of insulin. Following the action of proprotein convertase 1, a carboxypeptidase is required to remove the basic residues from the processing intermediate and generate the bioactive form of insulin. Another prohormone convertase, proprotein convertase 2 plays a more minor role in the first step of insulin biosynthesis, but a greater role in the first step of glucagon biosynthesis. The knockout of proprotein convertase 1 is not lethal in mice or humans, most likely due to the presence of the second convertase, although mice lacking proprotein convertase 1 activity show a number of defects including slow growth.[citation needed]

Proprotein convertase 1 is a calcium (Ca2+) activated serine endoprotease (meaning that a serine residue is part of the active site that hydrolyzes the peptide bond within the substrate). It is related to the bacterial enzyme known as subtilisin. There are nine subtilisin homologs in mammals; in addition to proprotein convertase 1 and 2, other members of this enzyme family include furin, PACE4, PC4, PC5/6, PC7/8, PCSK9, and SKI1/S1P.

Proprotein convertase 1 converts prorenin into renin.

Clinical significance

Variants in the PCSK1 gene may be associated with obesity.

See also

  1. GRCh38: Ensembl release 89: ENSG00000175426 – Ensembl, May 2017
  2. GRCm38: Ensembl release 89: ENSMUSG00000021587 – Ensembl, May 2017
  3. “Human PubMed Reference:”. National Center for Biotechnology Information, U.S. National Library of Medicine.
  4. “Mouse PubMed Reference:”. National Center for Biotechnology Information, U.S. National Library of Medicine.
  5. Seidah NG, Mattei MG, Gaspar L, Benjannet S, Mbikay M, Chrétien M (September 1991). “Chromosomal assignments of the genes for neuroendocrine convertase PC1 (NEC1) to human 5q15-21, neuroendocrine convertase PC2 (NEC2) to human 20p11.1-11.2, and furin (mouse 7[D1-E2] region)“. Genomics. 11 (1): 103–7. doi:10.1016/0888-7543(91)90106-O. PMID 1765368.
  6. Jansen E, Ayoubi TA, Meulemans SM, Van de Ven WJ (June 1995). “Neuroendocrine-specific expression of the human prohormone convertase 1 gene. Hormonal regulation of transcription through distinct cAMP response elements“. J. Biol. Chem. 270 (25): 15391–7. doi:10.1074/jbc.270.25.15391. PMID 7797529.
  7. “EC 3.4.21.93”. www.qmul.ac.uk.
  8. Renström F, Payne F, Nordström A, et al. (April 2009). “Replication and extension of genome-wide association study results for obesity in 4923 adults from northern Sweden“. Hum. Mol. Genet. 18 (8): 1489–96. doi:10.1093/hmg/ddp041. PMC 2664142. PMID 19164386.

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Proprotein convertase 2 (PC2) also known as prohormone convertase 2 or neuroendocrine convertase 2 (NEC2) is a serine protease and proprotein convertase PC2, like proprotein convertase 1 (PC1), is an enzyme responsible for the first step in the maturation of many neuroendocrine peptides from their precursors, such as the conversion of proinsulin to insulin intermediates. To generate the bioactive form of insulin (and many other peptides), a second step involving the removal of C-terminal basic residues is required; this step is mediated by carboxypeptidases E and/or D. PC2 plays only a minor role in the first step of insulin biosynthesis, but a greater role in the first step of glucagon biosynthesis compared to PC1. PC2 binds to the neuroendocrine protein named 7B2, and if this protein is not present, proPC2 cannot become enzymatically active. 7B2 accomplishes this by preventing the aggregation of proPC2 to inactivatable forms. The C-terminal domain of 7B2 also inhibits PC2 activity until it is cleaved into smaller inactive forms that lack carboxy-terminal basic residues. Thus, 7B2 is both an activator and an inhibitor of PC2. PC2 has been identified in a number of animals, including C. elegans.

  • Gomez-Saladin E, Wilson DL, Dickerson IM (1994). “Isolation and in situ localization of a cDNA encoding a Kex2-like prohormone convertase in the nematode Caenorhabditis elegans”. Cellular and Molecular Neurobiology. 14 (1): 9–25. doi:10.1007/bf02088586. PMID 7954663. S2CID 43015859

In humans, proprotein convertase 2 is encoded by the PCSK2 gene. It is related to the bacterial enzyme subtilisin, and altogether there are 9 different subtilisin-like genes in mammals: furin, PACE4, PC4, PC5/6, PC7/8, PCSK9, and SKI1/S1P.

Further reading

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Proprotein convertase subtilisin/kexin type 4 is an enzyme that in humans is encoded by the PCSK4 gene.

Further reading

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Main Article References

  1. Andrew W. Artenstein; Steven M. Opal (December 29, 2011). “Proprotein Convertases in Health and Disease”. N Engl J Med. 365 (26): 2507–2518. doi:10.1056/NEJMra1106700. PMID 22204726.
  2. New Drugs for Lipids Set Off Race, By ANDREW POLLACK, New York Times, November 5, 2012
  3. The Role of Proprotein Convertases in Animal Models of Skin Carcinogenesis, by Daniel Bassi, Morgan & Claypool Publishers, 2012, DOI: doi:10.4199/C00060ED1V01Y201206PAC001
  4. Steiner DF, Cunningham D, Spigelman L, Aten B (August 1967). “Insulin biosynthesis: evidence for a precursor“. Science. 157 (3789): 697–700. Bibcode:1967Sci…157..697S. doi:10.1126/science.157.3789.697. PMID 4291105. S2CID 29382220.
  5. Chrétien M, Li CH (July 1967). “Isolation, purification, and characterization of gamma-lipotropic hormone from sheep pituitary glands“. Can. J. Biochem. 45 (7): 1163–74. doi:10.1139/o67-133. PMID 6035976.
  6. Therapeutic Potential of Furin Inhibition: An Evaluation Using a Conditional Furin Knockout Mouse Model, by Jeroen Declercq and Prof. Dr. J.W.M. Creemers, Morgan & Claypool Publishers, 2012, DOI:10.4199/C00068ED1V01Y201211PAC004
  7. Lin L, Nemeth E, Goodnough JB, Thapa DR, Gabayan V, Ganz T (2008). “Soluble hemojuvelin is released by proprotein convertase-mediated cleavage at a conserved polybasic RNRR site”. Blood Cells Mol. Dis. 40 (1): 122–31. doi:10.1016/j.bcmd.2007.06.023. PMC 2211380. PMID 17869549.
  8. Kuninger D, Kuns-Hashimoto R, Nili M, Rotwein P (2008). “Pro-protein convertases control the maturation and processing of the iron-regulatory protein, RGMc/hemojuvelin“. BMC Biochem. 9: 9. doi:10.1186/1471-2091-9-9. PMC 2323002. PMID 18384687.
  9. Peptide Biosynthesis: Prohormone Convertases 1/3 and 2, by A. Hoshino and I. Lindberg, Morgan Claypool Publishers, 2012, ISBN 978-161504-364-4, DOI 10.4199/C00050ED1V01Y201112NPE001
  10. Proprotein Convertases in Gynecological Cancers, by A.J. Klein-Szanto, 2012, Morgan & Claypool Publishers, DOI:10.4199/C00068ED1V01Y201211PAC004
  11. Brenner C, Fuller RS (1992). “Structural and Enzymatic Characterization of a Purified Prohormone-Processing Enzyme: Secreted, Soluble Kex2 Protease”. Proc. Natl. Acad. Sci. 89 (3): 922–926. Bibcode:1992PNAS…89..922B. doi:10.1073/pnas.89.3.922. PMC 48357. PMID 1736307.
  12. Seidah NG, Chrétien M (November 1999). “Proprotein and prohormone convertases: a family of subtilases generating diverse bioactive polypeptides”. Brain Res. 848 (1–2): 45–62. doi:10.1016/S0006-8993(99)01909-5. PMID 10701998. S2CID 22831526.
  13. Fugère M, Day R (June 2005). “Cutting back on pro-protein convertases: the latest approaches to pharmacological inhibition”. Trends Pharmacol. Sci. 26 (6): 294–301. doi:10.1016/j.tips.2005.04.006. PMC 7119077. PMID 15925704.

External links

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Endopeptidases: serine proteases/serine endopeptidases (EC 3.4.21)
Enzymes
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