# PPOX

## Overview
The PPOX gene encodes the enzyme protoporphyrinogen oxidase, which is essential in the biosynthesis of heme, a critical component of hemoglobin, myoglobin, and various cytochromes. Protoporphyrinogen oxidase, the protein product of the PPOX gene, catalyzes the oxidation of protoporphyrinogen IX to protoporphyrin IX, the penultimate step in heme production. This enzyme is localized in the mitochondria, reflecting its integral role in cellular energy metabolism. The structure of the protein includes a homodimer formation, crucial for its enzymatic function, with each monomer binding a flavin adenine dinucleotide (FAD) cofactor necessary for its activity (Boateng2015Characterisation; Maneli2003Kinetic). Mutations in the PPOX gene can lead to variegate porphyria, a disorder characterized by photosensitivity, skin lesions, and neurovisceral symptoms, underscoring the enzyme's importance in human health (Roberts1998Molecular; Frank1998Homozygous).

## Structure
The human protoporphyrinogen oxidase (PPOX) protein is a crucial enzyme in the biosynthesis of heme. Structurally, PPOX is a homodimer, where each monomer binds a noncovalently attached flavin adenine dinucleotide (FAD) cofactor (Maneli2003Kinetic). The protein is divided into two domains as defined by a single protease digestion site, with the N-terminal domain containing the FAD binding fold and the C-terminal domain containing the active site, located at the interface between the two domains (Maneli2003Kinetic). This arrangement suggests a significant role of these domains in the enzyme's function, particularly in the binding of the FAD cofactor and the catalytic activity at the active site.

The secondary structure of PPOX includes both alpha-helices and beta-sheets, contributing to the stability and functionality of the enzyme. The FAD binding region is characterized by a conserved sequence that forms a Rossman fold, important for binding the adenine moiety of FAD (Boateng2015Characterisation). Specific interactions in this region include hydrogen bonds and hydrophobic interactions involving residues such as Val-251, Val-15, Pro-284, Ala-283, and Glu-39, which interact with different parts of the FAD molecule (Boateng2015Characterisation).

Overall, the molecular structure of PPOX, including its primary, secondary, and tertiary structures, as well as its quaternary dimeric state, plays a critical role in its function as an enzyme in heme biosynthesis.

## Function
The PPOX gene encodes the enzyme protoporphyrinogen oxidase, which plays a pivotal role in the heme biosynthesis pathway by catalyzing the oxidation of protoporphyrinogen IX to protoporphyrin IX. This reaction represents the penultimate step in the production of heme, an essential component for various biological functions including oxygen transport and cellular respiration (Novakova2022Generation).

Located in the inner membrane of mitochondria, the primary site of heme synthesis, PPOX is active in several tissues such as the liver, lymphocytes, and cultured fibroblasts. The enzyme's activity is crucial for the proper function of mitochondrial respiratory complexes, particularly complex III and complex IV, which are integral to cellular respiration and energy production (Novakova2022Generation; TAKETANI1995The).

Mutations or deficiencies in PPOX can lead to variegate porphyria, a condition characterized by a significant reduction in enzyme activity, underscoring the enzyme's critical role in maintaining normal heme levels within cells. This disruption can result in diminished mitochondrial function and altered cellular energy metabolism (Novakova2022Generation; TAKETANI1995The).

## Clinical Significance
Mutations in the PPOX gene are primarily associated with variegate porphyria (VP), a disorder characterized by a partial defect in the activity of protoporphyrinogen oxidase, leading to a buildup of porphyrins or their precursors. This condition manifests with a range of symptoms including severe photosensitivity, skin lesions, and, in some cases, acute neurovisceral attacks (Roberts1998Molecular; Frank1998Homozygous). The homozygous variant of VP, although rare, presents with more severe symptoms such as early childhood onset of photosensitization, skin lesions, and skeletal abnormalities (Roberts1998Molecular).

Specific mutations identified in the PPOX gene include missense mutations and deletions that significantly reduce the enzyme's activity, contributing to the clinical features observed in patients. For instance, mutations like G169E and G358R have been linked to severely reduced PPO enzyme activity and elevated protoporphyrin levels, confirming the diagnosis of VP in affected individuals (Frank1998Homozygous). Additionally, the presence of mutations such as R59W, Y348C, and R138P in South African families has been correlated with severe symptoms and early onset of the disease, indicating a substantial reduction in PPOX activity (Corrigall2000Homozygous; Corrigall2000Homozyous).

The identification and characterization of these mutations are crucial for the diagnosis and management of VP, as evidenced by studies reporting a high diagnostic sensitivity for detecting mutations in the PPOX gene among patients suspected of having porphyria (Whatley2009Diagnostic).

## Interactions
PPOX (protoporphyrinogen oxidase) is known to interact with various molecules, crucial for its function in the heme biosynthetic pathway. Notably, PPOX forms a homodimer, a structural arrangement essential for its enzymatic activity (Maneli2003Kinetic). This dimerization is supported by the presence of a hydrophobic crystal packing interface, which suggests interactions through van der Waals forces and potential stabilization by charged lipid head groups (Corradi2006Crystal).

Additionally, PPOX interacts with the flavin adenine dinucleotide (FAD) cofactor. This interaction involves multiple noncovalent bonds, including hydrogen bonds and hydrophobic interactions, which are critical for stabilizing the flavin for catalysis (Boateng2015Characterisation). The enzyme's interaction with FAD is further facilitated by a conserved FAD-binding motif within the PPOX superfamily, highlighting its structural importance (Boateng2015Characterisation).

Moreover, PPOX has been found to interact with RNA editing factors, suggesting a regulatory role in plastid RNA editing. This indicates potential interactions with nucleic acids, although specific details of these interactions are not provided (Wang2021The). These interactions underline the multifaceted role of PPOX in cellular metabolism and regulation.


## References


[1. (Boateng2015Characterisation) Mavis O. Boateng, Anne V. Corrigall, Edward Sturrock, and Peter N. Meissner. Characterisation of the flavin adenine dinucleotide binding region of myxococcus xanthus protoporphyrinogen oxidase. Biochemistry and Biophysics Reports, 4:306–311, December 2015. URL: http://dx.doi.org/10.1016/j.bbrep.2015.10.006, doi:10.1016/j.bbrep.2015.10.006. (0 citations) 10.1016/j.bbrep.2015.10.006](https://doi.org/10.1016/j.bbrep.2015.10.006)

[2. (Maneli2003Kinetic) Mbulelo H. Maneli, Anne V. Corrigall, Horst H. Klump, Lester M. Davids, Ralph E. Kirsch, and Peter N. Meissner. Kinetic and physical characterisation of recombinant wild-type and mutant human protoporphyrinogen oxidases. Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics, 1650(1–2):10–21, August 2003. URL: http://dx.doi.org/10.1016/s1570-9639(03)00186-9, doi:10.1016/s1570-9639(03)00186-9. (38 citations) 10.1016/s1570-9639(03)00186-9](https://doi.org/10.1016/s1570-9639(03)00186-9)

[3. (TAKETANI1995The) SHIGERU TAKETANI, JOHJI INAZAWA, TATSUO ABE, TAKAKO FURUKAWA, HIRAO KOHNO, RIKIO TOKUNAGA, KOICHI NISHIMURA, and HACHIRO INOKUCHI. The human protoporphyrinogen oxidase gene (ppox): organization and location to chromosome 1. Genomics, 29(3):698–703, October 1995. URL: http://dx.doi.org/10.1006/geno.1995.9949, doi:10.1006/geno.1995.9949. (96 citations) 10.1006/geno.1995.9949](https://doi.org/10.1006/geno.1995.9949)

[4. (Roberts1998Molecular) A. G. Roberts, H. Puy, T. A. Dailey, R. R. Morgan, S. D. Whatley, H. A. Dailey, P. Martasek, Y. Nordmann, J.-C. Deybach, and G. H. Elder. Molecular characterization of homozygous variegate porphyria. Human Molecular Genetics, 7(12):1921–1925, November 1998. URL: http://dx.doi.org/10.1093/hmg/7.12.1921, doi:10.1093/hmg/7.12.1921. (48 citations) 10.1093/hmg/7.12.1921](https://doi.org/10.1093/hmg/7.12.1921)

[5. (Novakova2022Generation) Zora Novakova, Mirko Milosevic, Zsofia Kutil, Marketa Ondrakova, Barbora Havlinova, Petr Kasparek, Cristian Sandoval-Acuña, Zuzana Korandova, Jaroslav Truksa, Marek Vrbacky, Jakub Rohlena, and Cyril Barinka. Generation and characterization of human u-2 os cell lines with the crispr/cas9-edited protoporphyrinogen oxidase ix gene. Scientific Reports, October 2022. URL: http://dx.doi.org/10.1038/s41598-022-21147-x, doi:10.1038/s41598-022-21147-x. (1 citations) 10.1038/s41598-022-21147-x](https://doi.org/10.1038/s41598-022-21147-x)

[6. (Corrigall2000Homozygous) A.V. Corrigall, R.J. Hift, L.M. Davids, V. Hancock, D. Meissner, R.E. Kirsch, and P.N. Meissner. Homozygous variegate porphyria in south africa: genotypic analysis in two cases. Molecular Genetics and Metabolism, 69(4):323–330, April 2000. URL: http://dx.doi.org/10.1006/mgme.2000.2975, doi:10.1006/mgme.2000.2975. (21 citations) 10.1006/mgme.2000.2975](https://doi.org/10.1006/mgme.2000.2975)

[7. (Whatley2009Diagnostic) Sharon D Whatley, Nicola G Mason, Jacqueline R Woolf, Robert G Newcombe, George H Elder, and Michael N Badminton. Diagnostic strategies for autosomal dominant acute porphyrias: retrospective analysis of 467 unrelated patients referred for mutational analysis of the hmbs, cpox, or ppox gene. Clinical Chemistry, 55(7):1406–1414, July 2009. URL: http://dx.doi.org/10.1373/clinchem.2008.122564, doi:10.1373/clinchem.2008.122564. (102 citations) 10.1373/clinchem.2008.122564](https://doi.org/10.1373/clinchem.2008.122564)

[8. (Corradi2006Crystal) Hazel R. Corradi, Anne V. Corrigall, Ester Boix, C.Gopi Mohan, Edward D. Sturrock, Peter N. Meissner, and K.Ravi Acharya. Crystal structure of protoporphyrinogen oxidase from myxococcus xanthus and its complex with the inhibitor acifluorfen. Journal of Biological Chemistry, 281(50):38625–38633, December 2006. URL: http://dx.doi.org/10.1074/jbc.m606640200, doi:10.1074/jbc.m606640200. (90 citations) 10.1074/jbc.m606640200](https://doi.org/10.1074/jbc.m606640200)

[9. (Wang2021The) Baifan Wang, Zijuan Zhang, Hao Zhu, Congwei Niu, Xin Wen, and Zhen Xi. The hydrogen bonding network involved arg59 in human protoporphyrinogen ix oxidase is essential for enzyme activity. Biochemical and Biophysical Research Communications, 557:20–25, June 2021. URL: http://dx.doi.org/10.1016/j.bbrc.2021.03.124, doi:10.1016/j.bbrc.2021.03.124. (5 citations) 10.1016/j.bbrc.2021.03.124](https://doi.org/10.1016/j.bbrc.2021.03.124)

[10. (Frank1998Homozygous) Jorge Frank, John McGrath, HaMut Lam, Robert M. Graham, John L.M. Hawk, and Angela M. Christiano. Homozygous variegate porphyria: identification of mutations on both alleles of the protoporphyrinogen oxidase gene in a severely affected proband. Journal of Investigative Dermatology, 110(4):452–455, April 1998. URL: http://dx.doi.org/10.1046/j.1523-1747.1998.00148.x, doi:10.1046/j.1523-1747.1998.00148.x. (37 citations) 10.1046/j.1523-1747.1998.00148.x](https://doi.org/10.1046/j.1523-1747.1998.00148.x)