View source for Argininosuccinate lyase

{{enzyme
| Name = Argininosuccinate lyase
| EC_number = 4.3.2.1
| CAS_number = 9027-34-3
| IUBMB_EC_number = 4/3/2/1
| GO_code = 0004056
| image = Tetramer ASLsmaller.png
| width = 
| caption = Crystal structure of duck argininosuccinate lyase with bound argininosuccinate.<ref name="pmid15320872">{{PDB|1TJW}}; {{cite journal | vauthors = Sampaleanu LM, Codding PW, Lobsanov YD, Tsai M, Smith GD, Horvatin C, Howell PL | title = Structural studies of duck delta2 crystallin mutants provide insight into the role of Thr161 and the 280s loop in catalysis | journal = Biochem. J. | volume = 384 | issue = Pt 2 | pages = 437–47 |date=December 2004 | pmid = 15320872 | pmc = 1134128 | doi = 10.1042/BJ20040656 | url = | issn = }}</ref>
}}
{{Infobox protein
   |Name=Argininosuccinate lyase
   |image= ASLMonomerwithdomains.png
   |caption= Crystallographic structure of the human ASL monomer with labeled domains.<ref name="Sampaleanu_2001"/>
   |Symbol=ASL 
   |AltSymbols=
   |HGNCid=746
   |Chromosome=7
   |Arm=p
   |Band=
   |LocusSupplementaryData=ter-q22
   |ECnumber=4.3.2.1
   |OMIM=608310
   |EntrezGene=435
   |RefSeq=NM_000048
   |UniProt=P04424
   |PDB=
}}

'''ASL''' ('''argininosuccinate lyase''', also known as '''argininosuccinase''') is an [[enzyme]] that catalyzes the reversible breakdown of [[argininosuccinic acid|argininosuccinate]] (ASA) producing the amino acid [[arginine]] and dicarboxylic acid [[fumaric acid|fumarate]]. Located in liver cytosol, ASL is the fourth enzyme of the [[urea cycle]] and involved in the biosynthesis of arginine in all species and the production of urea in [[ureotelic]] species.<ref name="Sampaleanu_2001">{{PDB|1K62}}; {{cite journal | vauthors = Sampaleanu LM, Vallée F, Thompson GD, Howell PL | title = Three-dimensional structure of the argininosuccinate lyase frequently complementing allele Q286R | journal = Biochemistry | volume = 40 | issue = 51 | pages = 15570–80 |date=December 2001 | pmid = 11747432 | doi = 10.1021/bi011525m| url = | issn = }}</ref> Mutations in ASL, resulting low activity of the enzyme, increase levels of urea in the body and result in various side effects.

The ASL gene is located on [[chromosome 7 (human)|chromosome 7]] between the [[centromere]] (junction of the long and short arm) and the long (q) arm at position 11.2, from [[base pair]] 64,984,963 to base pair 65,002,090.

ASL is related to [[intragenic complementation]].<ref name="pmid9256435">{{cite journal | vauthors = Turner MA, Simpson A, McInnes RR, Howell PL | title = Human argininosuccinate lyase: a structural basis for intragenic complementation | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 94 | issue = 17 | pages = 9063–8 |date=August 1997 | pmid = 9256435 | pmc = 23030 | doi = 10.1073/pnas.94.17.9063 | url = | issn = }}</ref><ref name="pmid11092456">{{cite journal | vauthors = Yu B, Howell PL | title = Intragenic complementation and the structure and function of argininosuccinate lyase | journal = Cell. Mol. Life Sci. | volume = 57 | issue = 11 | pages = 1637–51 |date=October 2000 | pmid = 11092456 | doi = 10.1007/PL00000646 | url = | issn = }}</ref><ref name="pmid11747433">{{cite journal | vauthors = Yu B, Thompson GD, Yip P, Howell PL, Davidson AR | title = Mechanisms for intragenic complementation at the human argininosuccinate lyase locus | journal = Biochemistry | volume = 40 | issue = 51 | pages = 15581–90 |date=December 2001 | pmid = 11747433 | doi = 10.1021/bi011526e | url = | issn = }}</ref>

==Structure==
ASL is composed of four identical monomers; each monomer consisting of a single polypeptide chain between 49 and 52 kDa,<ref name= "Sampaleanu_2002"/> between 196 and 208 kDa for the entire tetrameric enzyme. Each monomer has three highly conserved regions remote from one another, but these regions cluster together in the tetramer to form four active sites. Therefore, each ASL homotetramer has four active sites to catalyze the breakdown of argininosuccinate.

Each monomer in the ASL homotetramer is composed of three structural domains; all three are primarily alpha helical. Domains 1 and 3 are similar in structure as they both consist of helix-turn-helix motifs. Domain 1 of the monomer contains the amino terminus. Domain 2 contains one small beta sheet, nine alpha helices, and the carboxyl terminus. Three of the nine alpha helices on one monomer are engaged mainly in hydrophobic interactions with another monomer to form a dimer. Two dimers then associate by way of alpha helix, one from each monomer, to form a central 20-helix core. The association of all four monomers allows for the catalytic activity at each possible active site.<ref name="pmid11092456" />

==Mechanism==
[[Image:ASLMechanism.png|thumb|300px|left|Proposed ASL mechanism|alt=slide 1]]
[[Image:Active site of ASL with argininosuccinate.png|thumb|300px|left|Argininosuccinate (in yellow) in the Active Site of ASL|alt=active site]]

The enzyme's cleavage of the argininosuccinate, to form fumarate and arginine, occurs through an E1cb elimination reaction. The base initiates the reaction by deprotonating the carbon adjacent to the arginine, or leaving group. Recent mutagenic studies of ASL homologues have shown that Histidine 162 or Threonine 161 of ASL is responsible for the proton abstraction of the Cβ, either directly or indirectly through a water molecule.<ref name="Sampaleanu_2002">{{cite journal | vauthors = Sampaleanu LM, Yu B, Howell PL | title = Mutational analysis of duck delta 2 crystallin and the structure of an inactive mutant with bound substrate provide insight into the enzymatic mechanism of argininosuccinate lyase | journal = J. Biol. Chem. | volume = 277 | issue = 6 | pages = 4166–75 |date=February 2002 | pmid = 11698398 | doi = 10.1074/jbc.M107465200 | url = | issn = }}</ref> Lysine 289 is thought to stabilize the negatively charged carbanion intermediate. Although there is no consensus of the catalytic acid that donates the proton to the imine functional group of the arginine product, some mutagenesis studies show serine 283 may be involved.<ref name= "Sampaleanu_2002"/>

==Role in the urea cycle==
Ammonia (NH<sub>3</sub>) is a toxic substance for many aerobic organisms and must be excreted. Some aquatic organisms release the toxin right directly into their environment, while other ureotelic species must convert their toxic nitrogen waste into non-toxic components, like uric acid or urea, through a series of catalyzed steps better known as the urea cycle. ASL catalyzes the fourth step in the cycle, following the action of argininosuccinate synthetase (ASS) in the liver cytosol. While ASS catalyzes the formation of argininosuccinate from citrulline and aspartate, ASL breaks the newly formed argininosuccinate into L-arginine and fumarate. L-arginine continues through the urea cycle to form urea and ornithine, while fumarate can enter the citric acid cycle.<ref name="isbn0-470-12930-1">{{cite book | author = Pratt, Charlotte Amerley | author2 = Voet, Donald | author3 = Voet, Judith G. | others = | title = Fundamentals of biochemistry: life at the molecular level | edition = | language = | publisher = Wiley | location = New York | year = 2008 | origyear = | pages = | quote = | isbn = 0-470-12930-1 | oclc = | doi = | url = | accessdate = | chapter = Figure 20.8 }}</ref>

==δ-Crystallin==
ASL, δ-[[crystallin]], class II fumarase, aspartase, adenylosuccinase lyase, and 3-carboxy-cis and cis-muconate lactonizing enzyme are all members of the same homotetrameric superfamily of enzymes, in which most catalyze the same type of elimination reactions where a C-O or C-N bond is broken and fumarate is released as a product. δ-crystallins are the major structural eye lens water-soluble proteins of most birds, reptiles, and some other vertebrates.<ref name="pmid11092456"/>

Within the superfamily, ASL is most closely related to δ-crystallin in amino acid sequence and in protein fold structure. There are two isoforms of δ-crystallin, δI and δII. These two isoforms conserve 69% and 71% of the ASL amino acid sequence, respectively, but only the δII isoform retains the same enzymatic activity as ASL. The similarities have led researches to believe that these crystallins have evolved from the recruitment to the lens of preexisting metabolic enzymes, like ASL, by a process called 'gene sharing'. The same gene product functions as both a lens crystallin and an enzyme in other non-ocular tissues. Comparative studies of the δ-crystallins have been beneficial for understanding the enzymatic mechanism of the ASL reaction.<ref name="Chakraborty">{{cite journal | vauthors = Chakraborty AR, Davidson A, Howell PL | title = Mutational analysis of amino acid residues involved in argininosuccinate lyase activity in duck delta II crystallin | journal = Biochemistry | volume = 38 | issue = 8 | pages = 2435–43 |date=February 1999 | pmid = 10029537 | doi = 10.1021/bi982150g | url = | issn = }}</ref>

==Mutations and ASL deficiencies: argininosuccinic aciduria==
Mutations in the human ASL gene causes argininosuccinic aciduria, a rare autosomal recessive disorder, and results in deficiencies of the urea cycle. Argininosuccinate lyase is an intermediate enzyme in the urea synthesis pathway and its function is imperative to the continuation of the cycle. A non-functioning enzyme results in patients' accumulation of ammonia, argininosuccinate, and citrulline in the blood, and argininosuccinate is excreted in the urine.<ref name="Ficicioglu_2009">{{cite journal | vauthors = Ficicioglu C, Mandell R, Shih VE | title = Argininosuccinate lyase deficiency: longterm outcome of 13 patients detected by newborn screening | journal = Mol. Genet. Metab. | volume = 98 | issue = 3 | pages = 273–7 |date=November 2009 | pmid = 19635676 | doi = 10.1016/j.ymgme.2009.06.011 | url = | issn = | pmc = 2773214 }}</ref> Other resulting symptoms include lethargy, vomiting, hypothermia, hyperventilation, hepatomegaly and progressive encephalopathy in infant patients, and abnormal hair growth, hepatic fibrosis, episodic vomiting, growth and developmental delay,<ref name="Ficicioglu_2009"/> in patients experiencing the disorder later in childhood.

ASL is a key enzyme in the conversion of ammonia to urea through the urea cycle. Ammonia builds to toxic levels, resulting in hyperammonemia.<ref name= GeneticsHomeRef>{{cite web |url= https://ghr.nlm.nih.gov/gene/ASL|title=ASL gene argininosuccinate lyase |author=<!--Not stated--> |year=2007 |website=NIH  |publisher=U.S. Department of Health & Human Services |access-date= }}</ref> Ammonia is toxic in part because it affects the nervous system. There is biochemical evidence that shows rises in ammonia can inhibit glutaminase and therefore limit the rate of synthesis of neurotransmitters such as glutamate,<ref>{{cite journal | last = Jack | first = JJB | title = Actions of ammonia on the central nervous system | journal = Journal of Inherited Metabolic Disease | volume = 5 | issue = S2 | pages = 104 | year = 1982 | doi = 10.1007/BF01805572}}</ref> which can explain the developmental delay in argininosuccinic aciduria patients.

One mutation in patients with argininosuccinic aciduria occurs when glutamine 286 is mutated to arginine. The enzyme now has a positively charged arginine in place of a neutrally charged glutamine and studies suggest this change may sterically and/or electrostatically hinder a conformational change necessary for catalysis.

== References ==
{{Reflist|2}}

== External links ==
* [https://www.ncbi.nlm.nih.gov/books/NBK1217/ GeneReviews/NCBI/NIH/UW entry on Urea Cycle Disorders Overview]
* [https://www.ncbi.nlm.nih.gov/books/NBK51784/ GeneReviews/NCBI/NIH/UW entry on Argininosuccinate Lyase Deficiency]
* [https://www.ncbi.nlm.nih.gov/omim/207900,608310,207900,608310 OMIM entries on Argininosuccinate Lyase Deficiency] 
* [https://web.archive.org/web/20160304030540/http://www.genecards.org/cgi-bin/carddisp?ASL GeneCard]

{{Urea cycle enzymes}}
{{Carbon-nitrogen lyases}}

[[Category:Animal genes]]
[[Category:Lyases]]
[[Category:Urea cycle]]