Adenylosuccinate Synthetase
IntroductionIntroduction
Adenylosuccinate Synthetase (AdSS) is part of the Ligase family of enzymes[1]. Ligase enzymes commonly 'glue' two different molecules together and create a new chemical bond.
Its systematic name is IMP:L-aspartate ligase. It is mainly involved in purine bio-synthesis. It does this by catalyzing the GTP dependent changeover of IMP and aspartic acid to AMP in the presense of Mg2+.[2] In Humans, AdSS catalyzes the first committed step in the purine nucleotide cycle by the de novo synthesis of adenosine monophosphate.[3]
AdSS (3hid) was isolated from Yersinia pestis CO92and can be found in a variety of organisms ranging from yeast to bacteria to humans. Its crystal structure was determined using x-ray diffraction at a resolution of 1.60 Angstoms. The gene is located on chromosome 1 q44, in humans and is expressed in the majority of an organisms cells.
StructureStructure
The AdSS enzyme is a dimer consisting of two identical monomeric subunits. The main structural component of each monomer is a centrally located beta sheet that is comprised of 10 strands. Nine of the 10 strands are parallel while the 10th strand is anti-parallel with respect to the other 9 strands. There are also several other secondary structures including 2 small 3/10 helices and two anti-parallel sheets consisting of 2 and 3 strands respectively. Additionally there are 11 alpha helices.[4] AdSS has three major binding sites, one for GTP, one for IMP and its active site. The active site, in yellow, and ligand molecules in orange can be seen .
The major fold of this protein is unique and has not been documented in any other type of protein. The active site itself consists of the following residues: Gly12, Gly15, Gly17, Lys18, Ile19, and Lys331. Lysine 140 and Arg147 are located in the region between the two monomers. Asp231 is bonded to Lys140 in a salt bridge and its carbonyl atom is bonded to Arg147.
Helices are highlighted in green and beta sheets are shown in orange .
AdSS has an optimal pH of 6.5 and a denatures at pH 7.4.[5] It has a melting point of 85 degrees Celsius.[6]
Reaction MechanismReaction Mechanism
AdSS undergoes the following amination reaction:
GTP + IMP + L-Asp -> GDP + phosphate + N6-(1,2-dicarboxyethyl)-AMP[7]
+ 
+
-> 
+ 
+ 
The 6-O of inosine is displaced by aspartate which yields adenylosuccinate. The presence, or over abundance of AMP acts as a feedback inhibitor for AdSS.[8]
Binding of GTP alone is not sufficient to induce a conformational change in the enzyme. IMP, coupled with an acetate ion invoke a conformational change which helps prepare the active site for catalysis. The reaction proceeds as follows: The gamma phosphoryl group of GTP is transferred to the 6-keto group of IMP which causes the displacement of an inorganic phosphorus molecule, Pi, from L-Aspartate. This leads to the formation of an adenylosuccinate molecule.[9]
Medical ImplicationsMedical Implications
Defects in the gene product can result in a wide variety of diseases such as Acidosis, Gout, Neoplasms, Sarcoma, Leukemia and Lymphoma[10]. Since AdSS plays a crucial role in purine catalysis, it is a choice target for anti tumor drugs and antibiotics. A variety of enzymes are found to be improperly balanced in cancer cells, including AdSS which is found to be over expressed 5.5 times as much as it would normally be.[11]
ReferencesReferences
- ↑ http://www.pdb.org/pdb/explore/explore.do?structureId=3HID
- ↑ Mukhopadhyay RP, Chandra AL. Keratinase of a streptomycete. Indian J Exp Biol. 1990 Jun;28(6):575-7. PMID:1698173
- ↑ Pierloot RA. The treatment of psychosomatic disorders by the general practitioner. Int J Psychiatry Med. 1977-1978;8(1):43-51. PMID:649264
- ↑ Poland BW, Silva MM, Serra MA, Cho Y, Kim KH, Harris EM, Honzatko RB. Crystal structure of adenylosuccinate synthetase from Escherichia coli. Evidence for convergent evolution of GTP-binding domains. J Biol Chem. 1993 Dec 5;268(34):25334-42. PMID:8244965
- ↑ Ware JD, Bellini WJ, Ash RJ. Metabolic characteristics of cells infected with a herpesvirus of turkeys. J Natl Cancer Inst. 1975 Dec;55(6):1379-82. PMID:1548
- ↑ Katsarkas A, Kirkham TH. Paroxysmal positional vertigo--a study of 255 cases. J Otolaryngol. 1978 Aug;7(4):320-30. PMID:691098
- ↑ Van der Weyden MB, Kelly WN. Human adenylosuccinate synthetase. Partial purification, kinetic and regulatory properties of the enzyme from placenta. J Biol Chem. 1974 Nov 25;249(22):7282-9. PMID:4436310
- ↑ Bates PC, Millward DJ. Muscle growth and protein turnover in a fast growing rat strain. Proc Nutr Soc. 1978 May;37(1):19A. PMID:662843
- ↑ Iancu CV, Borza T, Choe JY, Fromm HJ, Honzatko RB. Recombinant mouse muscle adenylosuccinate synthetase: overexpression, kinetics, and crystal structure. J Biol Chem. 2001 Nov 9;276(45):42146-52. Epub 2001 Sep 17. PMID:11560929 doi:10.1074/jbc.M106294200
- ↑ Tyagi AK, Cooney DA. Identification of the antimetabolite of L-alanosine, L-alanosyl-5-amino-4-imidazolecarboxylic acid ribonucleotide, in tumors and assessment of its inhibition of adenylosuccinate synthetase. Cancer Res. 1980 Dec;40(12):4390-7. PMID:7438071
- ↑ Weber G. Enzymes of purine metabolism in cancer. Clin Biochem. 1983 Feb;16(1):57-63. PMID:6861338
| Please do NOT make changes to this Sandbox until after April 23, 2010. Sandboxes 151-200 are reserved until then for use by the Chemistry 307 class at UNBC taught by Prof. Andrea Gorrell. |