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Glutamate-Aspartate Aminotransferase

Introduction: Aspartate Aminotransferase(AST)also known as serum glutamic oxaloacetic transaminase (SGOT)is an enzyme that functions within the Amino Acid Biosynthesis pathway to interconvert Glutamate and Aspartate. Aminotransferases work to transform amino acids using transamination reactions. This is a process which utilizes the exchange of an α-keto acid to alter an amino acid usually Glutamate and oxaloacetate to a second amino acid in this case Aspartate and α-ketoglutarate. The reaction is dependent on pyridoxial phosphate (PLP) a cofactor which switches between the Pyridoxial Phosphate(PLP) form and the Pyridoxamine Phosphate (PMP) form. The reaction is important in both amino acid synthesis and degradation. When looking at the formation of aspartate it is synthesized from oxaloacetate which is a key step of the citric acid cycle. Also glutamate can be degraded into ammonium ions through the oxidative deamination process. Aspartate Aminotransferae is generally found and has been studied in E.coli, pig heart cytosol, and chicken mitochondira. It is also present in other microorganisms such as Thermus thermophilus and Haloferax. However within these organisms it is commonly found in the muscles, tissues, heart,liver, and kidneys. This is why Aspartate Aminotransferase is so important in tests that monitor liver disease and damage to any of the above organs. [2]

Structure: The structure of Aspartate Aminotransferase has been determined through studies using X-ray crystallography. AST contains both α-helices packed on either side of a central sheet of β-strands in each of its domains. It is seen that AST has two identical subunits. Each subunit is approximately 300 residues long and each is composed of both a small and large domain. In addition a third domain is present which consists of the N-terminal residues 3-14; these residues link and stabilize the two subunits by forming a strand. Pyridoxial phosphate (PLP), the cofactor for AST binds to the large domain, interacting with the amino group of the Lys258 residue. It also interacts through hydrogen bonding with the Asp222 and Tyr 225 residues within the large domain. The small domain is responsible for the shift of the enzyme from an open conformation to closed conformation. This shift is based on the binding of the substrate. are located between the two domains near the interface. There are two independent active sites each contains two arginine residues which are responsible for the enzyme’s specificity for the substrate. The first is Arg 292 which forms complexes with carboxylate side chains and the second is ARg286 which interacts with the alpha carobxylate group of the substrate.[2]

Mechanism of Structure: Alpha ketoglutarate is an enzyme from Kreb’s Cycle, which exhibits several biochemical pathways. It reacts with a free amino acid that comes in the form of ketoacids. In addition, a cofactor Pyridoxal-5’-phosphate is added into the reaction in order to yield Glutamate and alpha ketoglutarate within the reaction of transamination by adding an amine to the alpha carbon carbonyl. This reaction involves another amino acid aspartate that uses another enzyme from the TCA cycle oxaloacetate, a reversible step enzyme that interconverts the appropriate amino acid while adding an amine to the actual precursor amino acid. Both of these reactions change the cofactor Pyridoxal-5’-phosphate to Pyridoxal-5’-amine which alters several conformational changes of the protein binding site. The actual cofactors P5P, PLP bind covalently and reversibly to an active-site in order to facilitate enzymatic reaction while the amino group is being added in to precursor used as a substrate. This helps catalyze the reaction by accepting the amino group from the amino acid then transferring it to the carbonyl compound of alpha keto glutarate and a keto acid to yield glutamate , then oxaloacetate and another form alpha keto acid to form an aspartate. There are other cofactors used in this reaction as intermediates through amination for yielding amino acids and regenerating the enzyme precursor from the Kreb’s cycle. [3][5]

Implications of Possible Actions: Asparatate Aminotransferase is found in the liver, heart, muscles, kidneys, and other organs. When any of these organs are damaged AST is released and this results in elevated levels of the enzyme in serum. These levels parallel the severity of damage within the organ. Increases in AST levels are common after Heart attack, muscle injury, and Liver disease. In some cases pregnancy can decrease AST levels.Liver damage is commonly detected through a liver function panel which measures the levels of both AST and Alanine Aminotransferase (ALT). When levels of AST exceed the levels of ALT this indicates that the liver disease is most likely alcohol-induced damager, liver tumors, or cirrhosis. When such damage occurs AST levels can take three to six months to return to normal levels.[4]

References: Islam, M M., and Goto. "Crystal Structure of E.coli AspAT Complexed with N-phosphopyridoxyl-2-methyl-L-glutamic Acid." RCS Protein Data Bank, 205. Web. 18 Nov. 2012. <http://www.rcsb.org/pdb/explore/explore.do?structureId=1x29>.[1]

Kirsch, Jack F. "Mechanism of Action of Aspartate Aminotransferase Proposed on the Basis of Its Spatial Structure." Journal Of Molecular Biology. Science Direct, 15 Apr. 1984. Web. 20 Nov. 2012. <http://www.sciencedirect.com/science/article/pii/0022283684903334>.[2]

Rose, Bob. "Amino Acid Biosynthesis." Moodle, 15 Nov. 2011. Web. 20 Nov. 2012. <http://moodle.wolfware.ncsu.edu/file.php/30769/Day23_aa_biosynthesis_2012.pdf>.[3]

Thompson, Gregory. "Aspartate Aminotransferase (AST)." Aspartate Aminotransferase. WebMD, 04 Nov. 2011. Web. 20 Nov. 2012. <http://www.webmd.com/digestive-disorders/aspartate-aminotransferase-ast>.[4]

Aspartate Transaminase. Wikipedia, n.d. Web. 24 Nov. 2012. <http://en.wikipedia.org/wiki/Aspartate_transaminase>.[5]