Sandbox Reserved 346

This Sandbox is Reserved from January 10, 2010, through April 10, 2011 for use in BCMB 307-Proteins course taught by Andrea Gorrell at the University of Northern British Columbia, Prince George, BC, Canada.

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spinqualitylabelsall models PDB ID 1b4x Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image
1b4x, resolution 2.45Å ()
Ligands:	,
Activity:	Aspartate transaminase, with EC number 2.6.1.1
Structural annotation: Resources: CATH : 1B4Xa01, 1B4Xa02 InterPro : Ipr000796, Ipr004839, Ipr015421, Ipr015424, Ipr004838 Pfam : PF00155 SCOP : d1b4xa_ UniProt : P00509
Functional annotation: Cellular component: cytoplasm Biological process: biosynthetic process amino acid metabolic process Molecular function: pyridoxal phosphate binding catalytic activity transferase activity, transferring nitrogenous groups aspartate transaminase activity transaminase activity protein binding transferase activity
Evolutionary conservation: Toggle Conservation Colors: Rows = identical sequences: Further Information: Caveats, Explanation, Complete Results at ConSurfDB
Resources:	FirstGlance, OCA, PDBsum, RCSB
Coordinates:	save as pdb, mmCIF, xml

Aspartate AminotransferaseAspartate Aminotransferase

General InformationGeneral Information

Aspartate Aminotransferase (AST), also know as Glutamic aspartic transaminase, glutamic oxaloacetic transaminase, and transaminase A., is an enzyme that is a member of the class-I pyridoxal-phosphate-dependent aminotransferase family.It is coded by the gene GOT1. It is a homodimer that is 413 amino acids long and serves a critical role in amino acid metabolism. Within prokaryote cells it is exclusively found in the cytosol, but in eukaryotic cells there are cytosol and mitochondrial isozymes.

In the human body it is produced by the brain, skeletal muscles, liver, pancreas, red blood cells, and kidneys. The wide range of tissues in which it is made, separates it from the similar enzyme alanine transaminase (ALT) which is found primarily in the liver. The level of AST in the body can be used as a marker for tissue disease or damage. As well, AST and ALT levels can be compared to pinpoint whether tissue damage is primarily found within the liver.

StructureStructure

spinqualitylabelsall models Ligand Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

is a homodimer that contains 16 alpha helices and a Beta-sheet formed from 7 parallel and antiparallel strands.Each subunit contain an equivalent active site. The subunits connect between their large domains and between the the N-terminal residues and the large domain of the other subunit. This structure of AST varies minutely among organisms rangin from E. coli to humans. The structure of the active site is highly conserved, and their is a sequence homology of 25%.

Each subunit of the homo dimer is further divided into a samll and large domain. The is comprises of the amino acids from the N-terminus to the Pro 48 residue and from the Met 326 residue to the C-terminus. The remaining amino acids make up the , and the are connected by a long alpha helicx consisting of 32 amino acids.

The large domain is where the active site of AST is found and to accomadate this the core is contains many alpha/beta supersecondary structures. This is contrasted with the core of the small subunit which is fromed from two alpha hellices and two beta strands. In multicellular organisms there is a kink at the 325th residue which acts as a hinge for the samll domain, which allows for the resulting conformational changes that take place upon the binding of inhibitors to the enzyme.

As was stated above, the active site of AST is situated on the large domain of the subunit. Within the active site is the amino residue Lys 258, also known as the intternal aldimine, which binds with the cofactor Pyridoxal 5'-phosphate () forming what is called a Schiff base. Upon addition of an amino acid substrate a new Schiiff base forms between PLP and the amino acid

FunctionFunction

AST catalyzes the reversible transamination of the alpha-amino group from L-aspartate to alpha-ketoglutarate forming oxaloacetate and alpha-ketoglutamate.This reactivity is lower in E.coli than in higher eukaryotes, and had borader substrate specificity. However, the reaction takes place in the same way. Upon introduction of an amino acid substrate, a new Schiff base will form between it and the PLP cofactor. This causes the amino acid to lose a hydrogen to form the quinoid intermediate, and reprotanation takes place resulting in a ketimine. Next, the structure is hydrolyzed forming an alpha-keto acid and pyridoxamine phosphate. 2-methyl asparate acts as an inhibitor of AST when it forms a Schiif base with the PLP cofactor, rather than aspartate. This results in the process stopping at the step prior to the alpha protein elimination.

This reaction is essential to maintaining homeostasis in organisms. The four different molecules that can form as a result of this transanimation (oxaloacetate, alpha-ketoglutarate, aspartate, L-glutamate) our critical to a number of metabolic processes. Oxaloacetate and alpha-ketoglutarate play a critical role in the Krebs cycle and the varying forms of aspartate are important molecules in the urea cycle and participates in gluconeogenesis.

Clinical ApplicationsClinical Applications

The levels of AST in the body are indicitive of tissue damage and disease. Normally AST is found in minimal amounts within the blood, however when the organs mentioned above are damaged, AST is released into the blood. The amount released is proportional to the level of damage sustained. AST levels have beenn shown to rise substantially within 6 hours of the initial tissue degradation and can stay elevated for up to 4 days.

AST levels when compared with the levels of other enzymes can be used by physicians to determine where in the body the damge has taken place. Comparisons with ALT have proven particularly useful in identifying liver damage such as cirrhosis and hepatitis.