Sandbox Reserved 346: Difference between revisions

Aspartate Aminotransferase (AST), also known 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 <ref name ="AST family and name">PMID:20977429</ref>. It is coded by the gene GOT1<ref name ="AST gene">PMID:4193185</ref>. It is a homodimer that is 413 amino acids long and serves a critical role in amino acid and carbohydrate metabolism, ureogenesis, and the transfer of reducing equivalents into the mitochondria and chloroplast<ref name ="AST ROLES AND STRUCTURE">PMID:10708649</ref>. Within prokaryote cells it is exclusively found in the cytosol, but in eukaryotic cells there are cytosol, mitochondrial, and chloroplast isozymes<ref name ="AST family and name"/><ref name ="AST Structure"/>.  

In the human body it is produced in the brain, skeletal muscles, liver, pancreas, red blood cells, and kidneys <ref name ="AST ORGANS">PMID:2569674</ref><ref name ="Liver damage"/>. 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<ref name ="Liver damage">PMID:10831269</ref>. The level of AST in the body can be used as a marker for tissue disease or damage<ref name ="Liver damage"/>. As well, AST and ALT levels can be compared to pinpoint whether tissue damage is primarily found within the liver<ref name ="Liver damage">PMID:12546613</ref>.

<Structure load='1b4x' size='300' frame='true' align='left' caption='Figure 1: Asymetric unit of Aspartate aminotransferase, with highlighted  small and large domain and PLP cofactor' scene='Sandbox_Reserved_346/Ast/1'/>

<scene name='Sandbox_Reserved_346/Ast/1'>AST</scene> is a homodimer that contains 16 α-helices and a β-sheet formed from 7 parallel and antiparallel strands<ref name ="AST Structure"/>. Each subunit contains an equivalent active site<ref name ="AST Structure">PMID:2121725</ref>. The subunits connect at two sites: between their large domains and between the N-terminal residues and the large domain on the other subunit<ref name ="AST Structure"/>. This structure of AST varies minutely among organisms ranging from ''E. coli'' to humans<ref name ="AST Structure"/><ref name ="AST ROLES AND STRUCTURE"/>. As well, the structure of the active site is highly conserved with a sequence homology of 25%<ref name ="AST Structure"/>.

 

 

 

As was stated above, the active site of AST is situated on the large domain of the subunit<ref name ="AST Structure"/>. Within the active site is the amino residue Lys 258, also known as the internal aldimine, which binds with the cofactor Pyridoxal 5'-phosphate (<scene name='Sandbox_Reserved_346/Plp/5'>PLP</scene>) forming what is called a [http://en.wikipedia.org/wiki/Schiff_base Schiff base]<ref name ="AST Structure"/><ref name ="AST ROLES AND STRUCTURE"/>. Upon the addition of an amino acid substrate, a new Schiiff base forms between PLP and the amino acid<ref name ="AST Structure"/>.

 

 

[[Image:Ast-reaction final copy.JPG|right|thumb|upright=3|Figure 2: Transamination reaction of L-aspartate and α-ketoglutarate catalyzed by aspartate aminotransferase]]

AST catalyzes the reversible transamination of the α-amino group from L-aspartate to α-ketoglutarate forming oxaloacetate and glutamate<ref name ="AST ROLES AND STRUCTURE"/>. This reactivity is lower in E.coli than in higher eukaryotes, and has broader substrate specificity<ref name ="AST Structure"/>. However, the reaction takes place in the same way<ref name ="AST Structure"/>. Upon introduction of an amino acid substrate, a new Schiff base will form between it and the PLP  cofactor<ref name ="AST Structure"/><ref name ="TRANSAMINATION">PMID:5450225</ref>. This causes the amino acid to lose a hydrogen and form a quinoid intermediate, and reprotanation takes place resulting in a ketimine<ref name ="AST Structure"/><ref name ="TRANSAMINATION"/>. Next, the structure is hydrolyzed forming an α-keto acid and pyridoxamine phosphate<ref name ="TRANSAMINATION"/>. 2-methyl aspartate acts as an inhibitor of AST when it forms a Schiif base with the PLP cofactor, rather than aspartate<ref name ="TRANSAMINATION"/><ref name ="AST Structure"/>. This results in the process stopping at the step prior to the alpha protein elimination<ref name ="TRANSAMINATION"/><ref name ="AST Structure"/>.

This reaction is essential to maintaining homeostasis in organisms. The four different molecules that can form as a result of this transanimation (oxaloacetate, α-ketoglutarate, aspartate, L-glutamate) our critical to a number of metabolic processes<ref name ="OXALOACETATE">PMID:11124972</ref><ref name ="ALPHA-KETOGLUTARATE">PMID:11124972</ref><ref name ="ASPARTATE">PMID:1557428</ref><ref name ="AST ROLES AND STRUCTURE"/><ref name ="glutamate"/>.  Oxaloacetate and α-ketoglutarate play a critical role in the Krebs cycle, varying forms of aspartate are important molecules in the urea cycle and participate in gluconeogenesis, and glutamate is an important molecule in metabolic pathways associated with memory<ref name ="OXALOACETATE">PMID:11124972</ref><ref name ="ALPHA-KETOGLUTARATE">PMID:11124972</ref><ref name ="ASPARTATE">PMID:1557428</ref><ref name ="AST ROLES AND STRUCTURE"/><ref name ="glutamate">PMID:15929064</ref>.

The levels of AST in the body are indicative of tissue damage and disease<ref name ="TISSUE DAMAGE">PMID:8432855</ref>. Normally AST is found in minimal amounts within the blood, however when the organs mentioned above are damaged, AST is released into the blood<ref name ="TISSUE DAMAGE"/>. The amount released is proportional to the level of damage sustained<ref name ="TISSUE DAMAGE"/>. AST levels have been shown to rise substantially within 6 hours of the initial tissue degradation and can stay elevated for up to 4 days<ref name ="TISSUE DAMAGE"/>. AST levels when compared with the levels of other enzymes can be used by physicians to determine where in the body the damage has taken place<ref name ="Liver damage"/>. Comparisons with ALT have proven particularly useful in identifying liver damage such as cirrhosis and hepatitis<ref name ="Liver damage"/>. Under normal condition, AST levels within men are 6-34 IU/L and for women it is 8-40 IU/L<ref name ="TISSUE DAMAGE"/>.