This enzyme belongs to the family of isomerases, specifically those intramolecular oxidoreductases interconverting aldoses and ketoses. The systematic name of this enzyme class is N-anthranilate aldose-ketose-isomerase. Other names in common use include:
PRA isomerase,
PRAI,
IGPS:PRAI, and
N-anthranilate ketol-isomerase.
xPRAI
PRAI
Reaction
Phosphoribosylanthranilate isomerase is one of the many enzymes within the biosynthesis pathway of tryptophan. The upstream* pathway substrates and intermediates are shown below. As seen in Fig. 3, N--anthranilate via this enzyme is converted into 1--1-deoxribulose 5-phosphate. As the name phosphoribosylanthranilate isomerase suggests, it functions as an isomerase, rearranging the parts of the molecule without adding or removing molecules or atoms. The reaction seen in Fig. 3, is an intramolecular redox reaction. Its first step involves a proton transfer. This product intermediate, an enolamine, is fluorescent, which is useful for kinetic studies within this pathway. However, this product is unstable, and quickly isomerases into an α-amino keto.
Note: Upstream/Downstream are relative to the compounds/molecules directly involved in phosphoribosylanthranilate isomerase reaction
Kinetics
data, is given in the table below for PRAI and indole-glycerol-phosphate synthase.
Enzyme
Temperature
Km
kcat
tPRAI
25
0.280
3.7
45
0.390
13.5
60
0.730
38.5
80
1.030
116.8
tIGPS
25
0.006
0.11
45
0.014
0.75
60
0.053
3.24
80
0.123
15.4
Structure
Depending on the microorganism PRAI's structure can vary between a mono-functional enzyme or a stable bi-functional dimeric enzyme. Within Saccharomyces cerevisiae, Bacillus subtilis, Pseudomonas putida, and Acinetobacter calcoaceticus the enzyme is monmeric. In contrast, in hyperthermophileThermotoga maritima,Escherichia coli, Salmonella typhimurium, and Aerobacter aerogenes, and Serratia marcescens, it is a bi-functional enzyme with indoleglycerol phosphate synthase as the paired enzyme. The crystal structure has been characterized for a variety of the above listed microorganisms. The known 2.0 A structure of PRAI from Pyrococcus furiosus shows that tPRAI has a TIM-barrel fold. PRAI derived from Thermococcus kodakaraensis also expresses a similar TIM-barrel fold structure. The subunits of tPRAI associate via the N-terminal faces of their central beta-barrels. Two long, symmetry-related loops that protrude reciprocally into cavities of the other subunit provide for multiple hydrophobic interactions. Moreover, the side chains of the N-terminal methionines and the C-terminal leucines of both subunits are immobilized in a hydrophobic cluster, and the number of salt bridges is increased in tPRAI. These features appear to be mainly responsible for the high thermostability of tPRAI. The bi-functional version of this enzyme isolated from E. Coli performs two steps within the Tryptophan pathway. Referencing Fig. 7, the N-terminal catalyzes the IGPS reaction, and the C-terminal domain performs the PRAI reaction. Although these domains overlap, the active sites are not overlapping, and studies have shown that mono-functional enzymes composing of these two domains are still able to produce a functional tryptophan bio-synthetic pathway. The βα loops are responsible for the activity of this enzyme, and the αβ loops are involved in the protein's stability. More details on the discovery of this enzyme's structure can be found in Willmann's paper.
Active site
Specifically, for phosphoribosyl anthranilate isomerase, TkTrpF, from Thermococcus kodakaraensis. The active site for the Amadori rearrangement, was determined to be Cys8 and Asp135.
Inhibitors
An enzyme inhibitor is molecule that binds to an enzyme that therefore decreases the activity of the protein. The following molecules have been shown to inhibit PRAI acivity: Reduced 1-; Indoleglycerol phosphate ; Indolepropanol phosphate ; CuS04 ; More
There are homologous genes which produce this enzyme in plant species such as Arabidopsis thaliana and Oryza sativa. One form of bacterium it is found in Thermotoga maritima. Phosphoribosylanthranilate isomerase is also found in various forms of fungi such as Kluyveromyces lactis, Saccharomyces cerevisiae, and Ashbya gossypii. A list of genes encoding for PRAI can also be found on KEGG Enzyme database.
