Ribose-5-phosphate isomerase

RpiA in human beings is encoded on the second chromosome on the short arm (p arm) at position 11.2. Its encoding sequence is nearly 60,000 base pairs long.[1] The only known naturally occurring genetic mutation results in ribose-5-phosphate isomerase deficiency, discussed below. The enzyme is thought to have been present for most of evolutionary history. Knock-out experiments conducted on the genes of various species meant to encode RpiA have indicated similar conserved residues and structural motifs, indicating ancient origins of the gene. [2]

A structural diagram of the enzyme ribose-5-phosphate isomerase by Zhang, et al.

Rpi exists as two distinct proteins, termed RpiA and RpiB. Although RpiA and RpiB catalyze the same reaction, they show no sequence or overall structural homology. According to Jung et al.,[3] an assessment of RpiA using SDS-PAGE shows that the enzyme is a homodimer of 25 kDa subunits. The molecular mass of the RpiA dimer was found to be 49 kDa [3] by gel filtration. Recently, the crystal structure of RpiA was determined. (please see http://www3.interscience.wiley.com/cgi-bin/fulltext/97516673/PDFSTAR)

Due to its role in the pentose phosphate pathway and the Calvin cycle, RpiA is highly conserved in most organisms, such as bacteria, plants, and animals. RpiA plays an essential role in the metabolism of plants and animals, as it is involved in the Calvin cycle which takes place in plants, and the pentose phosphate pathway which takes place in plants as well as animals.

All orthologs of the enzyme maintain an asymmetric tetramer quaternary structure with a cleft containing the active site. Each subunit consists of a five stranded β-sheet. These β-sheets are surrounded on both sides by α-helices.[4] This αβα motif is not uncommon in other proteins, suggesting possible homology with other enzymes.[5] The separate molecules of the enzyme are held together by highly polar contacts on the external surfaces of the monomers. It is presumed that the active site is located where multiple β-sheet C termini come together in the enzymatic cleft. This cleft is capable of closing upon recognition of the phosphate on the pentose (or an appropriate phosphate inhibitor). The active site is known to contain conserved residues equivalent to the E. coli residues Asp81, Asp84, and Lys94. These are directly involved in catalysis.[6]

In the non-oxidative part of the pentose phosphate pathway, RPIA converts Ru5P to R5P which then is converted by ribulose-phosphate 3-epimerase to xylulose-5-phosphate (figure 3).[11] The end result of the reaction essentially is the conversion of the pentose phosphates to intermediates used in the glycolytic pathway. In the oxidative part of the pentose phosphate pathway, RpiA converts Ru5P to the final product, R5P through the isomerization reaction (figure 3). The oxidative branch of the pathway is a major source for NADPH which is needed for biosynthetic reactions and protection against reactive oxygen species.[12]

In the Calvin cycle, the energy from the electron carriers is used in carbon fixation, the conversion of carbon dioxide and water into carbohydrates. RPIA is essential in the cycle, as Ru5P generated from R5P is subsequently converted to ribulose-1,5-bisphosphate (RuBP), the acceptor of carbon dioxide in the first dark reaction of photosynthesis (Figure 3).[13] The direct product of RuBP carboxylase reaction is glyceraldehyde-3-phosphate; these are subsequently used to make larger carbohydrates.[14] Glyceraldehyde-3-phosphate is converted to glucose which is later converted by the plant to storage forms (e.g., starch or cellulose) or used for energy.[15]

Human ribose-5-phosphate isomerase A (RpiA) plays a role in human hepatocellular carcinoma (HCC).[18] A significant increase in RpiA expression was detected both in tumor biopsies of HCC patients and in a liver cancer tissue array. Importantly, the clinicopathological analysis indicated that RpiA mRNA levels were highly correlated with clinical stage, grade, tumor size, types, invasion and alpha-fetoprotein levels in the HCC patients. In addition, the ability of RpiA to regulate cell proliferation and colony formation in different liver cancer cell lines required ERK signaling as well as the negative modulation of PP2A activity and that the effects of RpiA could be modulated by the addition of either a PP2A inhibitor or activator. It suggests that RpiA overexpression can induce oncogenesis in HCC.[19]

RpiA generated attention when the enzyme was found to play an essential role in the pathogenesis of the parasite Plasmodium falciparum, the causative agent of malaria. Plasmodium cells have a critical need for a large supply of the reducing power of NADPH via PPP in order to support their rapid growth. The need for NADPH is also required to detoxify heme, the product of hemoglobin degradation.[20] Furthermore, Plasmodium has an intense requirement for nucleic acid production to support its rapid proliferation. The R5P produced via increased pentose phosphate pathway activity is used to generate 5-phospho-D-ribose α-1-pyrophosphate (PRPP) needed for nucleic acid synthesis. It has been shown that PRPP concentrations are increased 56 fold in infected erythrocytes compared with uninfected erythrocytes.[17] Hence, designing drugs that target RpiA in Plasmodium falciparum could have therapeutic potential for patients that suffer from malaria.