APRT

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Introductions

APRT is a purine remediation enzyme that recovers 5'- adenosine from adenine. In APRT deficiency, adenine is replaced by xanthine dehydrogenase (also known as xanthine oxidase) into DHA, which is cleared by the kidneys but is largely insoluble in the urine and deposited in the renal tubules, leading to obstructive nephropathy and crystalloid nephropathy.

Structure

APRT is a homodimer with 179 amino acid residues per monomer. Residues A131, L159, V25 and R27 are important for purine specificity in human APRT. Catalytic site of APRT with reactants adenine and PRPP resolved. The Hood is believed to be important for purine specificity, while the flexible loop is thought to contain the molecules within the active site. The core is highly conserved in many APRT. The hood containing the adenine binding site has more variability in the enzyme family. A13-residue motif contains the PRPP-binding region and involves two adjacent acidic residues and at least one surrounding hydrophobic residue.

Figure 1. Structure of APRT.

Function

APRT catalyze Mg²⁺-dependent reactions that convert purine bases to the corresponding nucleotides. They are found in a variety of organisms, including plants, mammals, and parasitic protozoa. APRT transports the ATP produced by oxidative phosphorylation in the mitochondrial matrix to the mitochondria via the inner membrane, and then transports the ADF from the mitochondria to the mitochondria. Therefore, APRT not only couples the energy production process of mitochondria to the energy consumption process of cells, but also affects the energy production of tissue cells, which in turn affects the function of tissue cells.

Mechanism

APRT proceeds by a double-ordered sequential mechanism involving the formation of a ternary complex. The enzyme binds first to PRPP and then to adenine. After phosphoribosyl transfer occurs, pyrophosphate leaves first, followed by AMP. kinetic studies have shown that phosphoribosyl transfer is relatively fast, while product release (especially of AMP) is rate-limiting. In human APRT, the N9 proton of adenine is extracted by Glu104 to form the oxygen-carbon transition state. This acts as a nucleophilic reagent, attacking the heterocapsid carbon of PRPP, forming AMP and displacing pyrophosphate from PRPP. the mechanism of APRT is generally consistent with that of other APRT, which retain the ability to displace α-1-pyrophosphate from PRPP using a nitrogen nucleophilic reagent in SN1 or SN2 attack.

Diseases

APRT deficiency (APRTD) is a rare autosomal recessive metabolic disorder due to a mutation of the APRT gene. APRT is a purine-metabolism enzyme that catalyzes the formation of 5'-adenosine monophosphate (5'-AMP) and pyrophosphate (PP) from adenine and 5-phosphoribosyl-1-pyrophosphate. In patients with complete APRT deficiency, adenine is oxidized by xanthine oxidase (XO) to the highly insoluble and nephrotoxic derivative 2,8-dihydroxyadenine (2,8-DHA), leading to urolithiasis and renal failure caused by intratubular crystalline precipitation. The APRT gene, located on chromosome 16q24, is approximately 2.6 kb long, contains five exons and four introns, and encodes a protein of 180 amino acid residues. The human enzyme, present in a variety of cell types including erythrocyte, is a homodimer composed of two identical subunit species with a molecular weight of about 19.481 Da. Currently, there are two isoforms produced by alternative splicing: the isoform 1 (P07741-1) and the isoform 2 (P07741-2); the isoform 1 has been considered as the 'canonical' one.