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Function(s) and Biological Relevance

(IMPDH) is the enzyme that catalyzes the rate-limiting step in the de novo guanine nucleotide biosynthetic pathway, converting inosine monophosphate (IMP) to xanthosine monophosphate (XMP) with the reduction of nicotinamide adenine dinucleotide (NAD). Organisms that undergo the purine nucleotide biosynthetic pathway have IMPDH, including humans. This particular form of IMPDH described and highlighted comes from the recently studied fungus Ashbya gossypii ^[1]. Additional ligands of IMPDH include nicotinamide adenine dinucleotide (NAD), acetate (ACT), and guanosine-5’ monophosphate (5GP). IMPDH is a regulator of the intracellular guanine nucleotide pool amount and helps control control cell division and proliferation, and therefore related to malignant transformation, tumor cell proliferation, and intracellular and extracellular pathogenic infections. Purine dinucleoside polyphosphates are found to bind to the Bateman domain of Ashbya gossypii IMPDH to allosterically regulate the catalytic activity by competing against purine mononucleotides^[2].

Broader Implications

The disease pathophysiology is extensive as the improper IMPDH regulation may lead to uncontrolled cell division and proliferation, affecting the immune system's ability to fight off pathogens and signal tumor cells to apoptose. Therapeutic studies using dinucleoside polyphosphates may allosterically regulate the inhibition of IMPDH activity. Dinucleoside polyphosphates have physiological functions including cell division, neurotransmission, apoptosis, vasoconstriction, platelet aggregation, and cellular process variety enhancement from DNA replication to repair^[3]. Selected IMPDH inhibitors composed of dinucleoside polyphosphates may be used to make IMPDHs targets for immunosuppressive, antiviral, and anticancer drugs^[4], with antibacterial and chemotherapeutic strategies feasibile^[5].

Structural Highlights and Structure-Function Relationships

The shows alpha helices (pink) and beta sheets (orange). The Ashbya gossypii IMPDH is 31% helical and 15% beta sheet, which the other percentages including random coils and residue structures. The active site is located towards the C-terminus within the TIM barrel, containing 8 alpha-helices and 8 beta sheets.

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Energy Transformation

IMPDH is activated by monovalent cations, such as K+ and Na+, within the triose-phosphate isomerase (TIM) barrel. The Arg320-Asn306-Asp272 catalytic triad (Arg322-Asn303-Asp274 in human type II IMPDH) works inter-dependently and synergistically in the TIM barrel active site to make the nucleophilic component, Cysteine, highly reactive to form a temporary covalent bond with the substrate^[6]. Substrates bind randomly to IMPDH as the hydride transfer is quick and NAD is reduced to hydrolyze the covalent intermediate within the enzyme-substrate complex. A covalent intermediate, E-XMP*, is formed, which decreases the energy needed in later nucleophilic and covalent catalysis steps^[7]. Based on normal physiological conditions, the IMPDH mechanism is often not kinetically favorable. The Bateman domains within the TIM barrel are composed of cystathionine beta-synthase motifs that perceive metal ion concentration, cellular energy status, and ionic strength; and will allosterically regulate IMPDH activity^[8]. Eukaryotic IMPDHs have three nucleotide-binding sites in the Bateman domain that allosterically modulate catalytic activity. These three nucleotide-binding sites bind adenine/guanine dinucleoside polyphosphates, and the affinity for these sites increases for these dinucleoside polyphosphates as the activity of IMPDH increases. Purine dinucleoside polyphosphates compete with purine mononucleotides within these sites, so the Bateman domain sites make IMPDH more sensitive to inhibition^[9]. Enzyme catalysis is able to finish with the energy need reduced as the covalent bond is broken later in the reaction to regenerate the enzyme.