The Kinetic Mechanism of the Human Bifunctional Enzyme ATIC (5-Amino-4-imidazolecarboxamide Ribonucleotide Transformylase/Inosine 5′-Monophosphate Cyclohydrolase)

Abstract

) is ϳ2-3-fold faster than the forward rate (2.9 s ؊1 ), whereas the cyclohydrolase reaction is essentially unidirectional in the forward sense. The cyclohydrolase reaction thus draws the overall bifunctional reaction toward the production of inosine monophosphate. 3) There was no kinetic evidence of substrate channeling of the intermediate, the formylaminoimidazole carboxamide ribonucleotide, between the formyltransferase and the cyclohydrolase active sites.5-Amino-4-imidazolecarboxamide ribonucleotide transformylase/inosine monophosphate cyclohydrolase (ATIC) 1 is a bifunctional enzyme that catalyzes the penultimate and final steps in the de novo purine nucleotide biosynthetic pathway. AICAR formyl transferase (AICARFT) catalyzes the transfer of the formyl group from a reduced folate cofactor, N 10

Crystal Structures of Human Bifunctional Enzyme Aminoimidazole-4-carboxamide Ribonucleotide Transformylase/IMP Cyclohydrolase in Complex with Potent Sulfonyl-containing Antifolates

“…1B) in which the transformylase and cyclohydrolase active sites were separated by ϳ50 Å. However, no evidence of channeling or tunneling of 5-formyl-AICAR between the two domains has been forthcoming (5,8), although strong electropositive patches that connect the two active sites may sequester 5-formyl-AICAR to the vicinity of the binding sites.…”

Interdomain Communications in Bifunctional Enzymes: How Are Different Activities Coordinated?

“…At the same time, there is mounting evidence that substrate channeling is not a common property of bifunctional enzymes catalyzing sequential reactions of a metabolic pathway (3,4,23). These findings suggest that bifunctionality has other potential advantages.…”

“…In vitro, different salts stimulate the LKR activity of this bifunctional enzyme isolated from rice and maize (27,28). To investigate the structural basis of these effects, different forms of the enzyme were prepared (26), including: [1] Native enzyme (LKR-LR-SDH, the two domains connected by the linker); [2] Isolated N-terminal domain (LKR); [3] …”

“…In many cases, such enzymes contain two different active centers located within separate domains linked on a single polypeptide and encoded by a single gene. Three types of bifunctional enzyme are known: those catalyzing consecutive reactions of a metabolic pathway (1)(2)(3)(4); those which catalyze two non-consecutive reactions (5), and those catalyzing opposing reactions (6). The role played by specific interdomain interactions in the functioning of different active centers within a macromolecule presents a problem of considerable interest.…”

“…Another goal is to discuss the information available on the structural basis of communication between active centers that does not involve substrate channeling. Such is the case of a number of bifunctional enzymes catalyzing consecutive reactions (3,4). Lysine-ketoglutarate reductase/saccharopine dehydrogenase was chosen to demonstrate the regulatory role of interdomain interactions in bifunctional enzymes of this type.…”