Zinc-Independent Folate Biosynthesis: Genetic, Biochemical, and Structural Investigations Reveal New Metal Dependence for GTP Cyclohydrolase IB

Abstract

GTP cyclohydrolase I (GCYH-I) is an essential Zn2؉ -dependent enzyme that catalyzes the first step of the de novo folate biosynthetic pathway in bacteria and plants, the 7-deazapurine biosynthetic pathway in Bacteria and Archaea, and the biopterin pathway in mammals. We recently reported the discovery of a new prokaryoticspecific GCYH-I (GCYH-IB) that displays no sequence identity to the canonical enzyme and is present in ϳ25% of bacteria, the majority of which lack the canonical GCYH-I (renamed GCYH-IA). Genomic and genetic analyses indicate that in those organisms possessing both enzymes, e.g., Bacillus subtilis, GCYH-IA and -IB are functionally redundant, but differentially expressed. Whereas GCYH-IA is constitutively expressed, GCYH-IB is expressed only under Zn 2؉ -limiting conditions. These observations are consistent with the hypothesis that GCYH-IB functions to allow folate biosynthesis during Zn 2؉ starvation. Here, we present biochemical and structural data showing that bacterial GCYH-IB, like GCYH-IA, belongs to the tunneling-fold (T-fold) superfamily. However, the GCYH-IA and -IB enzymes exhibit significant differences in global structure and activesite architecture. While GCYH-IA is a unimodular, homodecameric, Zn 2؉ -dependent enzyme, GCYH-IB is a bimodular, homotetrameric enzyme activated by a variety of divalent cations. The structure of GCYH-IB and the broad metal dependence exhibited by this enzyme further underscore the mechanistic plasticity that is emerging for the T-fold superfamily. Notably, while humans possess the canonical GCYH-IA enzyme, many clinically important human pathogens possess only the GCYH-IB enzyme, suggesting that this enzyme is a potential new molecular target for antibacterial development.The Zn 2ϩ

Characterization of the Response to Zinc Deficiency in the Cyanobacterium Anabaena sp. Strain PCC 7120

“…Thus, the derepression of all4725 under zinc limitation would probably lead to the replacement of a zinc-dependent porphobilinogen synthase (All4380) by a zinc-independent one (All4725), under conditions in which the former is most probably inactive. Replacement of zinc-dependent proteins by zinc-independent isoforms seems to be a common strategy in a variety of prokaryotes (5,16,21,31,50).…”

Bacterial Strategies to Maintain Zinc Metallostasis at the Host-Pathogen Interface

“…Enzymes involved in this process include threonyl-tRNA synthetase, GTP cyclohydrolase I (FolE), porphobilinogen synthase (PBGS), and DksA (81)(82)(83)(84). In the case of the GTP cyclohydrolases, the Zn-requiring (Ia) enzyme is a better catalyst with a very high affinity for Zn; the 1b enzyme, on the other hand, has modest affinity for many divalent metal ions, each of which support variable levels of activity (82). In this case, Zn sparing is clearly a "workaround" to access a crucial metabolic process, i.e.…”

Structural Basis of Biological Nitrile Reduction

“…Gel filtration studies of an enzyme from each subfamily, the B. subtilis and E. coli QueF, were consistent with homododecameric and dimer structures for the unimodular and bimodular enzymes, respectively (5). The latter result was surprising as it implies that the enzyme lacks the canonical T-fold quaternary architecture of all other bimodular T-fold enzymes (9,10), but recent crystallographic analysis of the bimodular enzyme from Vibrio cholerae also revealed a homodimer structure for this subfamily (11), confirming that this enzyme does lack the canonical quaternary structure of the superfamily.…”