Investigating the interaction between GTP-cyclohydrolase1 and its feedback regulatory protein

Abstract

GTP-cyclohydrolase-1 (GTPCH1) catalyses the rate-limiting step in the biosynthesis of tetrahydrobiopterin (BH4), an essential cofactor for enzymes including aromatic amino acid hydroxylases and nitric oxide synthases. Strategies that increase vascular BH4 biosynthesis represent a promising therapeutic approach for the treatment of endothelial dysfunction. GTPCH1 is subject to feedback and feed-forward regulation by BH4 and L-phenylalanine (L-phe) respectively, via an allosteric protein interaction with GTPCH1 feedback regulatory protein (GFRP). The aim of this thesis was to investigate the GTPCH1-GFRP interaction using recombinant proteins, and to validate the functional significance of the interaction in the vasculature using animal models. Human GTPCH1 and GFRP proteins were recombinantly expressed. Studies compared the activity and protein interactions of native GTPCH1 with a truncated mutant. A kinetic GTPCH1 activity assay was modified to enable a high-throughput screen of a fragment library. GTPCH1-GFRP interactions were assessed using surface plasmon resonance (SPR). Finally, in-vivo and ex-vivo functional studies in rodents investigated the effects of L-phe on vascular function and BH4 levels. Studies using recombinant proteins revealed the activity of truncated GTPCH1 exceeds that of native GTPCH1. A high-throughput screen successfully identified four compounds that modulate GTPCH1 activity. Biophysical analysis (SPR) demonstrated that both native and truncated GTPCH1 bind to GFRP in the absence of natural ligands. The kinetics and binding rate constants have been reported for the first time. In functional studies, oral L-phe supplementation in rodents led to a rise of BH4 levels within aortic tissue, and reversed vascular dysfunction observed in vessels obtained from spontaneously hypertensive rats. This thesis demonstrates that modulation of the GTPCH1-GFRP interactions represents a novel therapeutic target to regulate endogenous BH4 levels. SPR data suggests that GTPCH1 and 2 GFRP are constitutively bound in-vivo and indicate that the N-terminal region of GTPCH1 may directly interact with GFRP and modulate basal enzyme activity. The functional effects of L-phe on vascular BH4 levels and function, validate the potential of the GTPCH1-GFRP pathway as a therapeutic target for cardiovascular disease. 3

l-Phenylalanine Restores Vascular Function in Spontaneously Hypertensive Rats Through Activation of the GCH1-GFRP Complex

“…GCH1 binds to GFRP to form a protein complex that is receptive to allosteric regulation by both L-phe (feed forward) and BH 4 (feedback) regulation (21) . Our results have confirmed numerous previous reports that L-phe only enhances GCH1 activity when it is bound to GFRP 24 , 30 .…”

“…As previously described 24 , 30 , the combination of purified recombinant GFRP with GCH1 protein had a higher basal activity than GCH1 alone in vitro. The addition of L-phe (2 mmol·l −1 ) had no effect on purified GCH1 activity alone but caused a significant rise in GCH1 activity when coincubated with GFRP, confirming that L-phe is an allosteric regulator of the GCH1-GFRP complex only ( Figure 1A ).…”

“…A kinetic microplate assay was used to determine the effects of L-phe (1 mmol·l −1 ) on the activity of recombinantly expressed human GCH1 (0.1 μmol·l −1 ) either alone or when coincubated with recombinantly expressed human GFRP (1 μmol·l −1 ) (24) . This assay measures the accumulation of the intermediate reaction product, dihydroneopterin triphosphate (H 2 NTP), by monitoring an increase in A 340 over time (30) ( Supplemental Methods ).…”

“…Additionally, the primary source of BH 4 appears to be derived from GCH1 localized within the vascular endothelium, and GFRP is coexpressed within these cells 28 , 29 . Finally, oral L-phe elicits a rise in vascular BH 4 —an effect that is absent in mice lacking endothelial GCH1 (24) .…”

“…This GCH1-GFRP complex is operative in humans because oral challenge with L-phe leads to a 3-fold rise in plasma biopterin levels (a correlate of BH 4 )—an effect that is attenuated in patients with a loss-of-function GCH1 mutation (23) . Targeting endogenous BH 4 biosynthesis, by activating the GCH1-GFRP axis pharmacologically, thus represents a method to enhance vascular BH 4 levels at the level of the endothelium, circumventing the poor bioavailability following oral BH 4 administration 14 , 24 .…”