Predicted deficiency of the GTP cyclohydrolase I feedback regulatory protein (GFRP): a disorder of dynamic tetrahydrobiopterin homeostasis.

Tetrahydrobiopterin (BH4) disorders are defined by pathogenic variants affecting enzymes of cofactor synthesis or regeneration. The GTP cyclohydrolase I feedback regulatory protein (GFRP), encoded by GCHFR, occupies a different position: it does not synthesize BH4 but regulates GTP cyclohydrolase I (GTPCH), the first and rate-limiting enzyme of the pathway. Phenylalanine-bound GFRP reduces the positive cooperativity of GTPCH toward GTP and increases activity at subsaturating GTP, whereas BH4 binds allosterically to GTPCH and GFRP stabilizes and sensitizes the inhibitory state. Because GTPCH remains catalytically active and intrinsically allosteric without GFRP, complete GFRP deficiency is predicted to preserve biosynthetic capacity while disrupting dynamic regulation. The proposed state combines basal derepression, caused by reduced physiological sensitivity to BH4 feedback, with impaired inducibility, caused by loss of phenylalanine-dependent stimulation. Fasting phenylalanine and pterins may therefore be normal or increased, and newborn screening may be unremarkable. The strongest predicted biochemical feature is an adequate rise in phenylalanine accompanied by an absent or disproportionately small rise in BH4 or total biopterin. Tissue consequences are expected to vary with GTPCH abundance, GTP availability, phosphorylation, downstream synthetic capacity and redox state. This review defines predicted GCHFR variant classes, proposes a diagnostic and functional-validation framework, identifies decisive null-model and rescue experiments, and presents explicit falsification criteria together with a synthetic illustrative case.