The Study of Carbamoyl Phosphate Synthetase 1 Deficiency Sheds Light on the Mechanism for Switching On/Off the Urea Cycle

Abstract

Carbamoyl phosphate synthetase 1 (CPS1) deficiency (CPS1D) is an inborn error of the urea cycle having autosomal (2q34) recessive inheritance that can cause hyperammonemia and neonatal death or mental retardation. We analyzed the effects on CPS1 activity, kinetic parameters and enzyme stability of missense mutations reported in patients with CPS1 deficiency that map in the 20-kDa C-terminal domain of the enzyme. This domain turns on or off the enzyme depending on whether the essential allosteric activator of CPS1, N-acetyl-L-glutamate (NAG), is bound or is not bound to it. To carry out the present studies, we exploited a novel system that allows the expression in vitro and the purification of human CPS1, thus permitting site-directed mutagenesis. These studies have clarified disease causation by individual mutations, identifying functionally important residues, and revealing that a number of mutations decrease the affinity of the enzyme for NAG. Patients with NAG affinity-decreasing mutations might benefit from NAG site saturation therapy with N-carbamyl-L-glutamate (a registered drug, the analog of NAG). Our results, together with additional present and prior site-directed mutagenesis data for other residues mapping in this domain, suggest an NAG-triggered conformational change in the β4-α4 loop of the C-terminal domain of this enzyme. This change might be an early event in the NAG activation process. Molecular dynamics simulations that were restrained according to the observed effects of the mutations are consistent with this hypothesis, providing further backing for this structurally plausible signaling mechanism by which NAG could trigger urea cycle activation via CPS1.

Neonatal-onset carbamoyl phosphate synthetase I deficiency

“…Just as other literatures showed that most cases of CPS1D are sporadic. [1–3] Brain MRI data, which were limited in other case reports, were recorded in this case. Meanwhile, a new mutation founded in this case had not previously been described.…”

“…The morbidity rate is 1/50,000 to 1/300,000. [1,2] The CSP1D could be onset in different ages. The normal procedure of urea cycle is interrupted as a result of the lack of carbamoyl phosphate synthetase I (CPS1).…”

“…[3–5] It is easy to be misdiagnosed because of atypical symptoms, sudden onset, rapid progression and low morbidity, especially the neonatal-onset types. [1,2] Thus, clinical data, especially the brain magnetic resonance imaging (MRI) was often limited. We present a case of neonatal-onset CSP1D, with nonspecific clinical manifestations and a rapid progression.…”

Amino acid homeostasis and signalling in mammalian cells and organisms

“…The most well-known examples in mammalian cells are carbamoylphosphate synthetase (CPS1) [45] and glutamate dehydrogenase (GDH) [46]. CPS1 controls the entry of amino groups into the urea cycle and adjusts urea production to the prevalence of glutamate and arginine in the liver [47].…”

Precision medicine in rare disease: Mechanisms of disparate effects of N -carbamyl- l -glutamate on mutant CPS1 enzymes

“…The newly developed baculovirus/insect cell expression system, which can produce a large amount of the recombinant CPS1 with desired mutations, provides a useful tool to evaluate the residual activity, protein yield and stability of CPS1 mutants. This tool has been successfully used to study the disease-causing roles and pathogenic mechanism of various mutations in CPS1 [36,38,51–54]. …”