Carbamoyl phosphate synthetase I (CPSI) deficiency, a recessively inherited error of the urea cycle, causes life-threatening hyperammonaemia. CPSI is a multidomain 1500-residue liver mitochondrial matrix protein that is allosterically activated by N-acetyl-l-glutamate, and which synthesises carbamoyl phosphate (CP) in three steps: bicarbonate phosphorylation by ATP, carbamate synthesis from carboxyphosphate and ammonia, and carbamate phosphorylation by ATP. Several missense mutations of CPSI have been reported in patients with CPSI deficiency, but the actual pathogenic potential and effects on the enzyme of these mutations remain non-characterised. Since the structure of Escherichia coli CPS is known and systems for its overexpression and purification are available, we have constructed and purified eight site-directed mutants of E.coli CPS affecting the enzyme large subunit (A126M, R169H, Q262P, N301K, P360L, V640R, R675L, S789P) that are homologous to corresponding missense mutations found in patients with CPSI deficiency, studying their stability and their ability to catalyse the CPS reaction as well as the partial reactions that reflect the different reactional steps, and analysing the substrate kinetics for the overall and partial reactions. The results show that all the mutations significantly decrease CP synthesis without completely inactivating the enzyme (as reflected in the catalysis of at least one partial reaction), that one of these mutations (Q262P) causes marked enzyme instability, and validate the use of E.coli CPS as a pathogenicity testing model for CPSI deficiency. The causality of the reported clinical mutations is supported and the derangements caused by the mutations are identified, revealing the specific roles of the residues that are mutated. In particular, the findings highlight the importance for carbamate phosphorylation and for allosteric activation of a loop that coordinates K(+), stress the key role of intersubunit interactions for CPS stability, and suggest that lid opening at both phosphorylation sites is concerted.