The role of substrate specificity and metal binding in defining the activity and structure of an intracellular subtilisin.

The dimeric intracellular subtilisin proteases (ISPs) found throughout Gram-positive bacteria are a structurally distinct class of the subtilase family. Unlike the vast majority of subtilisin-like proteases, the ISPs function exclusively within the cell, contributing the majority of observed cellular proteolytic activity. Given that they are active within the cell, little is known about substrate specificity and the role of stress signals such as divalent metal ions in modulating ISP function. We demonstrate that both play roles in defining the proteolytic activity of Bacillus clausii ISP and propose the molecular basis of their effects. Enzyme kinetics reveal that one particular synthetic tetrapeptide substrate, Phe-Ala-Ala-Phe-pNA, is hydrolysed with a catalytic efficiency ∼100-fold higher than any other tested. Heat-denatured whole proteins were found to be better substrates for ISP than the native forms. Substrate binding simulations suggest that the S1, S2 and S4 sites form defined binding pockets. The deep S1 cavity and wide S4 site are fully occupied by the hydrophobic aromatic side-chains of Phe. Divalent metal ions, probably Ca(2+), are proposed to be important for ISP activity through structural changes. The presence of >0.01 mM EDTA inactivates ISP, with CD and SEC suggesting that the protein becomes less structured and potentially monomeric. Removal of Ca(2+) at sites close to the dimer interface and the S1 pocket are thought to be responsible for the effect. These studies provide a new insight into the potential physiological function of ISPs, by reconciling substrate specificity and divalent metal binding to associate ISP with the unfolded protein response under stress conditions.

Regulation of an intracellular subtilisin protease activity by a short propeptide sequence through an original combined dual mechanism.

A distinct class of the biologically important subtilisin family of serine proteases functions exclusively within the cell and forms a major component of the bacilli degradome. However, the mode and mechanism of posttranslational regulation of intracellular protease activity are unknown. Here we describe the role played by a short N-terminal extension prosequence novel amongst the subtilisins that regulates intracellular subtilisin protease (ISP) activity through two distinct modes: active site blocking and catalytic triad rearrangement. The full-length proenzyme (proISP) is inactive until specific proteolytic processing removes the first 18 amino acids that comprise the N-terminal extension, with processing appearing to be performed by ISP itself. A synthetic peptide corresponding to the N-terminal extension behaves as a mixed noncompetitive inhibitor of active ISP with a K(i) of 1 µM. The structure of the processed form has been determined at 2.6 Å resolution and compared with that of the full-length protein, in which the N-terminal extension binds back over the active site. Unique to ISP, a conserved proline introduces a backbone kink that shifts the scissile bond beyond reach of the catalytic serine and in addition the catalytic triad is disrupted. In the processed form, access to the active site is unblocked by removal of the N-terminal extension and the catalytic triad rearranges to a functional conformation. These studies provide a new molecular insight concerning the mechanisms by which subtilisins and protease activity as a whole, especially within the confines of a cell, can be regulated.

Crystal structure of an intracellular subtilisin reveals novel structural features unique to this subtilisin family.

The intracellular subtilisin proteases (ISPs) are the only known members of the important and ubiquitous subtilisin family that function exclusively within the cell, constituting a major component of the degradome in many Gram-positive bacteria. The first ISP structure reported herein at a spacing of 1.56 A reveals features unique among subtilisins that has enabled potential functional and physiological roles to be assigned to sequence elements exclusive to the ISPs. Unlike all other subtilisins, ISP from B. clausii is dimeric, with residues from the C terminus making a major contribution to the dimer interface by crossing over to contact the partner subunit. A short N-terminal extension binds back across the active site to provide a potential novel regulatory mechanism of intrinsic proteolytic activity: a proline residue conserved throughout the ISPs introduces a kink in the polypeptide backbone that lifts the target peptide bond out of reach of the catalytic residues.

Safety profile of a nicotine lozenge compared with that of nicotine gum in adult smokers with underlying medical conditions: a 12-week, randomized, open-label study.

BACKGROUND: Nicotine polacrilex lozenges deliver 25% to 27% more nicotine compared with equivalent doses of nicotine polacrilex gum. The increased nicotine exposure from the lozenge has raised questions about the relative safety of the lozenge and gum. OBJECTIVE: The objective of this study was to compare the safety profiles of the 4-mg nicotine lozenge and 4-mg nicotine gum in smokers with selected label-restricted diseases. METHODS: This was a multicenter, randomized, open-label study in adult smokers with heart disease, hypertension not controlled by medication, and/or diabetes mellitus. Patients were randomized in a 1:1 ratio to receive the 4-mg nicotine lozenge or 4-mg nicotine gum. Safety assessments were made at baseline and at 2, 4, 6, and 12 weeks after the start of product use. RESULTS: Nine hundred one patients were randomized to treatment, 447 who received the lozenge and 454 who received the gum (safety population). The majority were women (52.7%). Patients' mean age was 53.9 years, their mean weight was 193.9 pounds, and they smoked a mean of 25.2 cigarettes per day at baseline. Five hundred fifty-three patients, 264 taking the lozenge and 289 taking the gum, used the study product for > or =4 days per week during the first 2 weeks (evaluable population). The nicotine lozenge and nicotine gum were equally well tolerated, despite increased nicotine exposure from the lozenge. The incidence of adverse events in the 2 groups was similar during the first 2 weeks of product use (evaluation population: 55.3% lozenge, 54.7% gum), as well as during the entire study (safety population: 63.8% and 58.6%, respectively). Stratification of patients by sex, age, extent of concurrent smoking, extent of product use, and severity of adverse events revealed no clinically significant differences between the lozenge and gum. The most common adverse events were nausea (17.2% and 16.1%; 95% CI, -3.7 to 6.0), hiccups (10.7% and 6.6%; 95% CI, 0.5 to 7.8), and headache (8.7% and 9.9%; 95% Cl, -5.0 to 2.6). Serious adverse events were reported in 11 and 13 patients in the respective groups. Fewer than 6% of patients in either group were considered by the investigator to have a worsening of their overall disease condition during the study. The majority of patients (>60%) experienced no change in their disease status from baseline. CONCLUSION: The 4-mg nicotine lozenge and 4-mg nicotine gum had comparable safety profiles in these patients with label-restricted medical conditions.