Sandbox Reserved 702: Difference between revisions

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The helical domain of the edema factor interacts with the adenylyl cyclase domain and switch C in the absence of calmodulin. This locks the enzyme in an inactive state. The binding of calmodulin on four discrete regions dicrupts most of those interaction allowing the enzyme to switch in an active form. This takes place in two steps.

The calcium-free, closed N-terminal domain of calmodulin binds to the edema factor thanks to an interaction with its helical domain. This interaction is due to hydrogen bonds and a salt bridge between helices I and II of the N-terminal domain of calmodulin and helices L and M of the helical domain of the edema factor. When the N-terminal domain is noud to the helical domain, the calcium-loaded C-terminal domain in its open conformation inserts between the helical domain and the catalytic core. This allows a conformational change of switch C that will stabilize the catalytic loop (switch B) of the enzyme in an active state. A rigid-body rotation of CB relative to CA also occurs. This changes the pocket formed between these two domains and allows the interaction of the edema factor with the phospates of ATP. More precisely, lysines of the active site (K346, K353 and K372) are moved upwards. Indeed, the edema factor helical domain undergoes a 15 Å translation and a 30° rotation away from the catalytic core.

The calcium-free, closed N-terminal domain of calmodulin binds to the edema factor thanks to an interaction with its helical domain. This interaction is due to hydrogen bonds and a salt bridge between helices I and II of the N-terminal domain of calmodulin and helices L and M of the helical domain of the edema factor. When the N-terminal domain is noud to the helical domain, the calcium-loaded C-terminal domain in its open conformation inserts between the helical domain and the catalytic core. This allows a conformational change of switch C that will stabilize the catalytic loop (switch B) of the enzyme in an active state. A rigid-body rotation of CB relative to CA also occurs. This changes the pocket formed between these two domains and allows the interaction of the edema factor with the phospates of ATP. More precisely, basic lysines of the active site (K346, K353 and K372) are moved upwards. Indeed, the edema factor helical domain undergoes a 15 Å translation and a 30° rotation away from the catalytic core.

=== Two magnesium ions are coordinated by the catalytic site ===

Adenylyl cyclase activity of the edema factor requires two magnesium ions. One of them is coordinated by two aspartic residues (D491 and D493) and the histidine 577. The other one is coordianted by the non-esterified oxygens of α, β and γ phosphates of ATP and by aspartic D493. The distance between these two magnesium ions is about 4.5 Å. One ion deprotonates the 3'OH of ATP whereas the other one stabilizes the penta-covalent intermediate during the transition state.

Histidine 351 also plays a key role in the adenylyl cyclase reaction. Histidine is a crucial amino acid because its pKa (6.8) is close to pH of cytoplasm. It may intervene in proton-transfer reactions. Histidine 351 has a critical role in the catalytic activity of edema factor but is not a catalytic base. Since it is about 6.0 Å away from the 3'O of cAMP, its role is to allow a water molecule to enter between them. Histidine does not act as a general base (acceptor of 3'OH proton), but is facilitates an increase in the concentration of HO- ions in the proximity of the 3'OH group.

Sandbox Reserved 702: Difference between revisions

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 The helical domain of the edema factor interacts with the adenylyl cyclase domain and switch C in the absence of calmodulin. This locks the enzyme in an inactive state. The binding of calmodulin on four discrete regions dicrupts most of those interaction allowing the enzyme to switch in an active form. This takes place in two steps.
-The calcium-free, closed N-terminal domain of calmodulin binds to the edema factor thanks to an interaction with its helical domain. This interaction is due to hydrogen bonds and a salt bridge between helices I and II of the N-terminal domain of calmodulin and helices L and M of the helical domain of the edema factor. When the N-terminal domain is noud to the helical domain, the calcium-loaded C-terminal domain in its open conformation inserts between the helical domain and the catalytic core. This allows a conformational change of switch C that will stabilize the catalytic loop (switch B) of the enzyme in an active state. A rigid-body rotation of CB relative to CA also occurs. This changes the pocket formed between these two domains and allows the interaction of the edema factor with the phospates of ATP. More precisely, lysines of the active site (K346, K353 and K372) are moved upwards. Indeed, the edema factor helical domain undergoes a 15 Å translation and a 30° rotation away from the catalytic core.
+The calcium-free, closed N-terminal domain of calmodulin binds to the edema factor thanks to an interaction with its helical domain. This interaction is due to hydrogen bonds and a salt bridge between helices I and II of the N-terminal domain of calmodulin and helices L and M of the helical domain of the edema factor. When the N-terminal domain is noud to the helical domain, the calcium-loaded C-terminal domain in its open conformation inserts between the helical domain and the catalytic core. This allows a conformational change of switch C that will stabilize the catalytic loop (switch B) of the enzyme in an active state. A rigid-body rotation of CB relative to CA also occurs. This changes the pocket formed between these two domains and allows the interaction of the edema factor with the phospates of ATP. More precisely, basic lysines of the active site (K346, K353 and K372) are moved upwards. Indeed, the edema factor helical domain undergoes a 15 Å translation and a 30° rotation away from the catalytic core.
 === Two magnesium ions are coordinated by the catalytic site ===
 Adenylyl cyclase activity of the edema factor requires two magnesium ions. One of them is coordinated by two aspartic residues (D491 and D493) and the histidine 577. The other one is coordianted by the non-esterified oxygens of α, β and γ phosphates of ATP and by aspartic D493. The distance between these two magnesium ions is about 4.5 Å. One ion deprotonates the 3'OH of ATP whereas the other one stabilizes the penta-covalent intermediate during the transition state.
+Histidine 351 also plays a key role in the adenylyl cyclase reaction. Histidine is a crucial amino acid because its pKa (6.8) is close to pH of cytoplasm. It may intervene in proton-transfer reactions. Histidine 351 has a critical role in the catalytic activity of edema factor but is not a catalytic base. Since it is about 6.0 Å away from the 3'O of cAMP, its role is to allow a water molecule to enter between them. Histidine does not act as a general base (acceptor of 3'OH proton), but is facilitates an increase in the concentration of HO- ions in the proximity of the 3'OH group.