User:Alexander Rudecki/Sandbox 1: Difference between revisions

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~~==Function==~~

~~[[Image:Cyclase_reaction.png|thumb|500px|right|Figure 5. Cyclization of a terminal glutamine residue via DromeQC.]]~~

The N-terminus of many proteins (ie gonadotropin releasing hormone and thyrotropin-releasing hormone) contain a pyroglutamic acid (pGlu) residue<ref>PMID: 196172</ref>. A pGlu ‘cap’ protects these proteins against degradation by aminopeptidases, and influences the conformation of the hormone or its associated receptor, leading to their activation<ref name="schilling"/>. Cyclization also leads to decreased basicity in the peptide. Though cyclization of Gln-tRNA to pGlu-tRNA has been shown to occur in papaya latex,<ref>PMID: 4881333</ref> N terminal pGlu formation must be post translational due to an essential methionine that initiates translation in all organisms.

==Function==

[[Image:Cyclase_reaction.png|thumb|500px|right|Figure 5. DromeQC-mediated cyclization of a terminal glutamine residue forming pyroglutamic acid (pGlu). The leaving amine group is labelled in red.]]

The N-terminus of many proteins (ie gonadotropin releasing hormone and thyrotropin-releasing hormone) contain a pyroglutamic acid (pGlu) residue<ref>PMID: 196172</ref> (Figure 5, right). A pGlu ‘cap’ protects these proteins against degradation by aminopeptidases, and influences the conformation of the hormone or its associated receptor, leading to their activation<ref name="schilling"/>. Cyclization also leads to decreased basicity in the peptide. Though cyclization of Gln-tRNA to pGlu-tRNA has been shown to occur in papaya latex,<ref>PMID: 4881333</ref> N terminal pGlu formation must be post translational due to an essential methionine that initiates translation in all organisms.

===Catalytic Mechanism===

===Catalytic Mechanism<ref name="mechanism">PMID: 18470930</ref>===

DromeQC is ~~the enzyme~~ responsible for ~~this~~ post-translational processing of polypeptides. DromeQC catalyzes the cyclization of N-terminal glutamine, and to a lesser extent glutamate, into pyroglutamic acid (5-oxo-L-proline, or <Glu) (Figure 5). This enzyme can be categorized as follows:

DromeQC is responsible for the post-translational processing of polypeptides. DromeQC catalyzes the cyclization of N-terminal glutamine, and to a lesser extent glutamate, into pyroglutamic acid (pGlu, 5-oxo-L-proline, or <Glu) (Figure 5). This enzyme can be categorized as follows:

:-transferase (2)

Line 64:

::::-acts on glutaminyl/glutamyl residues (2.3.2.5)

The cyclization reaction occurs via a nucleophilic attack of the α-amine on the γ carbon in the glutamine side chain. The enzymatic mechanism for DromeQC is still undetermined, but it seems plausible that it follows that of its human orthologue. In hQC, the N-terminus of the peptide substrate is inserted into the active site pocket, where the γ amide oxygen chelates the catalytic zinc ion~~<ref name="mechanism">PMID: 18470930</ref>~~. This ion-dipole interaction causes carbonyl polarization, making it a better electrophile. To facilitate the reaction, a conserved glutamate (Glu201) acts as both a general base and acid. Glu201 abstracts a proton from the α-~~amino~~ group, causing it to nucleophilically attack the γ amide oxygen. This produces a tetrahedral intermediate with a charged oxygen that is stabilized by Zn<sup>2+</sup>. Glu201 then protonates the γ amide nitrogen, and an amine group is expelled as the carbonyl reforms. Also essential to this mechanism is a conserved aspartate (Asp248) that coordinates/stabilizes the leaving amine group.

The cyclization reaction occurs via a nucleophilic attack of the N-terminal α-amine on the γ carbon in the glutamine side chain. The enzymatic mechanism for DromeQC is still undetermined, but it seems plausible that it follows that of its human orthologue. In hQC, the N-terminus of the peptide substrate is inserted into the active site pocket, where the γ amide oxygen chelates the catalytic zinc ion. This ion-dipole interaction causes carbonyl polarization, making it a better electrophile. To facilitate the reaction, a conserved glutamate (Glu201) acts as both a general base and acid. Glu201 abstracts a proton from the α-amine group, causing it to nucleophilically attack the γ amide oxygen. This produces a tetrahedral intermediate with a charged oxygen that is stabilized by Zn<sup>2+</sup>. Glu201 then protonates the γ amide nitrogen, and an amine group is expelled as the carbonyl reforms. Also essential to this mechanism is a conserved aspartate (Asp248) that coordinates/stabilizes the leaving amine group.

==Localization==

DromeQC is known to localize in the brain and peripheral nerves of ''D. melanogaster''<ref name="schilling"/>. This fact was unveiled by the discovery of adipokinetic hormone; this protein has an N-terminal pGlu, supporting a post translational modification via DromeQC<ref>PMID: 2117437</ref>. Adipokinetic hormone was found to localize in ~~neurone~~ and nerve endings by immunohistochemistry, ~~suggesting that it functions~~ as ~~a locally released modulator~~ in tissues such as heart and skeletal muscle<ref>PMID: 6342796</ref>. Thus, It has been suggested that DromeQC is part of the secretory pathway, being targeted to secretory vesicles where hormone maturation takes place<ref name="schilling"/>. This claim is supported by a 27 residue signal sequence contained at the N-terminus of DromeQC<ref name="schilling"/>. Further support stems from immunohistochemical evidence that DromeQC and ~~its~~ modified substrate are excreted ~~from the cell, where they can be found in~~ the extracellular medium<ref name="schilling"/>.

DromeQC is known to localize in the brain and peripheral nerves of ''D. melanogaster''<ref name="schilling"/>. This fact was unveiled by the discovery of adipokinetic hormone; this protein has an N-terminal pGlu, supporting a post translational modification via DromeQC<ref>PMID: 2117437</ref>. Adipokinetic hormone was found to localize in neurons and nerve endings by immunohistochemistry, functioning as neuromodulator in tissues such as heart and skeletal muscle<ref>PMID: 6342796</ref>. Thus, It has been suggested that DromeQC is part of the secretory pathway, being targeted to secretory vesicles where hormone maturation takes place<ref name="schilling"/>. This claim is supported by a 27 residue signal sequence contained at the N-terminus of DromeQC<ref name="schilling"/>. Further support stems from immunohistochemical evidence that DromeQC and modified substrate are excreted into the extracellular medium<ref name="schilling"/>.

==References==

@@ Line 49: / Line 49: @@
-==Function==
-[[Image:Cyclase_reaction.png|thumb|500px|right|Figure 5. Cyclization of a terminal glutamine residue via DromeQC.]]
-The N-terminus of many proteins (ie gonadotropin releasing hormone and thyrotropin-releasing hormone) contain a pyroglutamic acid (pGlu) residue<ref>PMID: 196172</ref>. A pGlu ‘cap’ protects these proteins against degradation by aminopeptidases, and influences the conformation of the hormone or its associated receptor, leading to their activation<ref name="schilling"/>. Cyclization also leads to decreased basicity in the peptide. Though cyclization of Gln-tRNA to pGlu-tRNA has been shown to occur in papaya latex,<ref>PMID: 4881333</ref> N terminal pGlu formation must be post translational due to an essential methionine that initiates translation in all organisms.
+==Function==
+[[Image:Cyclase_reaction.png|thumb|500px|right|Figure 5. DromeQC-mediated cyclization of a terminal glutamine residue forming pyroglutamic acid (pGlu). The leaving amine group is labelled in red.]]
+The N-terminus of many proteins (ie gonadotropin releasing hormone and thyrotropin-releasing hormone) contain a pyroglutamic acid (pGlu) residue<ref>PMID: 196172</ref> (Figure 5, right). A pGlu ‘cap’ protects these proteins against degradation by aminopeptidases, and influences the conformation of the hormone or its associated receptor, leading to their activation<ref name="schilling"/>. Cyclization also leads to decreased basicity in the peptide. Though cyclization of Gln-tRNA to pGlu-tRNA has been shown to occur in papaya latex,<ref>PMID: 4881333</ref> N terminal pGlu formation must be post translational due to an essential methionine that initiates translation in all organisms.
-===Catalytic Mechanism===
+===Catalytic Mechanism<ref name="mechanism">PMID: 18470930</ref>===
-DromeQC is the enzyme responsible for this post-translational processing of polypeptides. DromeQC catalyzes the cyclization of N-terminal glutamine, and to a lesser extent glutamate, into pyroglutamic acid (5-oxo-L-proline, or <Glu) (Figure 5). This enzyme can be categorized as follows:
+DromeQC is responsible for the post-translational processing of polypeptides. DromeQC catalyzes the cyclization of N-terminal glutamine, and to a lesser extent glutamate, into pyroglutamic acid (pGlu, 5-oxo-L-proline, or <Glu) (Figure 5). This enzyme can be categorized as follows:
 :-transferase (2)
@@ Line 64: / Line 64: @@
 ::::-acts on glutaminyl/glutamyl residues (2.3.2.5)
-The cyclization reaction occurs via a nucleophilic attack of the α-amine on the γ carbon in the glutamine side chain. The enzymatic mechanism for DromeQC is still undetermined, but it seems plausible that it follows that of its human orthologue. In hQC, the N-terminus of the peptide substrate is inserted into the active site pocket, where the γ amide oxygen chelates the catalytic zinc ion<ref name="mechanism">PMID: 18470930</ref>. This ion-dipole interaction causes carbonyl polarization, making it a better electrophile. To facilitate the reaction, a conserved glutamate (Glu201) acts as both a general base and acid. Glu201 abstracts a proton from the α-amino group, causing it to nucleophilically attack the γ amide oxygen. This produces a tetrahedral intermediate with a charged oxygen that is stabilized by Zn<sup>2+</sup>. Glu201 then protonates the γ amide nitrogen, and an amine group is expelled as the carbonyl reforms. Also essential to this mechanism is a conserved aspartate (Asp248) that coordinates/stabilizes the leaving amine group.
+The cyclization reaction occurs via a nucleophilic attack of the N-terminal α-amine on the γ carbon in the glutamine side chain. The enzymatic mechanism for DromeQC is still undetermined, but it seems plausible that it follows that of its human orthologue. In hQC, the N-terminus of the peptide substrate is inserted into the active site pocket, where the γ amide oxygen chelates the catalytic zinc ion. This ion-dipole interaction causes carbonyl polarization, making it a better electrophile. To facilitate the reaction, a conserved glutamate (Glu201) acts as both a general base and acid. Glu201 abstracts a proton from the α-amine group, causing it to nucleophilically attack the γ amide oxygen. This produces a tetrahedral intermediate with a charged oxygen that is stabilized by Zn<sup>2+</sup>. Glu201 then protonates the γ amide nitrogen, and an amine group is expelled as the carbonyl reforms. Also essential to this mechanism is a conserved aspartate (Asp248) that coordinates/stabilizes the leaving amine group.
 ==Localization==
-DromeQC is known to localize in the brain and peripheral nerves of ''D. melanogaster''<ref name="schilling"/>. This fact was unveiled by the discovery of adipokinetic hormone; this protein has an N-terminal pGlu, supporting a post translational modification via DromeQC<ref>PMID: 2117437</ref>. Adipokinetic hormone was found to localize in neurone and nerve endings by immunohistochemistry, suggesting that it functions as a locally released modulator in tissues such as heart and skeletal muscle<ref>PMID: 6342796</ref>. Thus, It has been suggested that DromeQC is part of the secretory pathway, being targeted to secretory vesicles where hormone maturation takes place<ref name="schilling"/>. This claim is supported by a 27 residue signal sequence contained at the N-terminus of DromeQC<ref name="schilling"/>. Further support stems from immunohistochemical evidence that DromeQC and its modified substrate are excreted from the cell, where they can be found in the extracellular medium<ref name="schilling"/>.
+DromeQC is known to localize in the brain and peripheral nerves of ''D. melanogaster''<ref name="schilling"/>. This fact was unveiled by the discovery of adipokinetic hormone; this protein has an N-terminal pGlu, supporting a post translational modification via DromeQC<ref>PMID: 2117437</ref>. Adipokinetic hormone was found to localize in neurons and nerve endings by immunohistochemistry, functioning as neuromodulator in tissues such as heart and skeletal muscle<ref>PMID: 6342796</ref>. Thus, It has been suggested that DromeQC is part of the secretory pathway, being targeted to secretory vesicles where hormone maturation takes place<ref name="schilling"/>. This claim is supported by a 27 residue signal sequence contained at the N-terminus of DromeQC<ref name="schilling"/>. Further support stems from immunohistochemical evidence that DromeQC and modified substrate are excreted into the extracellular medium<ref name="schilling"/>.
 ==References==
 <references/>