Sandbox Reserved 1734: Difference between revisions

Line 21:

The substrate of phenylalanine hydroxylase is the amino acid L-phenylalanine. Phenylalanine binds between the regulatory domain and the interacting catalytic domain, near the sequence binding motif. The activation of phenylalanine hydroxylase by L-phenylalanine induces a large conformational change, but a slow global conformational change. Full activation of PAH involves the shift and dimerization of the regulatory domains (6).

PAH is an iron (Fe2+) containing enzyme. The iron binds to 2 histidines at the active site (3 & 6). The cofactors of PAH include 6R-L-erythro-tetrahydrobiopterin (BH4) and oxygen (3). BH4 is sandwiched between hydrophobic residues and forms several hydrogen bonds with the N-terminal autoregulatory tail. BH4 binding causes a limited conformational change (mostly constrained to the N-terminal tail). PAH lacking this tail is not regulated by either BH4 or L-phenylalanine and is constitutively active. The BH4 binding site is flanked by the N-terminal (residues 21-32), the active-site lid (130-150), the Fe+2-coordinating residues, the Beta 6-alpha 7 loop (residues 245-251), and F254 (6).

PAH is an iron (Fe2+) containing enzyme. The iron binds to 2 histidines at the active site (3 & 6). The cofactors of PAH include 6R-L-erythro-tetrahydrobiopterin (BH4) and oxygen (3). <scene name='91/919043/Cofactor_and_neg_bh4/2'>BH4</scene> is sandwiched between hydrophobic residues and forms several hydrogen bonds with the N-terminal autoregulatory tail. BH4 binding causes a limited conformational change (mostly constrained to the N-terminal tail). PAH lacking this tail is not regulated by either BH4 or L-phenylalanine and is constitutively active. The BH4 binding site is flanked by the N-terminal (residues 21-32), the active-site lid (130-150), the Fe+2-coordinating residues, the Beta 6-alpha 7 loop (residues 245-251), and F254 (6).

Tetrahydrobiopterin induces a negative heterotropic allosteric effect on the enzyme, which is observed as the activation rate is slower for the BH4 holoprotein than compared to the unbound enzyme. Prior to BH4 binding, (PAH unbound state) a polar and salt-bridge interaction network links the three PAH domains.

== Function ==

The genetic information that codes for the production of phenylalanine hydroxylase is found on the long arm of chromosome 12 and contains 13 exons (1, 7, & 8). PAH is a metabolic enzyme contained in liver cells that catalyzes the hydroxylation reaction of the amino acid L-Phenylalanine to L-Tyrosine (2, 4, & 9). This protein specifically catalyzes the rate-limiting step in the phenylalanine catabolism, which is the para-hydroxylation step of the aromatic side chain (4 & 9). This catalysis is done by hydroxylation of its substrate by incorporation of one oxygen atom into the aromatic ring, and the final reaction includes the reduction of the second oxygen atom to water using electrons supplied by tetrahydrobiopterin (BH4). ~~<scene name='91/919043/Cofactor_and_neg_bh4/1'>~~BH4~~</scene>~~ functions as a co-substrate that is hydroxylated at each turnover to pterin-4a-carbinolamine (4a-OH-BH4), with consequent dissociation from the enzyme (4).

The genetic information that codes for the production of phenylalanine hydroxylase is found on the long arm of chromosome 12 and contains 13 exons (1, 7, & 8). PAH is a metabolic enzyme contained in liver cells that catalyzes the hydroxylation reaction of the amino acid L-Phenylalanine to L-Tyrosine (2, 4, & 9). This protein specifically catalyzes the rate-limiting step in the phenylalanine catabolism, which is the para-hydroxylation step of the aromatic side chain (4 & 9). This catalysis is done by hydroxylation of its substrate by incorporation of one oxygen atom into the aromatic ring, and the final reaction includes the reduction of the second oxygen atom to water using electrons supplied by tetrahydrobiopterin (BH4). BH4 functions as a co-substrate that is hydroxylated at each turnover to pterin-4a-carbinolamine (4a-OH-BH4), with consequent dissociation from the enzyme (4).

The major regulatory mechanisms of phenylalanine hydroxylase include activation of phenylalanine inhibition by BH4 and additional activation by phosphorylation (5). Phosphorylation acts as a mediator of phenylalanine activation by decreasing the phenylalanine concentration required to activate enzyme phosphorylation at Ser16 (4). Substrate activation and positive homotropic allosteric for phenylalanine binding involve all three functional domains and all four subunits in the holoenzyme (5). The hypothesized cause of the phenylalanine activation mechanism is that the homotropic binding of phenylalanine at the active site and the regulatory domain is involved in cooperativity through the interactions with the catalytic and oligomerization domains. Phenylalanine binds to an allosteric site, beside the active site, on the regulatory domain, which induces large conformational changes (1 & 6). Allosteric regulation is necessary to maintain phenylalanine levels below neurotoxic levels (4). BH4 acts as a negative allosteric regulator by blocking phenylalanine activation, however, BH4 binding to a Phe-activated form of PAH results in positive cooperativity.

Sandbox Reserved 1734: Difference between revisions

Navigation menu

Page actions

Page actions

Personal tools

Navigation

Search

Tools

@@ Line 21: / Line 21: @@
 The substrate of phenylalanine hydroxylase is the amino acid L-phenylalanine. Phenylalanine binds between the regulatory domain and the interacting catalytic domain, near the sequence binding motif. The activation of phenylalanine hydroxylase by L-phenylalanine induces a large conformational change, but a slow global conformational change. Full activation of PAH involves the shift and dimerization of the regulatory domains (6).
-PAH is an iron (Fe2+) containing enzyme. The iron binds to 2 histidines at the active site (3 & 6). The cofactors of PAH include 6R-L-erythro-tetrahydrobiopterin (BH4) and oxygen (3). BH4 is sandwiched between hydrophobic residues and forms several hydrogen bonds with the N-terminal autoregulatory tail. BH4 binding causes a limited conformational change (mostly constrained to the N-terminal tail). PAH lacking this tail is not regulated by either BH4 or L-phenylalanine and is constitutively active. The BH4 binding site is flanked by the N-terminal (residues 21-32), the active-site lid (130-150), the Fe+2-coordinating residues, the Beta 6-alpha 7 loop (residues 245-251), and F254 (6).
+PAH is an iron (Fe2+) containing enzyme. The iron binds to 2 histidines at the active site (3 & 6). The cofactors of PAH include 6R-L-erythro-tetrahydrobiopterin (BH4) and oxygen (3). <scene name='91/919043/Cofactor_and_neg_bh4/2'>BH4</scene> is sandwiched between hydrophobic residues and forms several hydrogen bonds with the N-terminal autoregulatory tail. BH4 binding causes a limited conformational change (mostly constrained to the N-terminal tail). PAH lacking this tail is not regulated by either BH4 or L-phenylalanine and is constitutively active. The BH4 binding site is flanked by the N-terminal (residues 21-32), the active-site lid (130-150), the Fe+2-coordinating residues, the Beta 6-alpha 7 loop (residues 245-251), and F254 (6).
 Tetrahydrobiopterin induces a negative heterotropic allosteric effect on the enzyme, which is observed as the activation rate is slower for the BH4 holoprotein than compared to the unbound enzyme. Prior to BH4 binding, (PAH unbound state) a polar and salt-bridge interaction network links the three PAH domains.
 == Function ==
-The genetic information that codes for the production of phenylalanine hydroxylase is found on the long arm of chromosome 12 and contains 13 exons (1, 7, & 8). PAH is a metabolic enzyme contained in liver cells that catalyzes the hydroxylation reaction of the amino acid L-Phenylalanine to L-Tyrosine (2, 4, & 9). This protein specifically catalyzes the rate-limiting step in the phenylalanine catabolism, which is the para-hydroxylation step of the aromatic side chain (4 & 9). This catalysis is done by hydroxylation of its substrate by incorporation of one oxygen atom into the aromatic ring, and the final reaction includes the reduction of the second oxygen atom to water using electrons supplied by tetrahydrobiopterin (BH4). <scene name='91/919043/Cofactor_and_neg_bh4/1'>BH4</scene> functions as a co-substrate that is hydroxylated at each turnover to pterin-4a-carbinolamine (4a-OH-BH4), with consequent dissociation from the enzyme (4).
+The genetic information that codes for the production of phenylalanine hydroxylase is found on the long arm of chromosome 12 and contains 13 exons (1, 7, & 8). PAH is a metabolic enzyme contained in liver cells that catalyzes the hydroxylation reaction of the amino acid L-Phenylalanine to L-Tyrosine (2, 4, & 9). This protein specifically catalyzes the rate-limiting step in the phenylalanine catabolism, which is the para-hydroxylation step of the aromatic side chain (4 & 9). This catalysis is done by hydroxylation of its substrate by incorporation of one oxygen atom into the aromatic ring, and the final reaction includes the reduction of the second oxygen atom to water using electrons supplied by tetrahydrobiopterin (BH4). BH4 functions as a co-substrate that is hydroxylated at each turnover to pterin-4a-carbinolamine (4a-OH-BH4), with consequent dissociation from the enzyme (4).
 The major regulatory mechanisms of phenylalanine hydroxylase include activation of phenylalanine inhibition by BH4 and additional activation by phosphorylation (5). Phosphorylation acts as a mediator of phenylalanine activation by decreasing the phenylalanine concentration required to activate enzyme phosphorylation at Ser16 (4). Substrate activation and positive homotropic allosteric for phenylalanine binding involve all three functional domains and all four subunits in the holoenzyme (5). The hypothesized cause of the phenylalanine activation mechanism is that the homotropic binding of phenylalanine at the active site and the regulatory domain is involved in cooperativity through the interactions with the catalytic and oligomerization domains. Phenylalanine binds to an allosteric site, beside the active site, on the regulatory domain, which induces large conformational changes (1 & 6). Allosteric regulation is necessary to maintain phenylalanine levels below neurotoxic levels (4). BH4 acts as a negative allosteric regulator by blocking phenylalanine activation, however, BH4 binding to a Phe-activated form of PAH results in positive cooperativity.