Insights into Enzyme Catalysis and Thyroid Hormone Regulation of Cerebral Ketimine Reductase/μ-Crystallin Under Physiological Conditions

André Hallen, Arthur J L Cooper, Joanne F. Jamie, Peter Karuso

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Mammalian ketimine reductase is identical to μ-crystallin (CRYM)—a protein that is also an important thyroid hormone binding protein. This dual functionality implies a role for thyroid hormones in ketimine reductase regulation and also a reciprocal role for enzyme catalysis in thyroid hormone bioavailability. In this research we demonstrate potent sub-nanomolar inhibition of enzyme catalysis at neutral pH by the thyroid hormones l-thyroxine and 3,5,3′-triiodothyronine, whereas other thyroid hormone analogues were shown to be far weaker inhibitors. We also investigated (a) enzyme inhibition by the substrate analogues pyrrole-2-carboxylate, 4,5-dibromopyrrole-2-carboxylate and picolinate, and (b) enzyme catalysis at neutral pH of the cyclic ketimines S-(2-aminoethyl)-l-cysteine ketimine (owing to the complex nomenclature trivial names are used for the sulfur-containing cyclic ketimines as per the original authors’ descriptions) (AECK), Δ1-piperideine-2-carboxylate (P2C), Δ1-pyrroline-2-carboxylate (Pyr2C) and Δ2-thiazoline-2-carboxylate. Kinetic data obtained at neutral pH suggests that ketimine reductase/CRYM plays a major role as a P2C/Pyr2C reductase and that AECK is not a major substrate at this pH. Thus, ketimine reductase is a key enzyme in the pipecolate pathway, which is the main lysine degradation pathway in the brain. In silico docking of various ligands into the active site of the X-ray structure of the enzyme suggests an unusual catalytic mechanism involving an arginine residue as a proton donor. Given the critical importance of thyroid hormones in brain function this research further expands on our knowledge of the connection between amino acid metabolism and regulation of thyroid hormone levels.

LanguageEnglish
Pages1252-1266
Number of pages15
JournalNeurochemical Research
Volume40
Issue number6
DOIs
Publication statusPublished - 15 Jun 2015

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Catalysis
Thyroid Hormones
Enzymes
Pyrroline Carboxylate Reductases
Brain
Enzyme inhibition
Triiodothyronine
Substrates
Terminology
Thyroxine
Research
Sulfur
Metabolism
Computer Simulation
Biological Availability
Lysine
Names
Cysteine
Arginine
crystallin mu

Cite this

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title = "Insights into Enzyme Catalysis and Thyroid Hormone Regulation of Cerebral Ketimine Reductase/μ-Crystallin Under Physiological Conditions",
abstract = "Mammalian ketimine reductase is identical to μ-crystallin (CRYM)—a protein that is also an important thyroid hormone binding protein. This dual functionality implies a role for thyroid hormones in ketimine reductase regulation and also a reciprocal role for enzyme catalysis in thyroid hormone bioavailability. In this research we demonstrate potent sub-nanomolar inhibition of enzyme catalysis at neutral pH by the thyroid hormones l-thyroxine and 3,5,3′-triiodothyronine, whereas other thyroid hormone analogues were shown to be far weaker inhibitors. We also investigated (a) enzyme inhibition by the substrate analogues pyrrole-2-carboxylate, 4,5-dibromopyrrole-2-carboxylate and picolinate, and (b) enzyme catalysis at neutral pH of the cyclic ketimines S-(2-aminoethyl)-l-cysteine ketimine (owing to the complex nomenclature trivial names are used for the sulfur-containing cyclic ketimines as per the original authors’ descriptions) (AECK), Δ1-piperideine-2-carboxylate (P2C), Δ1-pyrroline-2-carboxylate (Pyr2C) and Δ2-thiazoline-2-carboxylate. Kinetic data obtained at neutral pH suggests that ketimine reductase/CRYM plays a major role as a P2C/Pyr2C reductase and that AECK is not a major substrate at this pH. Thus, ketimine reductase is a key enzyme in the pipecolate pathway, which is the main lysine degradation pathway in the brain. In silico docking of various ligands into the active site of the X-ray structure of the enzyme suggests an unusual catalytic mechanism involving an arginine residue as a proton donor. Given the critical importance of thyroid hormones in brain function this research further expands on our knowledge of the connection between amino acid metabolism and regulation of thyroid hormone levels.",
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Insights into Enzyme Catalysis and Thyroid Hormone Regulation of Cerebral Ketimine Reductase/μ-Crystallin Under Physiological Conditions. / Hallen, André; Cooper, Arthur J L; Jamie, Joanne F.; Karuso, Peter.

In: Neurochemical Research, Vol. 40, No. 6, 15.06.2015, p. 1252-1266.

Research output: Contribution to journalArticleResearchpeer-review

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