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L-threonine degradation III (to methylglyoxal)

General Background |FRAME: THR| is an indispensable amino acid. It is degraded by a complex network of pathways and regulatory signals |CITS: [9871012]|. Two of the major routes for the degradation of L-threonine are known to occur both in mammals and in microorganisms. In the first route threonine is catabolized by |FRAME:THREDEHYDCAT-CPLX| (EC to |FRAME: AMMONIA| and |FRAME:2-OXOBUTANOATE|, which is rapidly and irreversibly converted to |FRAME: PROPIONYL-COA| and |FRAME: FORMATE| (see |FRAME:PWY-5437|). A biosynthetic version of this enzyme has also been reported in certain anaerobic bacteria (see |FRAME:ILEUSYN-PWY|) |CITS: [13405870]|. In the second route threonine is catabolized by |FRAME:THREODEHYD-CPLX| (EC to form |FRAME:AMINO-OXOBUT|, which is mainly cleaved by |FRAME: AKBLIG-CPLX|, forming glycine and acetyl-CoA (see |FRAME:THREONINE-DEG2-PWY|). In the absence of the later activity |FRAME:AMINO-OXOBUT| is spontaneously converted to |FRAME:AMINO-ACETONE|, which is further metabolized into |FRAME: METHYL-GLYOXAL| (see |FRAME: THRDLCTCAT-PWY|). A third route has been demonstrated in several bacteria and fungi. This route is based on the enzyme |FRAME:CPLX-7266| (EC, which cleaves threonine directly into |FRAME:GLY| and |FRAME:ACETALD| (see |FRAME:PWY-5436|). About This Pathway |FRAME: THREODEHYD-CPLX| catalyzes the oxidation of threonine by NAD+ to |FRAME:AMINO-OXOBUT|, which is typically converted by |FRAME:AKBLIG-CPLX| into glycine and acetyl-CoA (see |FRAME:THREONINE-DEG2-PWY|). However, in certain bacteria, and in mammals when the ratio acetyl-CoA/CoA increases in nutritional deprivation (e.g., in diabetes) |FRAME:AKBLIG-CPLX| activity is lacking or is deminished, and |FRAME:AMINO-OXOBUT| is instead converted non-enzymatically to |FRAME: AMINO-ACETONE|. Aminoacetone, like many other endogenous (e.g., |FRAME: METHYLAMINE|) and xenobiotic (e.g., |FRAME:BENZYLAMINE|) amines, is oxidized by dioxygen in a reaction catalyzed by amine oxidases, a group of poorly understood Cu-dependent enzymes. In mammals amine oxidases can be either plasma circulating or membrane bound. The reaction yields an aldehyde, |FRAME:HYDROGEN-PEROXIDE| and |FRAME:AMMONIA| ions |CITS: [6833209][8920635]|. With aminoacetone as the substrate, the product is the cytotoxic and genotoxic |FRAME: METHYL-GLYOXAL|, which is detoxified into |FRAME:D-LACTATE|, as described in |FRAME:PWY-901| |CITS: [11559049]|. This pathway has been reported as the main route of threonine degradation in |FRAME: TAX-615| |CITS: [355220]|. In mammals this pathway spans the cytoplasmic and mitochondrial compartments. The first reaction occurs in the cytosol, but aminoacetone moves into the mitochondrial compartment, where the remaining reactions occur. While this pathway occurrs in mammals |CITS:[6437452]|, it plays only a minor role in humans |CITS: [10780944]|, potentially due to the fact that the human |FRAME:THREODEHYD-CPLX| is an expressed pseudogene |CITS: [12361482]|.

from BIOCYC source record: META_THRDLCTCAT-PWY
Type: pathway
Taxonomic scope
conserved biosystem

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