Metal ion inhibition of nonenzymatic pyridoxal phosphate catalyzed decarboxylation and transamination.
Nonenzymatic pyridoxal phosphate (PLP) catalyzed decarboxylations and transaminations have been revisited experimentally. Metal ions are known to catalyze a variety of PLP-dependent reactions in solution, including transamination. It is demonstrated here that the rate accelerations previously observed are due solely to enhancement of Schiff base formation under subsaturating conditions. A variety of metal ions were tested for their effects on the reactivity of the 2-methyl-2-aminomalonate Schiff bases. All were found to have either no effect or a small inhibitory one. The effects of Al(3+) were studied in detail with the Schiff bases of 2-methyl-2-aminomalonate, 2-aminoisobutyrate, alanine, and ethylamine. The decarboxylation of 2-methyl-2-aminomalonate is unaffected by metalation with Al(3+), while the decarboxylation of 2-aminoisobutyrate is inhibited 125-fold. The transamination reaction of ethylamine is 75-fold slower than that of alanine. Ethylamine transamination is inhibited 4-fold by Al(3+) metalation, while alanine transamination is inhibited only 1.3-fold. Metal ion inhibition of Schiff base reactivity suggests a simple explanation for the lack of known PLP dependent enzymes that make direct mechanistic use of metal ions. A comparison of enzyme catalyzed, PLP catalyzed, and uncatalyzed reactions shows that PLP dependent decarboxylases are among the best known biological rate enhancers: decarboxylation occurs 10(18)-fold faster on the enzyme surface than it does free in solution. PLP itself provides the lion's share of the catalytic efficiency of the holoenzyme: at pH 8, free PLP catalyzes 2-aminoisobutyrate decarboxylation by approximately 10(10)-fold, with the enzyme contributing an additional approximately 10(8)-fold.
Zabinski
R F
RF
Department of Chemistry, University of California-Davis, One Shields Avenue, Davis, California 95616, USA.
Toney
M D
MD
D000409:Alanine / Q000378:metabolism; D000535:Aluminum / Q000037:antagonists & inhibitors / Q000494:pharmacology; D000586:Amination / Q000187:drug effects; D000621:Aminoisobutyric Acids / Q000378:metabolism; D002262:Carboxy-Lyases / Q000737:chemistry / Q000378:metabolism*; D002384:Catalysis; D002412:Cations; D003653:Decarboxylation / Q000187:drug effects; D005021:Ethylamines / Q000378:metabolism; D007700:Kinetics; D011732:Pyridoxal Phosphate / Q000378:metabolism*
D000621:Aminoisobutyric Acids; D002412:Cations; D005021:Ethylamines; C100049:2-aminoisobutyric acid; D011732:Pyridoxal Phosphate; D000535:Aluminum; D002262:Carboxy-Lyases; D000409:Alanine; C041564:ethylamine