Pyridoxal-linked reactions

Pyridoxal-Linked Elimination and Replacement Reactions Leodis Davis and David E. Metzler... 

L-Lysine and cadaverine serve as precursors for the majority of piperidine alkaloids. The experimental results have been interpreted in terms of an attractive series of pyridoxal-linked intermediates derivable independently from both precursors see Scheme 1. The transformation of cadaverine into alkaloids involves stereospecific removal of one of the protons attached to C-1 (pro-S hydrogen) and probably involves a diamine oxidase cf. refs. 5 and 6. Examination of the diamine-oxidase-catalysed oxidation of cadaverine, with enzyme from hog kidney, and analysis by an excellent n.m.r. method, has shown that this reaction also involves removal of the 1-pro-5 proton from the diamine pea seedling diamine oxidase has been found to effect the conversion of benzylamine into benzaldehyde with similar stereochemistry. ... 

A brilliantly simple and largely satisfying solution [16] to the observations on lysine and cadaverine incorporation has been proposed. It is consistent in particular with the observed incorporation of lysine with distinction between C-2 and C-6, loss of nitrogen from C-2 but retention of the C-2 proton and it allows for normal incorporation of cadaverine 6.26). Central to the proposal is enzyme-catalysed decarboxylation of lysine (lysine decarboxylase) and oxidation of cadaverine (diamine oxidase) both involving pyridoxal phosphate as coenzyme. The proposed sequence involves orthodox pyridoxal-linked intermediates of which 625) and 6.27) are common to both enzyme-mediated reactions (Scheme 6.8). It is an important... 

Davis, L., and D. E. Metzler Pyridoxal-Linked Elimination and Replacement Reactions. In The Enzymes, Vol. VII, Paul D. Boyer, Third Ed. New York and London Academic Press. 1972. 

Pyridoxal Derivatives. Various aldehydes of pyridoxal (Table 3) react with hemoglobin at sites that can be somewhat controlled by the state of oxygenation (36,59). It is thereby possible to achieve derivatives having a wide range of functional properties. The reaction, shown for PLP in Figure 3, involves first the formation of a Schiff s base between the amino groups of hemoglobin and the aldehyde(s) of the pyridoxal compound, followed by reduction of the Schiff s base with sodium borohydride, to yield a covalendy-linked pyridoxyl derivative in the form of a secondary amine. 

Bis-Pyndoxal Tetraphosphate. A second class of bifunctional reagents, described in 1988, involves two pyridoxal groups linked by phosphates of different lengths (89). As shown in Table 4, the yield of intramolecularly cross-linked hemoglobin increases dramatically with increasing length of the phosphate backbone. It is beheved that the site of reaction of (bis-PL) is between the amino-terminal amino group of one P-chain and the... 

Muscle glycogen phosphorylase is a dimer of two identical subunits (842 residues, 97.44 kD). Each subunit contains a pyridoxal phosphate cofactor, covalently linked as a Schiff base to Lys °. Each subunit contains an active site (at the center of the subunit) and an allosteric effector site near the subunit interface (Eigure 15.15). In addition, a regulatory phosphorylation site is located at Ser on each subunit. A glycogen-binding site on each subunit facilitates prior association of glycogen phosphorylase with its substrate and also exerts regulatory control on the enzymatic reaction. 

Pyridoxal phosphate is a required coenzyme for many enzyme-catalyzed reactions. Most of these reactions are associated with the metabolism of amino acids, including the decarboxylation reactions involved in the synthesis of the neurotransmitters dopamine and serotonin. In addition, pyridoxal phosphate is required for a key step in the synthesis of porphyrins, including the heme group that is an essential player in the transport of molecular oxygen by hemoglobin. Finally, pyridoxal phosphate-dependent reactions link amino acid metabolism to the citric acid cycle (chapter 16). 

An clcctromeric displacement of electrons from bonds a h. or c (.see diagram below) would result in the relea.se of a cation (H, R. or COOH) and, sub.sequently. lead to the variety of reactions observed with pyridoxal. The extent to which one of these displacements predominates over others depends on the structure of the amino acid and the environment (pH. solvent, catulyst.s, enzymes, and such). When this mechanism applies in vivo, the pyridoxal component is linked to the enzyme through the phasphate of the hydroxymethyl group. 

The collagen molecules formed by removal of the propeptides spontaneously assemble into fibrils. At this stage, the fibrils are still immature and lack tensile strength, which is acquired by cross-linking. The initial step in cross-link formation is the oxidative deamination of a-amino groups in certain lysyl and hydroxyly-syl residues catalyzed by lysyl oxidase. The enzyme is a copper-dependent (probably cupric) protein, and the reaction requires molecular oxygen and pyridoxal phosphate for full activity. Only native collagen fibrils function as substrates. 

The enzyme shows a high substrate specificity for AdoMet, and affinity to the substrate is also high with a Km ranging from 12 to 60 pM, and the pH optimum is between 8.5 and 9.5. Interestingly, S-adenosylethionine shows some activity as a substrate. The enzyme reaction is competitively inhibited by AVG and AOA, which are inhibitors of pyridoxal phosphate-linked enzymes. 

The cofactor of glycogen phosphorylase is pyridoxal 5 -phosphate (PLP). This cofactor, linked via a Schiff base to a lysine residue (Lys680 in the rabbit sequence), is tightly bound to the enzyme and cannot be resolved from the apo-enzyme unless powerful denaturants are used. The use of PLP in the phosphorylase reaction is unusual and involves the 5 -phosphate group rather than the aldehydic group that is used more conunonly in, for example, transaminases (Johnson, 1 2). 

The more widespread route of catabolism could be via reaction 2. This is an extremely important reaction in higher plants as it is considered to be the primary source of CO2 in photorespiration (cf. Tolbert, this series, Vol. 2, Chapter 12 Keys, this volume. Chapter 9 Lorimer, Chapter 9, Vol. 8 this treatise). Reaction 2 can be carried out by particles from leaves of spinach (Kisaki et al., 1971) or tobacco (Bird et al., 1972a,b), and by pea cotyledon mitochondria (Clandinin and Cossins, 1975). There are some differences between the various systems in regard to the effect of oxygen and exogenous cofactors. The spinach and pea systems are stimulated by the addition of NAD, pyridoxal phosphate, and tetrahydrofolic acid. The tobacco leaf system does not require these exogenous cofactors however, electrons transported during the reaction can be linked to ATP synthesis with three ATP... 

Many transamination reactions are carried out in living systems using the pyridoxal coenzymes [equation (1)1. Pyridoxamine has been covalently linked to cyclomaltoheptaose, and keto acids containing aromatic rings undergo trans-... 

Present data indicates that chain growth in both gramicidin S and the tyrocidines commences at the D-phenylalanine residue adjacent to proline and that the first step in the synthesis is the conversion of L-phenylalanine to D-phenyldanine. In the case of the tyrocidines, the synthesis then proceeds in order from the amino to the carboxyl terminus to form a linear decapeptide ending with a thiol ester linked leucine. The peptide then cyclises relatively slowly to the final product. Yamada and Kurahashi showed that the epimerisation of L-phenylalanine in the initial step did not require pyridoxal phosphate or FAD and they suggested that the reaction occurs via the thiol ester enzyme bound form (101), Figure 3.18. 

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