A crosslinked cofactor in lysyl oxidase: redox function for amino acid side chains

Intra- and extracellular enzymes of collagen biosynthesis as biological and chemical targets in the control of fibrosis

The several steps in the pathway for the biosynthesis of fibrillar collagen are reviewed to illustrate potential sites for the chemotherapeutic control of fibrosis. Particular emphasis is placed upon the properties and inhibition of lysyl oxidase, the enzyme which initiates the covalent crosslinking of extracellular collagen molecules converting these to insoluble fibers, and upon the properties and inhibition of prolyl hydroxylase, the intracellular enzyme which hyroxylates proline residues within collagen.

Influence of Sodium Periodate and Tyrosinase on Binding of Alginate to Adlayers of Mytilus edulis Foot Protein 1

Mytilus edulis foot protein 1 (Mefp-1) is the most well-characterized component of this sea mussel's adhesive plaque. The plaque is a condensed, heterogeneous mixture consisting of a large proportion of cross-linked biopolymers that bonds the mussel to a chosen mooring. Mefp-1 is densely populated with lysine and L-3,4-dihyroxyphenylalanine (L-dopa) residues incorporated into a repeating amino acid sequence motif. It has been proposed that one plaque cross-linking reaction is the nucleophilic addition of the epsilon-amino groups of the lysine residues into the oxidized catechol (o-diphenol) functionality (quinone) of the L-dopa residues. In order to determine if this reaction occurs in adlayers of Mefp-1, a previously developed assay for epsilon-amino groups was applied. Adlayers of Mefp-1 were exposed to an oxidant, either the enzyme, mushroom tyrosinase, or sodium periodate. Binding of alginate to adlayers was used to probe for accessibility of epsilon-amino groups. It was found that lysine residues lose the ability to bind alginate after exposure to sodium periodate, but that this loss is not clearly due to a reaction with L-dopa residues. There is a slight decrease of binding of alginate to adlayers of Mefp-1 exposed to either active or thermally deactivated mushroom tyrosinase, probably due to the obstruction of binding sites by bound enzyme. Adsorption kinetics of mushroom tyrosinase onto adlayers of Mefp-1 for active and thermally inactivated enzyme were nearly identical. Attenuated total reflection Fourier transform infrared spectroscopy was used to characterize these interactions at a germanium (Ge) interface. Copyright 2000 Academic Press.

Ras-dependent and -independent regulation of reactive oxygen species by mitogenic growth factors and TGF-beta1

Mitogenic growth factors and transforming growth factor beta1 (TGF-beta1) induce the generation of reactive oxygen species (ROS) in nonphagocytic cells, but their enzymatic source(s) and regulatory mechanisms are largely unknown. We previously reported on the ability of TGF-beta1 to activate a cell surface-associated NADH:flavin:O(2) oxidoreductase (NADH oxidase) that generates extracellular H(2)O(2). In this study, we compared the ROS-generating enzymatic systems activated by mitogenic growth factors and TGF-beta1 with respect to the primary reactive species produced (O(2)(.-) vs. H(2)O(2)), the site of generation (intracellular vs. extracellular) and regulation by Ras. We find that the mitogenic growth factors PDGF-BB, FGF-2, and TGF-alpha (an EGF receptor ligand) are able to rapidly (within 5 min) induce the generation of intracellular O(2)(.-) without detectable NADH oxidase activity or extracellular H(2)O(2) release. In contrast, TGF-beta1 does not stimulate intracellular O(2)(.-) production and the delayed induction of extracellular H(2)O(2) release is not associated with O(2)(.-) production. Expression of dominant-negative Ras (N17Ras) protein by herpes simplex virus-mediated gene transfer blocks mitogen-stimulated intracellular O(2)(.-) generation but has no effect on TGF-beta1-induced NADH oxidase activation/H(2)O(2) production. These results demonstrate that there are at least two distinctly different ROS-generating enzymatic systems in lung fibroblasts regulated by mitogenic growth factors and TGF-beta1 via Ras-dependent and -independent mechanisms, respectively. In addition, these findings suggest that endogenous production of ROS by growth factors/cytokines may have different biological effects depending on the primary reactive species generated and site of production.

Comparative evaluation of the combined osteolathyritic effects of two nitrile combinations on xenopus embryos

Two nitrile combinations, beta-aminopropionitrile (beta APN) with aminoacetonitrile (AAN) and betaAPN with beta APN (as a sham combination), were evaluated using the frog embryo mixture toxicity assay to determine their combined osteolathyritic effects and to compare the results with theoretical effects for two combined effects models. In separate tests each nitrile was tested with copper sulfate to determine the importance of copper in osteolathyrogen-induced disruption of connective tissue cross-linking. Frog embryos (Xenopus laevis) were exposed for 96 h, with daily solution removal and replacement. Preserved tadpoles were evaluated for osteolathyritic lesions. For the nitrile:nitrile combinations, the chi(2) goodness-of-fit test was used to compare the resulting mixture-response curves to theoretical curves for dose-addition and independence. For beta APN with AAN, the combined osteolathyritic effect for five of the seven mixture curves generated was greater than expected for each of the combined effects models. For beta APN with beta APN, the combined effect for all seven mixture curves was consistent with dose-addition, the combined effect expected for chemicals inducing toxicity by the same mechanism. For the nitrile:copper combinations, the EC(50) for beta APN-induced osteolathyrism was increased two- to threefold (i.e. made less toxic) by co-administration with copper sulfate, while the EC(50) for AAN-induced osteolathyrism was unchanged. The results are consistent with the idea that beta APN and AAN induce osteolathyrism, at least in part, by different mechanisms.

Novel cofactors via post-translational modifications of enzyme active sites

Recent crystallographic and biochemical studies have revealed the existence of numerous novel post-translational modifications within enzyme active sites. These modifications create structural and functional diversity. Although the function and biosynthesis of some of these modifications are well understood, others need further investigation.

Lysyl oxidase and P-ATPase-7A expression during embryonic development in the rat

Lysyl oxidase activity is critical for the assembly and cross-linking of extracellular matrix proteins, such as collagen and elastin. Moreover, lysyl oxidase activity is sensitive to changes in copper status and genetic perturbations in copper transport, e.g., mutations in the P-type ATPase gene, ATP7A, associated with cellular copper transport. Lysyl oxidase may also serve as a vehicle for copper transport from extracellular matrix cells. Herein, we demonstrate that sufficient lysyl oxidase functional activity is present in the rat embryo at gestation day (GD) 9 to be detected in conventional enzyme assays. Estimation of embryonic lysyl oxidase functional activity, however, required partial purification in order to remove inhibitors. From GD 9 to GD 15, lysyl oxidase activity was relatively constant when expressed per unit of protein or DNA. In contrast, the steady-state levels of lysyl oxidase and ATP7A mRNA, measured by RT-PCR and expressed relative to total RNA and cyclophilin mRNA, increased approximately fourfold from GD 9 to 15. The pattern of temporal expression for ATP7A was consistent with its possible role in copper delivery to lysyl oxidase.

Structure, function, and applications of tryptophan tryptophylquinone enzymes

Tryptophan and tyrosine residues in proteins may be posttranslationally modified to form enzyme cofactors. Tryptophan tryptophylquinone (TTQ), the cofactor of methylamine dehydrogenase (MADH), is formed by covalent cross-linking of two tryptophan residues and incorporation of two oxygen atoms into one of the indole rings to form a quinone. MADH converts primary amines to their corresponding aldehydes plus ammonia. During the catalytic cycle, TTQ mediates electron transfer from substrate to a copper protein, amicyanin. These electrons are transferred to the respiratory chain via a c-type cytochrome. Structural, kinetic and site-directed mutagenesis studies have characterized protein-protein interactions, and mechanisms of catalysis and electron transfer by TTQ. Preliminary results obtained with MADH enzyme-electrodes demonstrate the potential for quinoprotein-based biosensors.

Copper-promoted overall transformation of 4-tert-butylphenol to its para-hydroxyquinonic derivative, 2-hydroxy-5-tert-butyl-1,4-benzoquinone. Biomimetic studies on the generation of topaquinone in copper amine oxidases

Topaquinone (TPQ) is a cofactor present at the active site of copper amine oxidases, derived from a Tyr residue inserted in the polypeptide chain through a copper-dependent but otherwise largely unknown mechanism. A simple model system was developed that permits to obtain the overall transformation of 4-tert-butylphenol, chosen as a model for Tyr, into a TPQ-like, para-hydroxyquinonic structure in the presence of Cu(II)-imidazole mononuclear complexes.

Probing the structure of photosystem II with amines and phenylhydrazine

Photosynthetic oxygen evolution is catalyzed at the manganese-containing active site of photosystem II (PSII). Amines are analogs of substrate water and inhibitors of oxygen evolution. Recently, the covalent incorporation of (14)C from [(14)C]methylamine and benzylamine into PSII subunits has been demonstrated (Ouellette, A. J. A., Anderson, L. B., and Barry, B. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 2204-2209). To obtain more information concerning these labeling reactions, t-[(14)C]butylamine and phenylhydrazine were employed as probes. Neither compound can be oxidized by a transamination or addition/elimination mechanism, but both can react with activated carbonyl groups, produced as a result of posttranslational modification of amino acid residues, to give amine-derived adducts. (14)C incorporation into the PSII subunits D2/D1 and CP47 was obtained upon treatment of PSII with either t-[(14)C]butylamine or [(14)C]phenylhydrazine. For t-butylamine and methylamine, the amount of labeling increased when PSII was treated with denaturing agents. Labeling of CP47, D2, and D1 with methylamine and phenylhydrazine approached a one-to-one stoichiometry, assuming that D2 and D1 each have one binding site. Evidence was obtained suggesting that reductive stabilization and/or access are modulated by PSII light reactions. These results support the proposal that PSII subunits D2, D1, and CP47 contain quinocofactors and that access to these sites is sterically limited.

Activation of chick tendon lysyl oxidase in response to dietary copper

Lysyl oxidase (EC 1.4.3.13), a cuproenzyme, can account for 10-30% of the copper present in connective tissue. Herein, we assess the extent to which tissue copper concentrations and lysyl oxidase activity are related because the functional activity of lysyl oxidase and the copper content of chick tendon are both related to dietary copper intake. Chicks (1-d old) were fed diets (basal copper concentration, 0.4 microg/g diet) to which copper was added from 0 to 16 microg/g diet. Liver and plasma copper levels tended to normalize in chickens that consumed from 1 to 4 microg copper/g of diet, whereas tendon copper concentrations suggested an unusual accumulation of copper in chickens that consumed 16 microg copper/g diet. The molecular weight of lysyl oxidase was also estimated using matrix-assisted laser desorption ionization/time-of-flight/mass spectrometry (MALDI/TOF/MS). A novel aspect of these measurements was estimation of protein mass directly from the surface of chick tendons and aortae. Whether copper deficiency (0 added copper) or copper supplementation (16 microg copper/g of diet) caused changes in the molecular weight of protein(s) in tendon corresponding to lysyl oxidase was addressed. The average molecular weight of the peak corresponding to lysyl oxidase in tendon and aorta from copper-deficient birds was 28,386 Da +/- 86, whereas the average molecular weight of corresponding protein in tendon from copper-supplemented birds was 28,639 Da +/- 122. We propose that the shift in molecular weight is due in part to copper binding and the formation of lysyl tyrosyl quinone, the cofactor at the active site of lysyl oxidase.

Active-site structure of the soluble quinoprotein glucose dehydrogenase complexed with methylhydrazine: a covalent cofactor-inhibitor complex

Soluble glucose dehydrogenase (s-GDH) from the bacterium Acinetobacter calcoaceticus is a classical quinoprotein. It requires the cofactor pyrroloquinoline quinone (PQQ) to catalyze the oxidation of glucose to gluconolactone. The precise catalytic role of PQQ in s-GDH and several other PQQ-dependent enzymes has remained controversial because of the absence of comprehensive structural data. We have determined the crystal structure of a ternary complex of s-GDH with PQQ and methylhydrazine, a competitive inhibitor of the enzyme. This complex, refined at 1.5-A resolution to an R factor of 16.7%, affords a detailed view of a cofactor-binding site of s-GDH. Moreover, it presents the first direct observation of covalent PQQ adduct in the active-site of a PQQ-dependent enzyme, thereby confirming previous evidence that the C5 carbonyl group of the cofactor is the most reactive moiety of PQQ.

Lysyl oxidase coupled with catalase in egg shell membrane

The activity of lysyl oxidase was found in egg shell membrane (ESM) of hens. The activity was determined by measuring the enzymatic conversion of n-butylamine and Nalpha-acetyl-L-lysine to n-butyraldehyde and Nalpha-acetyl-L-allysine, respectively. ESM lysyl oxidase was significantly inhibited by beta-aminopropionitrile, chelating agents, and deoxygenation, consistent with the known properties of lysyl oxidase. Nevertheless, ESM lysyl oxidase was insoluble in urea solution, suggesting that it complexes with ESM. These findings support previous reports indicating the presence of lysine-derived cross-links in ESM and the necessity of lysyl oxidase located in the isthmus of the hen oviduct for the biosynthesis of ESM. Lysyl oxidase secreted around the egg white from the isthmus may initiate the cross-linking reaction of ESM protein, and remain as the constituent of ESM. Moreover, the H(2)O(2) released by lysyl oxidase in ESM was completely decomposed by coexisting catalase activity. ESM lysyl oxidase activity was greatly elevated in the presence of H(2)O(2), probably due to the O(2) produced by catalase. These findings indicate that lysyl oxidase is coupled with catalase in ESM. This coupling enzyme system was considered to be involved in the biosynthesis of ESM and to protect the embryo against H(2)O(2).

Reverse engineering of bioadhesion in marine mussels

Marine mussels (Mytilus) are experts at bonding to a variety of solid surfaces in a wet, saline and turbulent environment. Bonding is rapid, permanent, versatile and protein-based. In mussels, adhesive bonding takes the form of a byssus--a bundle of extracorporeal threads--each connected to living tissues of the animal at one end and secured by an adhesive plaque at the other. We have investigated the composition and formation of byssal plaques and threads with the hope of discovering technologically relevant innovations in chemistry and materials science. All proteins isolated from the byssus to date share the quality of containing the unusual amino acid, 3,4-dihydroxyphenylalanine. This residue appears to have a dual functionality with significant consequences for adsorption and cohesion. On the one hand, it forms a diverse array of weaker molecular interactions such as metal chelates, H-bonds, and pi-cations: these appear to dominate in surface behavior (adsorption). On the other hand, 3,4-dihydroxyphenylalanine and its redox couple, dopaquinone, can mediate formation of covalent cross-links among byssal proteins (cohesion). One of the challenges in making functional biomimetic versions of byssal adhesion is to understand how these two reactivities are balanced.

Wheat prolamine crosslinking through dityrosine formation catalyzed by peroxidases: improvement in the modification of a poorly accessible substrate by "indirect" catalysis

"Enzyme-assisted" oxidative polymerization of wheat gliadins was performed in an attempt to obtain new protein-based networks. Two plant peroxidases (soybean and horseradish) were used to induce the dimerization of tyrosine residues. The results show that tyrosines are poorly modified by these enzymes in an aqueous medium (dityrosine corresponded to 2% of the total amount of tyrosine). Two approaches were tested to overcome problems relating to accessibility to the target tyrosines: First, the efficiency of protein crosslinking via tyrosine-tyrosine aromatic ring condensation was enhanced in water when the proteins were oxidized by a fungus peroxidase (manganese-dependent peroxidase from Phanerochaete chrysosporium), which acts according to an indirect catalysis mechanism (up to 12% of the total amount of tyrosine is recovered under a dimeric form). Second, when the gliadins were dispersed in a water/dioxane (3/1) mixed solvent system, the tyrosines were more accessible on the protein surface, and similar yields were obtained with both types of peroxidase. The two types of catalysis (contact and indirect) are considered from the standpoint of the accessibility of the target residues. Enzymatic oxidations were also performed on synthetic peptides mimicking the repeatitive domains of gliadins. The results show that exposure of tyrosine to the solvent may not be sufficient to induce dityrosine formation. The mechanical properties of some films obtained from peroxidase-treated gliadins were investigated to correlate protein crosslinking with a potential application. One effect of the enzymatic treatment was to increase the tensile strength of the films. Copyright 1999 John Wiley & Sons, Inc.

Mechanistic study on the roles of a bifidogenetic growth stimulator based on physicochemical characterization

2-Amino-3-carboxy-1,4-naphthoquinone, discovered as a novel bifidogenetic growth stimulator (BGS), has been characterized by determination of redox and acid-base equilibria, partition properties, and UV-vis and electron spin resonance spectral properties. BGS is proposed to function as an electron transfer mediator from NADH to O2. BGS is reduced by NADH-reduced diaphorase (or related enzymes) and the reduced BGS is reoxidized by autoxidation and a peroxidase-catalyzed reaction. The proposed reaction would spare pyruvate as an important metabolic intermediate, and minimize the cytotoxic effects of H2O2 generated by the autoxidation. Kinetic studies were performed in model enzymatic systems using 2-methyl-1,4-naphthoquinone (VK3) as a reference compound with a very weak growth-stimulating effect. The results support our proposal and reveal the superiority of BGS to VK3 as an electron transfer mediator in the proposed reactions.

Hepatic localisation of rat cysteine dioxygenase

BACKGROUND/AIM

Cysteine dioxygenase (CDO, E.C. 1.13.11.20) is the main catabolic enzyme of cysteine, metabolising cysteine to cysteinesulphinic acid. CDO abnormality has been implicated in a number of neurological and non-neurological diseases, with CDO deficiency possibly leading to excitotoxic damage to the brain and impaired Phase II metabolism in the liver.

METHODS

Two novel anti-CDO antibodies raised against linear synthetic peptides corresponding to two distinct epitopes on the 22 kDa gene product of the CDO-I gene were used for immunohistochemistry and Western blotting. These antibodies were characterised by their ability to both block and precipitate CDO enzyme activity as well as the ability of the respective antigenic peptides to absorb the antibodies and prevent the immunodetection of CDO.

RESULTS

The antibodies were found to detect the presence of a 68 kDa protein, which was subsequently shown to be CDO. Distribution was found to be centrilobular and did not alter when CDO was induced with cysteine or methionine; however, the intensity of staining increased, indicating an increase in the levels of CDO in that region.

CONCLUSIONS

These results suggest that the 68 kDa Type II is the predominant isoform in vitro and in vivo and that its centrilobular localisation may allow CDO to initiate the production of sulphate and taurine for Phase II conjugation in the liver.

Newly discovered redox cofactors: possible nutritional, medical, and pharmacological relevance to higher animals

Research spurred by the discovery of pyrroloquinoline quinone (PPQ) in 1979 led to the discovery of four additional oxidation-reduction (redox) cofactors, all of which result from transmogrification of amino acyl side chains in respective enzymes. These cofactors are (a) topa quinone in copper-containing amine oxidases, enzymes found in nearly all forms of life, including human; (b) lysyl topa quinone of the copper protein lysyl oxidase, an enzyme required for proper cross-linking of collagen and elastin; (c) tryptophan tryptophylquinone of alkylamine dehydrogenases from gram-negative soil bacteria; and (d) the copper-complexed cysteinyltyrosyl radical of fungal galactose oxidase. Originally, PQQ was thought to be a covalently bound cofactor in numerous enzymes from eukaryotes and prokaryotes. Today, PQQ is only found as a noncovalent cofactor in bacterial enzymes. The ubiquity of PQQ in the environment and its steady accessibility in the human diet has raised questions concerning its role as a vitamin, or an essential or helpful nutrient. The relevance to nutrition, medicine, and pharmacology of PQQ, topa quinone, lysyl topa quinone, tryptophan trytophylquinone, the galactose oxidase cofactor, and the enzymes harboring these cofactors are discussed in this review.

The structure and function of the PQQ-containing quinoprotein dehydrogenases

Bacterial methanol and glucose dehydrogenases containing a novel type of prosthetic group, subsequently identified as pyrrolo-quinoline quinone (PQQ), were first described about 30 years ago. Quinoproteins were originally defined as proteins containing PQQ but this definition has since been broadened to include those proteins containing other types of quinone-containing prosthetic groups, and the X-ray structures of representatives of each type of quinoprotein have recently been published. This review is mainly concerned with the structure and function of the PQQ-containing methanol dehydrogenase, whose structure has been determined at high resolution, and related proteins. Their basic structure consists of a 'propeller' fold superbarrel made up of 8-sheet 'propeller blades' which are held together by novel tryptophan-docking motifs. In methanol dehydrogenase the PQQ in the active site is coordinated to a Ca2+ ion and is maintained in position by a stacked tryptophan and a novel 8-membered ring structure made up of a disulphide bridge between adjacent cysteine residues. This review describes these features and discusses them in relation to previously proposed mechanisms for this enzyme.

The peculiar collagens of mussel byssus

The byssal collagens of marine mussels are extracorporeal collagens that function in byssal threads under tension. Each byssal thread resembles a shock absorber in its mechanical design: it is strong and stiff at one end and pliably elastic at the other. Primary structures of three of these collagens (preCols), deduced from cDNAs, reveal signal peptide sequences, but no N-glycosylation sites or propeptides typical of procollagens. The collagen domain (40-50 kDa) represents roughly half the mass of the mature molecules and is distinguished by its central location, abundant Gly-Gly-X repeats, and "flaws" (usually Gly deletions). Flanking the collagen domains on both sides are structural domains that resemble elastin in preCol-P, spider drag-line silk in preCol-D, and Gly-rich cell wall proteins in preCol-NG. Not surprisingly, studies of preCol distribution in byssal threads suggest preCol-P enhancement in the elastic proximal portion, while preCol-D predominates in the stiffer distal portion. PreCol-NG, in contrast, is evenly distributed. Although no data are yet available on the fibrillogenesis and cross-linking of the preCols, the quarter-stagger assembly of fibrillar interstitial collagens does not pertain since preCols lack the terminal peptides of tropocollagen. Metal-binding by histidines may mediate the initial inter- and intramolecular stabilization of preCols in the byssus.

Copper amine oxidase from Hansenula polymorpha: the crystal structure determined at 2.4 A resolution reveals the active conformation

BACKGROUND

Copper-containing amine oxidases (CAOs) are widespread in nature. These enzymes oxidize primary amine substrates to the aldehyde product, reducing molecular oxygen to hydrogen peroxide in the process. CAOs contain one type 2 copper atom and topaquinone (TPQ), a modified tyrosine sidechain utilized as a redox cofactor. The methylamine oxidase from the yeast Hansenula polymorpha (HPAO) is an isoform of CAO with a preference for small aliphatic amine or phenethylamine substrates. The enzyme is dimeric with a subunit molecular weight of 78 kDa. Structural studies are directed at understanding the basis for cofactor biogenesis and catalytic efficiency.

RESULTS

The X-ray crystal structure of HPAO has been solved at 2.4 A resolution by a combination of molecular replacement and single isomorphous replacement followed by refinement using sixfold symmetry averaging. The electron density at the catalytic site shows that the TPQ conformation corresponds to that of the active form of the enzyme. Two channels, one on either side of TPQ, are observed in the structure that provide access between the active site and the bulk solvent.

CONCLUSIONS

The structure shows TPQ in a position poised for catalysis. This is the first active CAO structure to reveal this conformation and may help further our understanding of the catalytic mechanism. On the substrate side of TPQ a water-containing channel leading to the protein surface can serve as an entrance or exit for substrate and product. On the opposite side of TPQ there is direct access from the bulk solvent of the dimer interface by which molecular oxygen may enter and hydrogen peroxide depart. In addition, a network of conserved water molecules has been identified which may function in the catalytic mechanism.

Identification of reaction products and intermediates of aromatic-amine dehydrogenase by 15N and 13C NMR

13C- and 15N-NMR studies of the reaction of aromatic amine dehydrogenase (AADH) with methylamine demonstrated that the products of the reductive half-reaction are an equivalent of formaldehyde hydrate and a reduced aminoquinol form of the tryptophan tryptophylquinone (TTQ) cofactor which contains covalently bound substrate-derived N. These data are consistent with the Ping Pong kinetic mechanism and aminotransferase-type chemical reaction mechanism which have been previously proposed for AADH. Comparison of the 15N-NMR spectra of the aminoquinol TTQ intermediates of AADH and methylamine dehydrogenase (MADH) revealed that the substrate-derived aminoquinol N of AADH and MADH exhibited distinct 15N chemical shifts which are separated by approx. 7 p.p.m. In each case, the signal for the substrate-derived aminoquinol N appears optimally with short pulse delay and exhibits a relaxation time and chemical shift which are consistent with 15N covalently bound to an aromatic ring (i.e. aminoquinol) which is attached to a rigid protein matrix. The aminoquinol of AADH is less stable against reoxidation than that of MADH. These data suggest that differences in the active-site mediated electrostatic environments of the aminoquinol N in the respective enzymes may influence both the observed 15N chemical shift and the relative reactivities of the TTQ aminoquinols towards oxygen. These data also demonstrate the utility of 13C- and 15N-NMR spectroscopy as a tool for monitoring the intermediates and products of enzyme-catalysed transformations.

Amine binding and oxidation at the catalytic site for photosynthetic water oxidation

Photosynthetic water oxidation occurs at the Mn-containing catalytic site of photosystem II (PSII). By the use of 14C-labeled amines and SDS-denaturing PAGE, covalent adducts derived from primary amines and the PSII subunits, CP47, D2/D1, and the Mn-stabilizing protein, can be observed. When PSII contains the 18- and 24-kDa extrinsic proteins, which restrict access to the active site, no 14C labeling is obtained. NaCl, but not Na2SO4, competes with 14C labeling in Mn-containing PSII preparations, and the concentration dependence of this competition parallels the activation of oxygen evolution. Formation of 14C-labeled adducts is observed in the presence or in the absence of a functional manganese cluster. However, no significant Cl- effect on 14C labeling is observed in the absence of the Mn cluster. Isolation and quantitation of the 14C-labeled aldehyde product, produced from [14C]benzylamine, gives yields of 1. 8 +/- 0.3 mol/mol PSII and 2.9 +/- 0.2 mol/mol in Mn-containing and Mn-depleted PSII, respectively. The corresponding specific activities are 0.40 +/- 0.07 micromol(micromol PSII-hr)-1 and 0.64 +/- 0.04 micromol(micromol PSII-hr)-1. Cl- suppresses the production of [14C]benzaldehyde in Mn-containing PSII, but does not suppress the production in Mn-depleted preparations. Control experiments show that these oxidation reactions do not involve the redox-active tyrosines, D and Z. Our results suggest the presence of one or more activated carbonyl groups in protein subunits that form the active site of PSII.

Lysyl oxidase: properties, regulation and multiple functions in biology

Lysyl oxidase (LO) is a copper-dependent amine oxidase that plays a critical role in the biogenesis of connective tissue matrices by crosslinking the extracellular matrix proteins, collagen and elastin. Levels of LO increase in many fibrotic diseases, while expression of the enzyme is decreased in certain diseases involving impaired copper metabolism. While the three-dimensional structure of the enzyme is not yet available, many of its physical-chemical properties, its novel carbonyl cofactor, and its catalytic mechanism have been described. Lysyl oxidase is synthesized as a preproprotein, secreted as a 50 kDa, N-glycosylated proenzyme and then proteolytically cleaved to the 32 kDa, catalytically active, mature enzyme. Within the past decade, the gene encoding LO has been cloned, facilitating investigations of the regulation of expression of the enzyme in response to diverse stimuli and in numerous disease states. Transforming growth factor-beta, platelet-derived growth factor, angiotensin II, retinoic acid, fibroblast growth factor, altered serum conditions, and shear stress are among the effectors or conditions that regulate LO expression. New, LO-like genes have also been identified and cloned, suggesting the existence of a multigene family. It has also become increasingly evident that LO may have other important biological functions in addition to its role in the crosslinking of elastin and collagen in the extracellular matrix.

Model sclerotization studies. 4. Generation of N-acetylmethionyl catechol adducts during tyrosinase-catalyzed oxidation of catechols in the presence of N-acetylmethionine

Incubation of catechol with mushroom tyrosinase in the presence of N-acetylmethionine resulted in the generation of an adduct. This product was identified to be N-acetylmethionyl catechol, on the basis of spectral characteristics and well-characterized chemical reaction of o-benzoquinone with N-acetylmethionine. Enzyme-catalyzed oxidation of catechol and the subsequent nonenzymatic addition of the resultant quinone to N-acetylmethionine accounted for the observed reaction. That the reaction is not confined to catechol alone, but is of general occurrence, can be demonstrated by the facile generation of similar adducts in incubation mixtures containing N-acetylmethionine, tyrosinase, and different N-acetylmethionines, such as 4-methylcatechol and N-acetyldopamine. Attempts to duplicate the reaction with insect cuticular phenoloxidases were not successful, as the excess N-acetylmethionine used in the reaction inhibited their activity. Nevertheless, occurrence of this nonenzymatic reactivity. Nevertheless, occurrence of this nonenzymatic reaction between N-acetylmethionine and mushroom tyrosinase-generated quinones indicates that a similar reaction between enzymatically generated quinones in the cuticle with protein-bound methionine moiety is likely to occur during in vivo quinone tanning as well.

Tough tendons. Mussel byssus has collagen with silk-like domains

The primary structure of the alpha-chain of preCol-D (molecular mass = 80 kDa), a tanned collagenous protein predominating in the distal portion of the byssal threads of the mussel Mytilus edulis, was deduced from cDNA to encode an unprecedented natural block copolymer with three major domain types: a central collagen domain flanked by fibroin-like domains and followed by histidine-rich termini. The fibroin-like domains have sequence motifs that strongly resemble the crystalline polyalanine-rich and amorphous glycine-rich regions of spider dragline silk fibroins. The terminal regions resemble the histidine-rich domains of a variety of metal-binding proteins. The silk domains may toughen the collagen by increasing its strength and extensibility. PreCol-D expression is limited to the mussel foot, which contains a longitudinal gradient of preCol-D mRNA. This gradient increases linearly in the proximal to distal direction and reaches a maximum just before the distal depression of the foot.

Irreversible inhibition of lysyl oxidase by homocysteine thiolactone and its selenium and oxygen analogues. Implications for homocystinuria

Homocysteine thiolactone, selenohomocysteine lactone, and homoserine lactone were found to be competitive, irreversible inhibitors of lysyl oxidase, with KI values of 21 +/- 3 microM, 8.3 +/- 2.2 microM, and 420 +/- 56 microM, respectively. The first order rate constants for inactivation (k2) of the enzyme varied over a much smaller range, ranging from 0.12 to 0.18 to 0.28 min-1 for the Se-, thio-, and O-lactones, respectively. Mutually exclusive labeling of the enzyme by [1-14C]beta-aminopropionitrile, [U-14C]phenylhydrazine, or [35S]homocysteine thiolactone was observed. These labeling results, together with the closely similar perturbations of the near UV-visible spectra of lysyl oxidase and of a model of its lysine tyrosylquinone cofactor by the thiolactone, indicate that the lactones likely derivatize and reduce the active site carbonyl cofactor. Substitution with deuterium at the alpha-carbon of the thiolactone caused a deuterium kinetic isotope effect on k2 of 3.2 +/- 0.2, consistent with the involvement of rate-limiting alpha-proton abstraction during lactone-induced inactivation of the enzyme. The activities of plasma amine oxidase and diamine oxidase were only minimally reduced at concentrations of the sulfur or selenium lactones that fully inhibited lysyl oxidase. Thus, these lactones constitute a new category of mechanism-based inactivators selective for lysyl oxidase. Further, these results may relate to the development of connective tissue defects seen in homocystinuria.

Characterization of the native lysine tyrosylquinone cofactor in lysyl oxidase by Raman spectroscopy

Lysine tyrosylquinone (LTQ) recently has been identified as the active site cofactor in lysyl oxidase by isolation and characterization of a derivatized active site peptide. Reported in this study is the first characterization of the underivatized cofactor in native lysyl oxidase by resonance Raman (RR) spectrometry. The spectrum is characterized by a unique set of vibrational modes in the 1200 to 1700 cm-1 region. We show that the RR spectrum of lysyl oxidase closely matches that of a synthetic LTQ model compound, 4-n-butylamino-5-ethyl-1,2-benzoquinone, in aqueous solutions but differs significantly from those of other topa quinone-containing amine oxidases under similar conditions. Furthermore, we have observed the same 18O shift of the C=O stretch in both the lysyl oxidase enzyme and the LTQ cofactor model compound. The RR spectra of different model compounds and their D shifts give additional evidence for the protonation state of LTQ cofactor in the enzyme. The overall similarity of these spectra and their shifts shows that the lysyl oxidase cofactor and the model LTQ compound have the same structure and properties. These data provide strong and independent support for the new cofactor structure, unambiguously ruling out the possibility that the structure originally reported had been derived from a spurious side reaction during the derivatization of the protein and isolation of the active site peptide.

Incorporation of copper into lysyl oxidase

Lysyl oxidase is a copper-dependent enzyme involved in extracellular processing of collagens and elastin. Although it is known that copper is essential for the functional activity of the enzyme, there is little information on the incorporation of copper. In the present study we examined the insertion of copper into lysyl oxidase using 67Cu in cell-free transcription/translation assays and in normal skin fibroblast culture systems. When a full-length lysyl oxidase cDNA was used as a template for transcription/translation reactions in vitro, unprocessed prolysyl oxidase appeared to bind copper. To examine further the post-translational incorporation of copper into lysyl oxidase, confluent skin fibroblasts were incubated with inhibitors of protein synthesis (cycloheximide, 10 microg/ml), glycosylation (tunicamycin, 10 microg/ml), protein secretion (brefeldin A, 10 microg/ml) and prolysyl oxidase processing (procollagen C-peptidase inhibitor, 2.5 microg/ml) together with 300 microCi of carrier-free 67Cu. It was observed that protein synthesis was a prerequisite for copper incorporation, but inhibition of glycosylation by tunicamycin did not affect the secretion of 67Cu as lysyl oxidase. Brefeldin A inhibited the secretion of 67Ci-labelled lysyl oxidase by 46%, but the intracellular incorporation of copper into lysyl oxidase was not affected. In addition, the inhibition of the extracellular proteolytic processing of prolysyl oxidase to lysyl oxidase had minimal effects on the secretion of protein-bound 67Cu. Our results indicate that, similar to caeruloplasmin processing [Sato and Gitlin (1991) J. Biol. Chem. 266, 5128-5134], copper is inserted into prolysyl oxidase independently of glycosylation.

Regulation of a novel gene encoding a lysyl oxidase-related protein in cellular adhesion and senescence

We report here a novel cDNA clone with a predicted protein sequence similar to lysyl oxidase. This full-length cDNA clone of 3432 base pairs (WS9-14) was isolated from human fibroblasts on the basis of its overexpression in senescent cells. It encodes an 87-kDa polypeptide, whose protein is a member of the scavenger receptor cysteine-rich family, because it contains four scavenger receptor cysteine-rich domains that are found in several secreted or cell surface proteins. The WS9-14 protein has a 48% identity with both lysyl oxidase and lysyl oxidase-like protein at a region corresponding to exons 2-6, implying the existence of a lysyl oxidase gene family. The pattern of WS9-14 gene expression by fibroblasts parallels pro-collagen I-alpha1 expression. Its mRNA level is induced by transforming growth factor beta-1 and indomethacin and inhibited by phorbol ester and retinoic acid. WS9-14 is abundantly expressed in all tumor cell lines examined that attach to culture dishes but not in cell lines that grow in suspension and is also up-regulated in senescent fibroblasts. These results suggest that WS9-14 gene encodes an extracellular protein that may be specifically involved in cell adhesion and senescence.

Expression of active, human lysyl oxidase in Escherichia coli

Lysyl oxidase (LO) is a copper amine oxidase of the extracellular matrix which initiates covalent cross-linking in collagens and elastin. Human LO was expressed in Escherichia coli. At 37 degrees C, large amounts of protein were obtained, but in the form of insoluble aggregates. Lowering the growth temperature, and reducing the amount of inducer, resulted in the production of soluble LO, which was active on a degrees [3H]lysine-labeled elastin substrate. LO was also targeted to the periplasm as a fusion protein with the pelb signal peptide. The periplasmic enzyme was soluble, active and inhibited by beta-aminopropionitrile. Production of the carbonyl co-factor is therefore not a limitation in the expression of active LO in bacteria.

Quinoprotein-catalysed reactions

This review is concerned with the structure and function of the quinoprotein enzymes, sometimes called quinoenzymes. These have prosthetic groups containing quinones, the name thus being analogous to the flavoproteins containing flavin prosthetic groups. Pyrrolo-quinoline quinone (PQQ) is non-covalently attached, whereas tryptophan tryptophylquinone (TTQ), topaquinone (TPQ) and lysine tyrosylquinone (LTQ) are derived from amino acid residues in the backbone of the enzymes. The mechanisms of the quinoproteins are reviewed and related to their recently determined three-dimensional structures. As expected, the quinone structures in the prosthetic groups play important roles in the mechanisms. A second common feature is the presence of a catalytic base (aspartate) at the active site which initiates the reactions by abstracting a proton from the substrate, and it is likely to be involved in multiple reactions in the mechanism. A third common feature of these enzymes is that the first part of the reaction produces a reduced prosthetic group; this part of the mechanism is fairly well understood. This is followed by an oxidative phase involving electron transfer reactions which remain poorly understood. In both types of dehydrogenase (containing PQQ and TTQ), electrons must pass from the reduced prosthetic group to redox centres in a second recipient protein (or protein domain), whereas in amine oxidases (containing TPQ or LTQ), electrons must be transferred to molecular oxygen by way of a redox-active copper ion in the protein.

Identification of the quinone cofactor in a lysyl oxidase from Pichia pastoris

A copper amine oxidase from Pichia pastoris is the only known non-mammalian lysyl oxidase [Tur, S.S. and Lerch, K. (1988) FEBS Lett. 238, 74-76]. Recently, the cofactor in mammalian lysyl oxidase has been identified as a novel lysine tyrosylquinone moiety [Wang, S.X., Mure, M., Medzihradszky, K.F., Burlingame, A.L., Brown, D.E., Dooley, D.M., Smith, A.J., Kagan, H.M. and Klinman, J.P. (1996) Science 273, 1078-1084]. In order to identify the cofactor in P. pastoris lysyl oxidase, we have isolated the phenylhydrazone-derivative of the active-site peptide. This peptide has the active-site sequence conserved among topa quinone containing amine oxidases. The resonance Raman spectra of the phenylhydrazone derivatives of the enzyme, active-site peptide, and a topa quinone model compound are essentially identical. Collectively, these results establish that P. pastoris lysyl oxidase is a topa quinone enzyme.