The core protein of fibroblast proteoheparan sulphate consists of disulphide-bonded subunits.

Fibroblast proteoheparan sulphate has a disulphide-bonded subunit structure. The core protein appears to consist of two polypeptides each of Mr 80 000-100 000. As shown elsewhere [Carlstedt, Cöster, Malmström & Fransson (1983) J. Biol. Chem. in the press], both polypeptide molecules carry four to six heparan sulphate side chains (approx. Mr 20 000) and an unknown number of oligosaccharide units, giving the whole macromolecule an Mr in the range 300 000-400 000.     

A simple procedure for the isolation of protein disulphide-isomerase.

A two-step procedure is described for the purification of protein disulphide-isomerase (PDI). This procedure is based on the previous finding that the beta-subunit of the prolyl 4-hydroxylase tetramer (alpha 2 beta 2) is identical with PDI [Koivu, Myllylä, Helaakoski, Pihlajaniemi, Tasanen & Kivirikko (1987) J. Biol. Chem. 262, 6447-6449; Pihlajaniemi, Helaakoski, Tasanen, Myllylä, Huhtala, Koivu & Kivirikko (1987) EMBO J. 6, 643-649]. The procedure involves purification of the prolyl 4-hydroxylase tetramer by a simple affinity chromatography and subsequent isolation of the beta-subunit from the dissociated tetramer by ion-exchange chromatography.     

Extended and globular protein domains in cartilage proteoglycans.

Electron microscopy after rotary shadowing and negative staining of the large chondroitin sulphate proteoglycan from rat chondrosarcoma, bovine nasal cartilage and pig laryngeal cartilage demonstrated a unique multidomain structure for the protein core. A main characteristic is a pair of globular domains (diameter 6-8 nm), one of which forms the N-terminal hyaluronate-binding region. They are connected by a 25 nm-long rod-like domain of limited flexibility. This segment is continued by a 280 nm-long polypeptide strand containing most chondroitin sulphate chains (average length 40 nm) in a brush-like array and is terminated by a small C-terminal globular domain. The core protein showed a variable extent of degradation, including the loss of the C-terminal globular domain and sections of variable length of the chondroitin sulphate-bearing strand. The high abundance (30-50%) of the C-terminal domain in some extracted proteoglycan preparations indicated that this structure is present in the cartilage matrix rather than being a precursor-specific segment. It may contain the hepatolectin-like segment deduced from cDNA sequences corresponding to the 3''-end of protein core mRNA [Doege, Fernandez, Hassell, Sasaki & Yamada (1986) J. Biol. Chem. 261, 8108-8111; Sai, Tanaka, Kosher & Tanzer (1986) Proc. Natl. Acad. Sci. 83, 5081-5085; Oldberg, Antonsson & Heinegård (1987) Biochem. J. 243, 255-259].     

An adenosine 3':5' monophosphate dependent protein kinase from sea urchin spermatozoa.

A cyclic AMP dependent protein kinase (EC 2.7.1.37) from sea urchin sperm as purified to near homogeneity and characterized. A 68-fold purification of the enzyme was obtained. This preparation had a specific activity of 389 000 units/mg protein with protamine as the substrate. On the basis of the purification required, it may be calculated that the protein kinase constitutes as much as 1.5% of the soluble protein in sperm. There appeared to be a single form of the enzyme in sea urchin sperm, based on the behavior of the enzyme during DEAE-cellulose and Sephadex G-200 column chromatography. Magnesium ion was required for enzyme activity. The rate of phosphorylation of protamine was stimulated 2.5-fold by an optimal concentration of 0.9 M NaCl. The Km for ATP (minus cyclic AMP) was 0.119 +/- 0.013 (S.D.) and 0.055 mM +/- 0.009 (S.D.) in the presence of cyclic AMP. The specificity of the enzyme toward protein acceptors, in decreasing order of phosphorylation, was found to be histone f1 protamine, histone f2b, histone f3 and histone f2a; casein and phosvitin were not phosphorylated. The holoenzyme was found to have an apparent molecular weight of 230 000 by Sephadex G-200 chromatography. In the presence of 5 - 10(-6) M cyclic AMP, the holoenzyme was dissociated on Sephadex G-200 to a regulatory subunit of molecular weight 165 000 and a catalytic subunit of Mr 73 000. The dissociation could also be demonstrated by disc gel electrophoresis in the presence and absence of cyclic AMP.     

Arginine residues are critical for the heparin-cofactor activity of antithrombin III.

A dilution/quench technique was used to monitor the time course of chemical modification on the heparin-cofactor (a) and progressive thrombin-inhibitory (b) activities of human antithrombin III. Treatment of antithrombin III (AT III) with 2,4,6-trinitrobenzenesulphonate at pH 8.3 and 25 degrees C leads to the loss of (a) at 60-fold more rapid rate than the loss of (b). This is consistent with previous reports [Rosenberg & Damus (1973) J. Biol. Chem. 248, 6490-6505; Pecon & Blackburn (1984) J. Biol. Chem. 259, 935-938] that lysine residues are involved in the binding of heparin to AT III, but not in thrombin binding. Treatment of AT III with phenylglyoxal at pH 8.3 and 25 degrees C again leads to a more rapid loss of (a) than of (b), with the loss of the former proceeding at a 4-fold faster rate. The presence of heparin during modification with phenylglyoxal significantly decreases the rate of loss of (a). Full loss of (a) correlates with the modification of seven arginine residues per inhibitor molecule, whereas loss of (b) does not commence until approximately four arginine residues are modified and is complete upon the modification of approximately eleven arginine residues per inhibitor molecule. This suggests that (the) arginine residue(s) in AT III are involved in the binding of heparin in addition to the known role of Arg-393 at the thrombin-recognition site [Rosenberg & Damus (1973) J. Biol. Chem. 248, 6490-6505; Jörnvall, Fish & Björk (1979) FEBS Lett. 106, 358-362].     

Domain structure of proteoheparan sulphate from confluent cultures of human embryonic skin fibroblasts.

Radiolabelled proteoheparan sulphates were isolated from confluent monolayers of fibroblasts and from their spent media. The cell-surface-associated proteoglycan (Mr 350 000) has a core protein of Mr 180 000 that is cleaved by reduction of disulphide bonds into polypeptides of Mr 90 000, both of which can bind transferrin [Fransson, Carlstedt, Cöster & Malmström (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 5657-5661]. Thrombin digestion of the proteoglycan yielded two major fragments. The larger one contained the heparan sulphate chains and glycoprotein-type oligosaccharides, whereas the smaller one contained interchain disulphide bond(s) and had affinity for transferrin as well as for octyl-Sepharose. The larger thrombic fragment was cleaved by trypsin into fragments containing the heparan sulphate chains and the oligosaccharides respectively. The smaller proteoheparan sulphate derived from the culture medium (Mr 150 000) had a core protein of Mr 30 000, which contained heparan sulphate-attachment and oligosaccharide-attachment regions, but no domains for binding of transferrin or for hydrophobic interactions.     

Purification and properties of a nuclear protein kinase associated with ribonucleic acid polymerase I.

A cyclic AMP-dependent nuclear protein kinase was found to be closely associated with rat liver nucleolar RNA polymerase I throughout most of its purification. This protein kinase was purified to near homogeneity. It exhibits a number of unusual catalytic properties, including the inability to utilize Mn2+ when RNA polymerase is the substrate and the ability to phosphorylate both acidic and basic substrates. Phosphorylation of RNA polymerase I by this protein kinase results in the formation of phosphoester bonds characteristic of phosphoserine and phosphothreonine. Radioautography of polyacrylamide-gel electrophoretograms of the phosphorylated RNA polymerase I revealed that the 32P was located primarily on enzyme subunits SA1, SA3, SA5, and SA6 [nomenclature of Kedinger, Gissinger & Chambon (1974) Eur. J. Biochem, 44, 421-436].     

Cartilage proteoglycans. Assembly with hyaluronate and link protein as studied by electron microscopy.

Aggregates formed by the interaction of cartilage proteoglycan monomers and fragments thereof with hyaluronate were studied by electron microscopy by use of rotary shadowing [Wiedemann, Paulsson, Timpl, Engel & Heinegård (1984) Biochem. J. 224, 331-333]. The differences in shape and packing of the proteins bound along the hyaluronate strand in aggregates formed in the presence and in the absence of link protein were examined in detail. The high resolution of the method allowed examination of the involvement in hyaluronate binding of the globular core-protein domains G1, G2 and G3 [Wiedemann, Paulsson, Timpl, Engel & Heinegård (1984) Biochem. J. 224, 331-333; Paulsson, Mörgelin, Wiedemann, Beardmore-Gray, Dunham, Hardingham, Heinegård, Timpl & Engel (1987) Biochem. J. 245, 763-772]. Fragments comprising the globular hyaluronate-binding region G1 form complexes with hyaluronate with an appearance of necklace-like structures, statistically interspaced by free hyaluronate strands. The closest centre-to-centre distance found between adjacent G1 domains was 12 nm. Another fragment comprising the binding region G1 and the adjacent second globular domain G2 attaches to hyaluronate only by one globule. Also, the core protein obtained by chondroitinase digestion of proteoglycan monomer binds only by domain G1, with domain G3 furthest removed from the hyaluronate. Globule G1 shows a statistical distribution along the hyaluronate strands. In contrast, when link protein is added, binding is no longer random, but instead uninterrupted densely packed aggregates are formed.     

Basement-membrane heparan sulphate with high affinity for antithrombin synthesized by normal and transformed mouse mammary epithelial cells.

Basement-membrane proteoglycans, biosynthetically labelled with [35S]sulphate, were isolated from normal and transformed mouse mammary epithelial cells. Proteoglycans synthesized by normal cells contained mainly heparan sulphate and, in addition, small amounts of chondroitin sulphate chains, whereas transformed cells synthesized a relatively higher proportion of chondroitin sulphate. Polysaccharide chains from transformed cells were of lower average Mr and of lower anionic charge density compared with chains isolated from the untransformed counterparts, confirming results reported previously [David & Van den Berghe (1983) J. Biol. Chem. 258, 7338-7344]. A large proportion of the chains isolated from normal cells bound with high affinity to immobilized antithrombin, and the presence of 3-O-sulphated glucosamine residues, previously identified as unique markers for the antithrombin-binding region of heparin [Lindahl, Bäckström, Thunberg & Leder (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 6551-6555], could be demonstrated. A significantly lower proportion of the chains derived from transformed cells bound with high affinity to antithrombin, and a corresponding decrease in the amount of incorporated 3-O-sulphate was observed.     

Determination of the range in binding-site densities of rat skin heparin chains with high binding affinities for antithrombin.

Rat skin heparin proteoglycans vary markedly in the proportions of their constituent polysaccharide chains that have high binding affinity for antithrombin. As the proportion of such chains in a proteoglycan rises, their degree of affinity for antithrombin also increases [Horner (1987) Biochem. J. 244, 693-698]. The antithrombin-binding-site densities of such chains have now been determined, by measuring heparin-induced enhancement of the intrinsic fluorescence of antithrombin and by chemical analysis for the disaccharide sequence glucuronosyl-N-sulphoglucosaminyl (3,6-di-O-sulphate), which is unique to this site in heparin [Lindahl, Bäckström, Thunberg & Leder (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 6551-6555]. Antithrombin-binding-site density ranged from one to five sites per chain.     

Substrates for cyclic AMP-dependent protein kinase in islets of Langerhans. Studies with forskolin and catalytic subunit.

To investigate substrates for cyclic AMP-dependent protein kinase in intact islets of Langerhans, batches of islets were incubated with [32P]Pi for 1 h in the presence of 10 mM-glucose; the adenylate cyclase activator forskolin, which in parallel experiments was shown to increase islet cyclic AMP content and insulin release, was then added. Islets were homogenized and subcellular fractions prepared by differential centrifugation. Phosphopeptides were electrophoresed on sodium dodecyl sulphate/polyacrylamide gels and quantified by autoradiography and densitometry. Within 5 min forskolin caused increased labelling of Mr-25 000 and -30 000 cytosolic and Mr-23 000 and -32 000 particulate peptides; a rapid decrease in phosphorylation of Mr-18 000 and -34 000 cytosolic peptides was also observed. In addition, rather slower phosphorylation occurred of the Mr-15 000 peptide previously identified as histone H3 [Christie & Ashcroft (1984) Biochem. J. 218, 87-99]. When similar subcellular fractions were incubated with [gamma-32P]ATP and purified catalytic subunit of cyclic AMP-dependent protein kinase, peptides phosphorylated included cytosolic species of Mr 25 000 and 30 000 and particulate species of Mr 23 000 and 32 000. The distribution of RNA in the subcellular fractions suggested that the Mr-32 000 species could be a ribosomal protein. The 24 000 g pellet was heterogeneous, as judged by marker assays, and was therefore fractionated further by Percoll-density-gradient centrifugation. The peak containing the Mr-23 000 peptide was resolved from marker enzymes for plasma membranes, mitochondria and endoplasmic reticulum and coincided with a peak for insulin: hence the Mr-23 000 peptide is likely to be a secretory-granule component. The study demonstrates that the potentiation of insulin release that occurs when islet cyclic AMP is increased is accompanied by rapid phosphorylation of specific islet substrates for cyclic AMP-dependent protein kinase. The data are consistent with the hypothesis that protein phosphorylation is involved in the regulation of insulin secretion.     

Quantitative analysis of cyclic beta-turn models.

The beta-turn is a frequently found structural unit in the conformation of globular proteins. Although the circular dichroism (CD) spectra of the alpha-helix and beta-pleated sheet are well defined, there remains some ambiguity concerning the pure component CD spectra of the different types of beta-turns. Recently, it has been reported (Hollósi, M., Kövér, K.E., Holly, S., Radics, L., & Fasman, G.D., 1987, Biopolymers 26, 1527-1572; Perczel, A., Hollósi, M., Foxman, B.M., & Fasman, G.D., 1991a, J. Am. Chem. Soc. 113, 9772-9784) that some pseudohexapeptides (e.g., the cyclo[(delta)Ava-Gly-Pro-Aaa-Gly] where Aaa = Ser, Ser(OtBu), or Gly) in many solvents adopt a conformational mixture of type I and the type II beta-turns, although the X-ray-determined conformation was an ideal type I beta-turn. In addition to these pseudohexapeptides, conformational analysis was also carried out on three pseudotetrapeptides and three pseudooctapeptides. The target of the conformation analysis reported herein was to determine whether the ring stress of the above beta-turn models has an influence on their conformational properties. Quantitative nuclear Overhauser effect (NOE) measurements yielded interproton distances. The conformational average distances so obtained were interpreted utilizing molecular dynamics (MD) simulations to yield the conformational percentages. These conformational ratios were correlated with the conformational weights obtained by quantitative CD analysis of the same compounds. The pure component CD curves of type I and type II beta-turns were also obtained, using a recently developed algorithm (Perczel, A., Tusnády, G., Hollósi, M., & Fasman, G.D., 1991b, Protein Eng. 4(6), 669-679). For the first time the results of a CD deconvolution, based on the CD spectra of 14 beta-turn models, were assigned by quantitative NOE results. The NOE experiments confirmed the ratios of the component curves found for the two major beta-turns by CD analysis. These results can now be used to enhance the conformational determination of globular proteins on the basis of their CD spectra.     

Modification and identification of glutamate residues at the arginine-recognition site in the catalytic subunit of adenosine 3′:5′-cyclic monophosphate-dependent protein kinase of rabbit skeletal muscle

It has been proposed that the active centre of cyclic AMP-dependent protein kinase contains an arginine-recognition site, which is considered to be essential for the function of the catalytic subunit of the kinase [Matsuo, Huang & Huang (1978) Biochem. J.173, 441-447]. The catalytic subunit can be inactivated by 3-(3-dimethylaminopropyl)-1-ethylcarbodi-imide and glycine ethyl ester at pH6.5. The enzyme can be protected from inactivation by preincubation with histone, a protein substrate of the enzyme. On the other hand, ATP, which also serves as a protein kinase substrate, does not afford protection. Polyarginine, a competitive inhibitor of protein kinase, which is known from kinetic studies to interact specifically with the arginine-recognition site, partially protects the catalytic subunit from inactivation by 3-(3-dimethylaminopropyl)-1-ethylcarbodi-imide. These results lead to the conclusion that the site of modification by carbodi-imide/glycine ethyl ester is most likely located at the arginine-recognition site of the active centre. A value of 1.7+/-0.2 (mean+/-s.d.) mol of carboxy groups per mol of catalytic subunit has been obtained for the number of essential carboxy groups for the function of protein kinase; a complete chemical modification of these essential carboxy groups results in total loss of catalytic activity. Finally, we have identified the essential carboxy group in the catalytic subunit of cyclic AMP-dependent protein kinase as being derived from glutamate residues. This is achieved by a three-step procedure involving an extensive proteolytic digestion of the [1-(14)C]glycine ethyl ester-modified enzyme and two successive high-voltage electrophoreses of the hydrolysate. It is concluded that 1.7mol of glutamyl carboxy groups per mol of catalytic subunit may be considered a component of the arginine-recognition site in the active centre of cyclic AMP-dependent protein kinase.     

Polymeric structure of the cyclic AMP-dependent protein kinase from the dimorphic fungus Mucor rouxii and purification of its catalytic subunit

Summary The polymeric structure of the cyclic AMP-dependent protein kinase (E.C.2.7.1.37) from the dimorphic fungus Mucor rouxii was analyzed through studies of gel filtration and sucrose gradient centrifugation of the holoenzyme and its subunits and by photoaffinity labeling of the regulatory subunit. It was demonstrated that it is a tetramer composed by two regulatory subunits (R) of mol. wt. 75 000 and two catalytic subunits (C) of mol. wt. 41 000 forming a holoenzyme R₂C₂ of mol. wt. 242 000. Frictional coefficients of 1.55 and 1.62 for the holoenzyme and for the regulatory dimer, respectively, indicate a significant degree of dimensional asymmetry in both molecules. A procedure for the purification of the catalytic subunit of the kinase is presented. The holoenzyme could be bound to a cyclic AMP-agarose column and the catalytic subunit could be eluted by 0.5 M NaCl, well resolved from the bulk of protein. This particular behaviour of the holoenzyme in cyclic AMP-agarose chromatography allowed the inclusion of this step in the purification of the catalytic subunit and corroborated that the holoenzyme was not dissociated by cyclic AMP alone. The isolated catalytic subunit displays Michaelis-Menten behaviour towards kemptide, protamine and histone and is inhibited by sulfhydryl reagents, indicating that the molecule has at least one cysteine residue essential for enzyme activity. The catalytic activity of the isolated C subunit is inactivated by the mammalian protein kinase inhibitor, and is inhibited by the regulatory subunit from homologous and heterologous sources. In general, the properties of the catalytic subunit suggest a structural similarity between Mucor and mammalian C subunits.Abbreviations C catalytic subunit monomer of protein kinase - R regulatory subunit monomer of protein kinase - 8-N₃-cyclic AMP 8-azido-cylic AMP - SDS sodium dodecyl sulfate - Pipes piperazine-N,N-bis(2-ethanesulfonic acid) See AcknowledgementsCareer Investigators from the CONICET     

Association of the type 1 protein phosphatase PP1 with the A-kinase anchoring protein AKAP220

The cyclic AMP (cAMP)-dependent protein kinase (PKA) and the type 1 protein phosphatase (PP1) are broad-specificity signaling enzymes with opposing actions that catalyze changes in the phosphorylation state of cellular proteins. Subcellular targeting to the vicinity of preferred substrates is a means of restricting the specificity of each enzyme [1] [2]. Compartmentalization of the PKA holoenzyme is mediated through association of the regulatory subunits with A-kinase anchoring proteins (AKAPs), whereas a diverse family of phosphatase-targeting subunits directs the location of the PP1 catalytic subunit (PP1c) [3] [4]. Here, we demonstrate that the PKA-anchoring protein, AKAP220, binds PP1c with a dissociation constant (KD) of 12.1 +/- 4 nM in vitro. Immunoprecipitation of PP1 from cell extracts resulted in a 10.4 +/- 3.8-fold enrichment of PKA activity. AKAP220 co-purified with PP1c by affinity chromatography on microcystin sepharos Immunocytochemical analysis demonstrated that the kinase, the phosphatase and the anchoring protein had distinct but overlapping staining patterns in rat hippocampal neurons. Collectively, these results provide the first evidence that AKAP220 is a multivalent anchoring protein that maintains a signaling scaffold of PP1 and the PKA holoenzyme.     

Proteolytic processing of a coleopteran-specific delta-endotoxin produced by Bacillus thuringiensis var. tenebrionis.

Insecticidal protein delta-endotoxin crystals harvested from sporulated cultures of Bacillus thuringiensis var. tenebrionis contain a major polypeptide of 67 kDa and minor polypeptides of 73, 72, 55 and 46 kDa. During sporulation, only the 73 kDa polypeptide could be detected at stage I. The 67 kDa polypeptide was first detected at stage II and increased in concentration throughout the later stages of sporulation and after crystal release, with a concomitant decrease in the 73 kDa polypeptide. This change could be blocked by the addition of proteinase inhibitors. Trypsin or insect-gut-extract treatment of the delta-endotoxin crystals after solubilization resulted in a cleavage product of 55 kDa with asparagine-159 of the deduced amino acid sequence of the toxin [Höfte, Seurinck, van Houtven & Vaeck (1987) Nucleic Acids Res. 15, 71-83; Sekar, Thompson, Maroney, Bookland & Adang (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 7036-7040; McPherson, Perlak, Fuchs, Marrone, Lavrik & Fischhoff (1988) Biotechnology 6, 61-66] at the N-terminus. This polypeptide was found to be as toxic in vivo as native delta-endotoxin.     

Phosphorylation of neurofilament proteins by protein kinase C

The low molecular mass (70 kDa) subunit of neurofilaments (NF-L) contains at least three phosphorylation sites in vivo and is phosphorylated by multiple kinases in a site-specific manner [(1987) J. Neurochem. 48, S101; Sihag, R.K. and Nixon, R.A. submitted]. In this study, we observed that the three subunits of neurofilament proteins from retinal ganglion cell neurons are substrates for purified mouse brain protein kinase C. Two-dimensional alpha-chymotryptic phosphopeptide map analyses of the NF-L subunit demonstrated that protein kinase C phosphorylates four polypeptide sites, two of which incorporate phosphate when retinal ganglion cells are pulse-radiolabeled with [32P]orthophosphate in vivo.     

Studies of rat adipose-tissue microsomal glycerol phosphate acyltransferase.

Incubation of rat adipose-tissue microsomal fractions with iodoacetate caused an inactivation of glycerol phosphate acyltransferase that could be prevented by the presence of palmitoyl-CoA. A microsomal protein of subunit Mr 54 000 was found to react with radioactively labelled iodoacetate in the absence, but not in the presence, of palmitoyl-CoA. It is suggested that this protein is a component of glycerol phosphate acyltransferase. Incubation of rat adipose-tissue microsomal fractions with the catalytic subunit of cyclic AMP-dependent protein kinase, ATP and Mg2+ caused an inactivation of glycerol phosphate acyltransferase whose magnitude depended on the conditions used for assay of the acyltransferase. Rat adipose tissue microsomal proteins were phosphorylated by using protein kinase and [gamma-32P]ATP. One of the phosphorylated proteins was very similar, but not identical, in mobility to the Mr-54 000 protein labelled by iodoacetate. In contrast with a previous report [Sooranna & Saggerson (1976) FEBS Lett. 64, 36-39], no changes could be detected in the activity of glycerol phosphate acyltransferase in adipocytes treated with adrenaline. Adipocytes were labelled with [32P]Pi and treated with adrenaline, but no 32P was incorporated into the Mr-54000 protein labelled by iodoacetate. The results suggest that the activity of adipose-tissue microsomal glycerol phosphate acyltransferase is not directly controlled by phosphorylation.     

Inhibition of rat brain tryptophan metabolism by ethanol withdrawal and possible involvement of the enhanced liver tryptophan pyrrolase activity.

The redox properties of the nitrogenase Mo-Fe protein from Klebsiella pneumoniae have been monitored by 57Fe Mössbauer spectroscopy between -460 and -160mV (relative to the normal hydrogen electrode). Two redox processes associated with the atoms of the protein were observed. One at -216mV (pH 8.7) was associated with the Fe-Mo cofactor centres in the protein and allowed identification of the Mössbauer parameters of the oxidized form of these centres. The other redox process at -340mV (pH 8.7) was associated with species M5 [Smith & Lang (1974) Biochem. J. 137, 169-180]. This latter redox process may be involved in enzyme turnover. The oxidized form of species M5 interacts magnetically with species M4. The structural implications of the data have been considered in relation to other published data. It is concluded that an unequivocal assignment of the M4 and M5 atoms to Fe-S cluster types is not yet possible.     

Further theoretical refinement on the internal thermodynamics of perfect enzymes.

By solving simultaneously the equation for ''uniform binding'' [Albery & Knowles (1976) Biochemistry 15, 5631-5640] and the equation for ''differential binding'' [Chin (1983) J. Am. Chem. Soc. 105, 6502-6503], I derived the following simple equation for perfect enzymes (with single substrate and single product) under irreversible conditions: K2 = beta(1 + Rs)/1-beta(1 + Rs) where K2 is the internal equilibrium constant and beta is the Brönsted coefficient of the elementary catalytic step, and Rs is defined as [S]0/Ks, with [S]0 being the physiological substrate concentration and Ks being the substrate dissociation constant. The equation suggests that the perfect enzyme can have different internal thermodynamic properties depending on physiological conditions.