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G A Radke

Publications and source records attributed to G A Radke.

At least 19 recordsLinked to original sources
PubMed1996

Lipoyl domain-based mechanism for the integrated feedback control of the pyruvate dehydrogenase complex by enhancement of pyruvate dehydrogenase kinase activity.

To conserve carbohydrate reserves, the reaction of the pyruvate dehydrogenase complex (PDC) must be down-regulated when the citric acid cycle is provided sufficient acetyl-CoA. PDC activity is reduced primarily through increased phosphorylation of its pyruvate dehydrogenase (E1) component due to E1 kinase activity being markedly enhanced by elevated intramitochondrial NADH:NAD+ and acetyl-CoA:CoA ratios. A mechanism is evaluated in which enhanced kinase activity is facilitated by the build-up of the reduced and acetylated forms of the lipoyl moieties of the dihydrolipoyl acetyltransferase (E2) component through using NADH and acetyl-CoA in the reverse of the downstream reactions of the complex. Using a peptide substrate, kinase activity was stimulated by these products, ruling out the possibility kinase activity is increased due to changes in the reaction state of its substrate, E1 (thiamin pyrophosphate). Each E2 subunit contains two lipoyl domains, an NH2-terminal (L1) and the inward lipoyl domain (L2), which were individually produced in fully lipoylated forms by recombinant techniques. Although reduction and acetylation of the L1 domain or free lipoamide increased kinase activity, those modifications of the lipoate of the kinase-binding L2 domain gave much greater enhancements of kinase activity. The large stimulation of the kinase generated by acetyl-CoA only occurred upon addition of the transacetylase-catalyzing (lipoyl domain-free) inner core portion of E2 plus a reduced lipoate source, affirming that acetylation of this prosthetic group is an essential mechanistic step for acetyl-CoA enhancing kinase activity. Similarly, the lesser stimulation of kinase activity by just NADH required a lipoate source, supporting the need for lipoate reduction by E3 catalysis. Complete enzymatic delipoylation of PDC, the E2-kinase subcomplex, or recombinant L2 abolished the stimulatory effects of NADH and acetyl-CoA. Retention of a small portion of PDC lipoates lowered kinase activity but allowed stimulation of this residual kinase activity by these products. Reintroduction of lipoyl moieties, using lipoyl protein ligase, restored the capacity of the E2 core to support high kinase activity along with stimulation of that activity up to 3-fold by NADH and acetyl-CoA. As suggested by those results, the enhancement of kinase activity is very responsive to reductive acetylation with a half-maximal stimulation achieved with approximately 20% of free L2 acetylated and, from an analysis of previous results, with acetylation of only 3-6 of the 60 L2 domains in intact PDC. Based on these findings, we suggest that kinase stimulation results from modification of the lipoate of an L2 domain that becomes specifically engaged in binding the kinase. In conclusion, kinase activity is attenuated through a substantial range in response to modest changes in the proportion of oxidized, reduced, and acetylated lipoyl moieties of the L2 domain of E2 produced by fluctuations in the NADH:NAD+ and acetyl-CoA:CoA ratios as translated by the rapid and reversible E3 and E2 reactions.

Acetyl Coenzyme A↗
PubMed1993

Partial activation of the pyruvate dehydrogenase kinase by the lipoyl domain region of E2 and interchange of the kinase between lipoyl domain regions.

The binding of the pyruvate dehydrogenase (E1) component and the E1-specific kinase to the core-forming dihydrolipoyl acetyltransferase (E2) component facilitates a severalfold enhancement in the rate at which the kinase phosphorylate E1 (i.e. versus free kinase phosphorylating free E1). The kinase and E1 associate with small exterior linker region-connected domains in the E2 structure. The kinase binds to one of two lipoyl domains, and the E1 component binds to a domain in E2"s structure between the lipoyl domain region and the inner domain. Sixty of the latter domains assemble to form a dodecahedron-shaped inner core. Binding of the kinase to a detached lipoyl domain region enhanced kinase activity. This bi-lipoyl domain fragment induced a 2-fold enhancement in the slow rate of phosphorylation of peptide substrate and intact E260 gave only a 50% higher rate. In contrast, the lipoyl domain fragment gave only a 40% enhancement in the faster rate of phosphorylation of E1; whereas the rate of phosphorylation of E1 was markedly increased (4-10-fold depending on conditions) by kinase interaction with the intact E2 core. Binding of E1 to an E2 structure lacking only the bi-lipoyl domain region did not enhance kinase activity. Thus, binding of the kinase to the lipoyl domain region elicits a structural change which enhances kinase activity; however, other processes are required to explain the very large enhancement in phosphorylation of E1 effected by intact E2 core. Among the latter is a need for a mechanism allowing one kinase molecule to phosphorylate many E1 tetramers, whereas both E1 and the kinase stay bound to the oligomeric E2 core (i.e. phosphorylation appears to be much faster than the dissociation of either the kinase or E1 tetramers from E260 core). Exposure of kinase bound to the lipoyl domain fragment to intact E2 core for 10 s allowed a transition to a maximal (7-fold) activation of the kinase. In the opposite direction, an increasing level of the free bi-lipoyl domain fragment rapidly reduced, in a concentration-dependent manner the activity of kinase bound initially to intact E2. The data strongly support kinase transfer between free lipoyl domains and the intact E2 core and fit about a 12-fold tighter binding of the kinase to intact E2 cores over binding to free lipoyl domains. Such an interchange of the kinase between these E2 structures was confirmed by sucrose gradient studies.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetyltransferases↗
PubMed1993

Critical role of a lipoyl cofactor of the dihydrolipoyl acetyltransferase in the binding and enhanced function of the pyruvate dehydrogenase kinase.

Mammalian pyruvate dehydrogenase kinase binds to the lipoyl domain region of the core structure forming dihydrolipoyl acetyltransferase (E2) subunits. The bound kinase has a greatly enhanced rate in phosphorylating E2-bound pyruvate dehydrogenase (E1) tetramers versus the rate at which resolved kinase phosphorylates dissociated E1. This E2-activated kinase function was completely prevented by selective alkylation of reduced lipoyl groups while kinase and E1 binding to the E2 core were retained. Selective removal of lipoyl cofactors from intact E2 by treatment with Enterococcus faecalis lipoamidase decreased kinase activity by 4-fold and caused selective release of a major portion of the kinase from E2 in a sucrose-step gradient procedure. Selective and reversible modification of the lipoyl groups of E2 subunits also allowed the kinase to be dissociated under mildly chaotropic conditions. Thus, the lipoyl prosthetic group on one of the two lipoyl domains of E2 subunits is critically important for maintaining E2-activated kinase function and contributes to binding of the kinase to E2. Since removal of the lipoyl group weakened kinase binding to E2 more than modifying lipoyl thiols, it is suggested that the hydrophobic inner portion of the lipoyl conjugate (i.e., lysine carbons and C1 to C5 of the lipoic acid) is important in the binding of the kinase.

Acetylation↗
PubMed1992

Additional binding sites for the pyruvate dehydrogenase kinase but not for protein X in the assembled core of the mammalian pyruvate dehydrogenase complex: binding region for the kinase.

A standard resolution of the bovine kidney pyruvate dehydrogenase complex yields a subcomplex composed of approximately 60 dihydrolipoyl transacetylase (E2) subunits, approximately 6 protein X subunits, and approximately 2 pyruvate dehydrogenase kinase heterodimers (KcKb). Using a preparation of resolved kinase in which Kc much greater than Kb, E2-X-KcKb subcomplex additionally bound at least 15 catalytic subunits of the kinase (Kc) and a much lower level of Kb. The binding of Kc to E2 greatly enhanced kinase activity even at high levels of bound kinase. Free protein X, functional in binding the E3 component, did not bind to E2-X-KcKb subcomplex. This pattern of binding Kc but not protein X was unchanged either with a preparation of E2 oligomer greatly reduced in protein X or with subcomplex from which the lipoyl domain of protein X was selectively removed. The bound inner domain of protein X associated with the latter subcomplex did not exchange with free protein X. These data support the conclusion that E2 subunits bind the Kc subunit of the kinase and suggest that the binding of the inner domain of protein X to the inner domain of the transacetylase occurs during the assembly of the oligomeric core. Selective release of a fragment of E2 subunits that contain the lipoyl domains (E2L fragment) releases the kinase (M. Rahmatullah et al., 1990, J. Biol. Chem. 265, 14,512-14,517). Sucrose gradient centrifugation yielded an E2L-kinase fraction with an increased ratio of the kinase to E2L fragment. A monoclonal antibody specific for E2L was attached to a gel matrix. Binding of E2L fragment also led to specific binding of the kinase. Extensive washing did not reduce the level of bound kinase. Thus, the kinase is tightly bound by the lipoyl domain region of E2.

Acetyltransferases↗
PubMed1991

Cryoelectron microscopy of mammalian pyruvate dehydrogenase complex.

Cryoelectron microscopy has been performed on frozen-hydrated pyruvate dehydrogenase complexes from bovine heart and kidney and on various subcomplexes consisting of the dihydrolipoyl transacetylase-based (E2) core and substoichiometric levels of the other two major components, pyruvate dehydrogenase (E1) and dihydrolipoyl dehydrogenase (E3). The diameter of frozen-hydrated pyruvate dehydrogenase complex (PDC) is 50 nm, which is significantly larger than previously reported values. On the basis of micrographs of the subcomplexes, it is concluded that the E1 and E3 are attached to the E2-core complex by extended (4-6 nm maximally) flexible tethers. PDC constructed in this manner would probably collapse and appear smaller than its native size when dehydrated, as was the case in previous electron microscopy studies. The tether linking E1 to the core involves the hinge sequence located between the E1-binding and catalytic domains in the primary sequence of E2, whereas the tether linking E3 is probably derived from a similar hinge-type sequence in component X. Tilting of the E2-based cores and comparison with model structures confirmed that their overall shape is that of a pentagonal dodecahedron. The approximately 6 copies of protein X present in PDC do not appear to be clustered in one or two regions of the complex and are not likely to be symmetrically distributed.

Acetyltransferases↗
PubMed1990

Changes in the core of the mammalian-pyruvate dehydrogenase complex upon selective removal of the lipoyl domain from the transacetylase component but not from the protein X component.

The dihydrolipoyl transacetylase (E2) component contains a COOH-terminal inner domain (E2I) and an extended NH2-terminal structure, which is composed of two lipoyl domains (the fragment containing both is designated as E2L) and a subunit-binding domain (E2B). The four domains are connected by hinge regions. A subcomplex, composed of an oligomer of E2 subunits, protein X (which also has an NH2-terminal lipoyl domain), and the [pyruvate dehydrogenase]-kinase catalytic and basic subunits (Kc and Kb, respectively) (i.e. E2.X.KcKb subcomplex), was treated with Clostridium histolyticum collagenase. E2 subunits were selectively cleaved at the NH2-terminal end of the E2B domain, releasing the E2L fragment. Complete release of E2 subunits also released the kinase subunits, indicating that the kinase is bound to the E2L portion of E2. The residual inner core subcomplex (designated E2IB.X) has a strong tendency to aggregate, but this can be reversed with heparin (1 mg/ml). The E2IB.X subcomplex binds the pyruvate dehydrogenase (E1) and dihydrolipoyl dehydrogenase (E3) components. The E1 component, which binds to the E2B domain, blocked collagenase cleavage of E2. We evaluated the capacity of the collagenase-treated E2.X.KcKb subcomplex, from which different portions of the E2L domains were removed, to support (in combination with excess levels of the E1 and E3 components) the overall reaction of the complex. Loss of activity occurred only after more than half of the E2L domains were removed. This delay is in sharp contrast to the effect of selective removal of the lipoyl domain of protein X, which leads to an immediate decrease in activity (Gopalakrishnan, S., Rahmatullah, M., Radke, G.-A., Powers-Greenwood, S. L., and Roche, T. E. (1989) Biochem. Biophys. Res. Commun. 160, 715-721). These results suggest that multiple lipoyl domains of the E2 component service the rate-limiting E1 component. After all the E2L domains were removed and the E2IB.X subcomplex was separated from free E2L, 10% activity was retained in the overall reaction. Thus, the lipoyl domain of protein X supported the overall reaction of the complex.

Acetyltransferases↗
PubMed1989

Role of protein X in the function of the mammalian pyruvate dehydrogenase complex.

Two lipoyl-bearing subunits--the dihydrolipoyl transacetylase and protein X--form the core of the mammalian pyruvate dehydrogenase complex. Selective removal of the lipoyl domain of protein X results in loss in the activity of the complex with a relationship suggesting the involvement of the lipoyl domain of protein X in a key but not rate limiting step. The dihydrolipoyl dehydrogenase component markedly reduces both the cleavage of protein X and the loss in activity. Using a microplate binding assay, we demonstrate that the lipoyl domain of protein X and the transacetylase component contribute to the binding of the dihydrolipoyl dehydrogenase component. These roles of protein X in the catalytic function and organization of the complex require new reactions and afford an explanation for the unusual stoichiometry of dihydrolipoyl dehydrogenase dimers in the complex.

Animals↗
PubMed1989

Separation of protein X from the dihydrolipoyl transacetylase component of the mammalian pyruvate dehydrogenase complex and function of protein X.

The dihydrolipoyl transacetylase (E2)-protein X-kinase subcomplex was resolved to produce an oligomeric transacetylase that was free of protein X and kinase subunits. We investigated the properties of this transacetylase E2 oligomer and of a form of the subcomplex from which only the lipoyl-bearing domain of protein X (XL) was removed. While retaining other catalytic and binding properties of the native subcomplex, the oligomeric transacetylase and the subcomplex lacking the XL domain had greatly reduced capacities both to support the overall reaction of the complex (upon reconstitution with other components) and to bind the dihydrolipoyl dehydrogenase component. Our results indicate that protein X, in part through its XL domain, contributes to the binding of the dihydrolipoyl dehydrogenase component and to the overall reaction of the complex.

Acetyltransferases↗
PubMed1989

Subunit associations in the mammalian pyruvate dehydrogenase complex. Structure and role of protein X and the pyruvate dehydrogenase component binding domain of the dihydrolipoyl transacetylase component.

We have further distinguished the structures and roles of the two lipoyl-bearing components of the pyruvate dehydrogenase complex, the dihydrolipoyl transacetylase (E2) component and the component designated as protein X. The amino acid sequences of the NH2-terminal regions of the lipoyl-bearing domain of the E2 component and protein X are different but related. The dihydrolipoyl dehydrogenase (E3) component but not the pyruvate dehydrogenase (E1) component protected protein X against proteolytic degradation by trypsin and protease Arg C. Protein X-specific polyclonal antibodies inhibit reconstitution of the overall reaction catalyzed by the complex (E2-X subcomplex recombined with the E1 and E3 components). The rate of development of this inhibition was reduced by pretreatment of E2-X subcomplex with the E3 component. These data strongly suggest the E3 component associates with protein X. The E1 component (an alpha 2 beta 2 tetramer), but not the E3 component, reduced trypsin cleavage of E2 subunits at 4 degrees C and altered the patterns of cleavage at 22 degrees C. At 22 degrees C a large (Mr congruent to 49,000) outer domain (E2LB) of the E2 component was produced. E2LB had the same NH2-terminal amino acid sequence as the smaller (Mr congruent to 38,000) lipoyl-bearing domain (E2L). E2LB, in contrast to E2L, interacted with both the E1 component and the beta subunit of the E1 component. Thus the E1 component is bound through an E1-binding domain that is located in E2 subunits between the inner domain and the outer, lipoyl-bearing domain.

Acetyltransferases↗
PubMed1989

Domain structures of the dihydrolipoyl transacetylase and the protein X components of mammalian pyruvate dehydrogenase complex. Selective cleavage by protease Arg C.

Treatment of the dihydrolipoyl transacetylase-protein X-kinase subcomplex (E2-X-KcKb) with protease Arg C selectively converted protein X into an inner domain fragment (Mr approximately equal to 35,000) and an outer (lipoyl-bearing) domain fragment (Mr approximately equal to 15,500). These fragments were larger and much smaller, respectively, than the inner domain and outer domain fragments derived from the E2 component, supporting the conclusion that protein X is distinct from the E2 component. Protease Arg C cleaved the Kb subunit more slowly than protein X. An increase in kinase activity correlated with this cleavage of the Kb subunits. An even slower cleavage of E2 subunits generated an inner domain fragment (Mr approximately equal to 31,500) and a lipoyl-bearing domain fragment (Mr approximately equal to 49,000) which had Mr values at least 3,000 and 10,000 larger, respectively, than the corresponding E2 fragments generated by trypsin treatment of the subcomplex. Following various extents of cleavage with protease Arg C or trypsin, residual oligomeric subcomplexes were isolated and characterized. We found that selective removal of the lipoyl-bearing domain of protein X did not alter lipoyl-mediated regulation of the kinase indicating that the lipoyl residues bound to E2 subunits are effective, that the inner domain of protein X remained associated with the inner domain of E2 subunits following the complete removal of the outer domains of both E2 and protein X, that, with only 10% of the E2 subunits intact, nearly half of the catalytic (Kc) subunits of the kinase were bound by the residual subcomplex, and that removal of the remaining outer domains from E2 subunits released the Kc subunits. Thus, protein X is unique among the subunits of the complex in binding tightly to the oligomeric inner domain of the transacetylase, and the outer domain of the transacetylase serves to bind to and facilitate the regulation of the catalytic subunit of the kinase.

Acyltransferases↗
PubMed1987

Measurement of protein using flow injection analysis with bicinchoninic acid.

We have used the bicinchoninic acid reagent developed by Pierce Chemical Co. to measure proteins in a simple flow injection analyzer. The sensitivity is comparable to that of the Lowry method and no pipetting of reagents is needed. Results are obtained in less than 1 min and samples may be run at a rate of 60/h. The response is linear over a range of protein concentration (0-10 micrograms) and sample size (5-20 microliters) convenient for most analytical requirements. A peristaltic pump, a controlled-temperature water bath, and a spectrophotometer with flow cuvette are the only special apparatus required.

Proteins↗
PubMed1986

Properties of the pyruvate dehydrogenase kinase bound to and separated from the dihydrolipoyl transacetylase-protein X subcomplex and evidence for binding of the kinase to protein X.

Studies were conducted on four pyruvate dehydrogenase kinase-containing fractions: purified pyruvate dehydrogenase complex, the dihydrolipoyl transacetylase-protein X-kinase subcomplex (E2.X.K), a kinase fraction (K fraction) prepared from the E2.X.K subcomplex, and a kinase fraction generated by limited trypsin-digestion of E2.X.K. We characterized the gel electrophoresis properties of dissociated subunits (one-dimensional and two-dimensional), the catalytic and ATP binding properties of kinase-containing fractions, and the subunit requirements for kinase binding to and being activated by the transacetylase-protein X subcomplex (E2.X). A significant portion of protein X was retained with the transacetylase core following release of virtually all the kinase. The K fraction had four major bands separated by sodium dodecyl sulfate-slab gel electrophoresis which corresponded to the dihydrolipoyl dehydrogenase, protein X, the trypsin-resistant catalytic subunit of the kinase and a chymotrypsin-resistant subunit which had a high pI and comigrated in one-dimensional systems with the chymotrypsin-sensitive alpha-subunit of the pyruvate dehydrogenase component. While purified kidney complex contained only about three molecules of kinase (determined by [14C]ATP binding), one molecule of E2.X subcomplex activated a large number (greater than 15) molecules of kinase associated with the protein X-containing K fraction. Sephadex G-200 chromatography of the K fraction in the presence of dithiothreitol led to coelution of protein X and kinase subunits. Limited trypsin digestion converted the transacetylase into subdomains and cleaved protein X and the high pI subunit of the kinase. Under those conditions, the intact catalytic subunit of the kinase did not bind to the large inner domain of the transacetylase but could be activated by untreated E2.X subcomplex. Thus, binding of the catalytic subunit of the kinase and its activation by E2.X required either protein X or the lipoyl-bearing outer domain of the transacetylase. In combination, our results suggest that protein X serves to anchor the kinase to the core of the complex.

Acetyltransferases↗
PubMed1984

Chemically coupled spectrophotometric assays based on flow injection analysis: determination of nitrogenase by assays for creatine, ammonia, hydrazine, phosphate, and dithionite.

Micromethods of direct chemical coupling have been developed for several different enzyme reactions, using the principles of flow injection analysis. Samples of 1-25 microliters are injected into a flowing stream of color-forming reagents and the peak of color change is measured after about 1 min. Alternatively, continuous slow infusion of a reacting system (5-100 microliters/min) gives a continuous change of color which can be monitored to derive enzyme reaction rates. These techniques are highly sensitive, requiring a few nanomoles of the substance being detected. Phosphate, ammonia, dithionite, creatine, and hydrazine have been measured. Consumption of reagents is less than 75 ml per hour; typical sample throughout is 30-40 samples per hour by the injection method, and 5 samples per hour by continuous infusion. The procedure has been applied to nitrogenase, continuously monitoring creatine produced from creatine phosphate by creatine kinase which is used to supply a constant level of ATP for nitrogenase. In this way nitrogenase activity can be determined over a wide range of enzyme concentrations. Production of inorganic phosphate directly from ATP, by injection of formaldehyde-quenched samples, was used when coupling to creatine kinase was not possible. Both injection of aliquots and continuous infusion were used for detection of hydrazine during nitrogenase reduction of azide, and the injection method has been used for ammonia assay during dinitrogen reduction. Dithionite oxidation was measured directly from decolorization of iodine, after trapping both dithionite and bisulfite with formaldehyde.

Ammonia↗
PubMed1983

Broad-zone active-enzyme chromatography. Keto-acid dehydrogenases as associating systems.

We have extended the method of active-enzyme chromatography to include the use of broad zones of enzyme. This allows examination of interacting systems in a way formally analogous to sedimentation velocity so that simulation of the observed activity profiles is possible. The method has been applied using pyridine nucleotide-linked active enzyme assays. At the concentrations presently accessible by this technique, hexokinase and glucose-6-phosphate dehydrogenase, both associating systems, show single symmetrical boundaries, as does isolated diaphorase, while pyruvate and alpha-ketoglutarate dehydrogenases show more complex patterns, with the position of the reaction boundary for diaphorase activity being dependent on enzyme concentration.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗