| 1. |
- Kvassman, Jan, et al.
(författare)
-
Mechanism of glyceraldehyde‐3‐phosphate transfer from aldolase to glyceraldehyde‐3‐phosphate dehydrogenase
- 1988
-
Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956 .- 1432-1033. ; 172:2, s. 427-431
-
Tidskriftsartikel (refereegranskat)abstract
- The catalytic interaction of glyceraldehyde‐3‐phosphate dehydrogenase with glyceraldehydes‐3‐phosphate has been examined by transient‐state kinetic methods. The results confirm previous reports that the apparent Km for oxidative phosphorylation of glyceraldehydes‐3‐phosphate decreases at least 50‐fold when the substrate is generated in a coupled reaction system through the action of aldolase on fructose 1,6‐bisphosphate, but lend no support to the proposal that glyceraldehydes 3‐phosphate is directly transferred between the two enzymes without prior release to the reaction medium. A theoretical analysis is presented which shows that the kinetic behaviour of the coupled two‐enzyme system is compatible in all respects tested with a free‐diffusion mechanism for the transfer of glyceraldehydes 3‐phosphate from the producing enzyme to the consuming one.
|
|
| 2. |
- Pettersson, Gösta, et al.
(författare)
-
A mathematical model of the Calvin photosynthesis cycle
- 1988
-
Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956 .- 1432-1033. ; 175:3, s. 661-672
-
Tidskriftsartikel (refereegranskat)abstract
- A mathematical model is presented for photosynthetic carbohydrate formation in C3 plants under conditions of light and carbon dioxide saturation. The model considers reactions of the Calvin cycle with triose phosphate export and starch production as main output processes, and treats concentrations of NADPH, NAD+, CO2, and H+ as fixed parameters of the system. Using equilibrium approximations for all reaction steps close to equilibrium, steady‐state and transient‐state relationships are derived which may be used for calculation of reaction fluxes and concentrations of the 13 carbohydrate cycle intermediates, glucose 6‐phosphate, glucose 1‐phosphate, ATP, ADP, and inorganic (Ortho)phosphate. Predictions of the model were examined with the assumption that photosynthate export from the chloroplast occurs to a medium containing orthophosphate as the only exchangeable metabolite. The results indicate that the Calvin cycle may operate in a single dynamically stable steady state when the external concentration of orthophosphate does not exceed 1.9 mM. At higher concentrations of the external metabolite, the reaction system exhibits overload breakdown; the excessive rate of photosynthate export deprives the system of cycle intermediates such that the cycle activity progressively approaches zero. Reactant concentrations calculated for the stable steady state that may obtain are in satisfactory agreement with those observed experimentally, and the model accounts with surprising accuracy for experimentally observed effects of external orthophosphate on the steady‐state cycle activity and rate of starch production. Control analyses are reported which show that most of the non‐equilibrium enzymes in the system have a strong regulatory influence on the steady‐state level of all of the cycle intermediates. Substrate concentration control coefficients for cycle enzymes may be positive, such that an increase in activity of an enzyme may raise the steady‐state concentration of the substrate is consumes. Under optimal external conditions (0.15–0.5 mM orthophosphate), reaction flux in the Calvin cycle is controlled mainly by ATP synthetase and sedoheptulose bisphosphatase; the cycle activity approaches the maximum velocity that can be supported by the latter enzyme. At lower concentrations of external orthophosphate the cycle activity is controlled almost exclusively by the phosphate translocator. At high external orthophosphate concentrations the phosphate translocator resumes predominant control, but also other non‐equilibrium enzymes gain strong flux control with one notable exception: ribulosebisphosphate carboxylase has no significant regulatory influence on the cycle activity under conditions of light and CO2 saturation, nor does it control the concentration of any cycle intermediate other than its substrate.
|
|
| 3. |
- Pettersson, Gösta, et al.
(författare)
-
A rapid‐equilibrium model for the control of the Calvin photosynthesis cycle by cytosolic orthophosphate
- 1987
-
Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956 .- 1432-1033. ; 169:2, s. 423-429
-
Tidskriftsartikel (refereegranskat)abstract
- A simple model based on rapid‐equilibrium assumptions is derived which relates the steady‐state activity of the Calvin cycle for photosynthetic carbohydrate formation in C3 plants to the kinetic properties of a single cycle enzyme (fructose bisphosphatase) and of the phosphate translocator which accounts for the export of photosynthate from the chloroplast. Depending on the kinetic interplay of these two catalysts, the model system may exhibit a single or two distinct modes of steady‐state operation, or may be unable to reach a steady state. The predictions of the model are analysed with regard to the effect of external orthophosphate on the steady‐state rate of photosynthesis in isolated chloroplasts under conditions of saturating light and CO2. Due to the possible existence of two distinct steady states, the model may account for the stimulatory as well as the inhibitory effects of external phosphate observed in experiments with intact chloroplasts. Stability arguments indicate, however, that only the steady‐state case corresponding to phosphate inhibition of the rate of photosynthesis could be of physiological interest. It is concluded that chloroplasts under physiological conditions most likely operate in a high‐velocity steady state characterized by a negative Calvin cycle flux control coefficient for the phosphate translocator. This means that any factor enhancing the export capacity of the phosphate translocator can be anticipated to decrease the actual steady‐state rate of photosynthate export due to a decreased steady‐state rate of cyelic photosynthate production.
|
|
| 4. |
- Pettersson, Gösta, et al.
(författare)
-
Dependence of the Calvin cycle activity on kinetic parameters for the interaction of non‐equilibrium cycle enzymes with their substrates
- 1989
-
Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956 .- 1432-1033. ; 186:3, s. 683-687
-
Tidskriftsartikel (refereegranskat)abstract
- Kinetic model studies and control analyses of the Calvin photosynthesis cycle have been performed to characterize the dependence of the cycle activity on maximum velocities and Kmvalues for the interaction of the non‐equilibrium cycle enzymes and ATP synthetase with their substrates under conditions of light and carbon dioxide saturation. The results show that Km values have no major influence on the cycle activity at optimal concentrations of external orthophosphate. The maximum cycle activity is controlled mainly by the catalytic capacities of ATP synthetase and sedoheptulose‐bisphosphatase, and is close to the maximum cycle flux that can be supported by these two enzymes.
|
|
| 5. |
- Pettersson, Gösta, et al.
(författare)
-
Effects of metabolite binding to ribulosebisphosphate carboxylase on the activity of the Calvin photosynthesis cycle
- 1988
-
Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956. ; 177:2, s. 351-355
-
Tidskriftsartikel (refereegranskat)abstract
- The regulatory implications of the interaction of ribulosebisphosphate carboxylase with metabolites participating in the Calvin photosynthesis cycle has been examined by control analysis based on our recently described kinetic model for photosynthetic carbohydrate formation in the chloroplast of C3 plants. The results provide clear evidence that the Calvin cycle activity under conditions of light and CO2 saturation is insignificantly affected by the inhibition of ribulosebisphosphate carboxylase caused by metabolites such as 3‐phosphoglycerate, fructose 1,6‐bisphosphate, sedoheptulose 1,7‐bisphosphate, NADPH, and inorganic orthophosphate. Due to the exceptionally high stromal concentration of the carboxylase, metabolite binding to the enzyme affects the Calvin cycle activity indirectly by reducing the pool of free orthophosphate and phosphorylated metabolites available for the cyclic reactions. This pool reduction corresponds typically to about 5 mM total phosphate and derives mainly from the binding of ribulose bisphosphate and orthophosphate. Substantial amounts of the metabolites interacting with ribulosebisphosphate carboxylase are present in an enzyme‐bound form. The bound form of the Calvin cycle intermediates sedoheptulose bisphosphate, fructose bisphosphate, and ribulose bisphosphate typically accounts for about 70, 80, and 90%, respectively, of the total stromal concentration of the intermediate.
|
|
| 6. |
- Pettersson, Gösta, et al.
(författare)
-
On the regulatory significance of inhibitors acting on non‐equilibrium enzymes in the Calvin photosynthesis cycle
- 1989
-
Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956 .- 1432-1033. ; 182:2, s. 373-377
-
Tidskriftsartikel (refereegranskat)abstract
- Control analyses and kinetic model studies have been performed in order to obtain quantitative information on the regulatory significance of 12 experimentally well‐documented inhibitory interactions of Calvin cycle intermediates with the four non‐equilibrium cycle enzymes. Evidence is presented to show that none of these interactions contributes significantly to the cycle flux control over the range of external orthophosphate concentrations where the reaction cycle shows close to optimal activity. Contrary to what has been generally supposed, the examined inhibitions appear to be of little interest for our understanding of the biological regulation of the Calvin photosynthesis cycle under conditions of light and carbon dioxide saturation.
|
|
| 7. |
- Ryde-Pettersson, Ulf
(författare)
-
A theoretical treatment of damped oscillations in the transient state kinetics of single‐enzyme reactions
- 1989
-
Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956 .- 1432-1033. ; 186:1-2, s. 145-148
-
Tidskriftsartikel (refereegranskat)abstract
- An extension of the available kinetic theory for reactions in the transient state is presented which establishes that single‐enzyme reactions may exhibit damped oscillations under the conditions of standard kinetic experiments performed by stopped‐flow techniques. Such oscillations may occur for reasonable magnitudes of rate constants in the enzymic reaction mechanism and at physiological concentrations of enzyme and substrate. In the simplest reaction systems, the oscillations will be strongly damped and lead to progress curves resembling those of a reaction governed by standard exponential transients; statistical regression methods may then have to be applied for their detection and characterization. The observation that single‐enzyme reactions may exhibit oscillatory behaviour points to a previously unrecognized possible source of the damped oscillations observed in metabolic systems such as the pathways of glycolysis or photosynthesis.
|
|
| 8. |
- Ryde-Pettersson, Ulf
(författare)
-
Identification of possible two‐reactant sources of oscillations in the Calvin photosynthesis cycle and ancillary pathways
- 1991
-
Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956 .- 1432-1033. ; 198:3, s. 613-619
-
Tidskriftsartikel (refereegranskat)abstract
- A systematic search for possible sources of experimentally observed oscillations in the photosynthetic reaction system has been performed by application of recent theoretical results characterizing the transient‐state rate behaviour of metabolic reactions involving two independent concentration variables. All subsystems involving two independent reactants in metabolically fundamental parts of the Calvin cycle and the ancillary pathways of starch and sucrose synthesis have been examined in order to decide on basis of their kinetic and stoichiometric structure whether or not they may trigger oscillations. The results show that no less than 20 possible oscillators can be identified in the examined reaction system, only three of which have been previously considered as potential sources of experimentally observed oscillations. This illustrates the superiority of the method now applied over those previously used to identify possible two‐reactant sources of metabolic oscillations and indicates that there should be no difficulty in complex metabolic pathways to point to a multitude of interactions that may trigger an oscillatory rate behaviour of the system.
|
|
| 9. |
- Ryde-Pettersson, Ulf
(författare)
-
On the mechanistic origin of damped oscillations in biochemical reaction systems
- 1990
-
Ingår i: European Journal of Biochemistry. - : Wiley. - 0014-2956 .- 1432-1033. ; 194:2, s. 431-436
-
Tidskriftsartikel (refereegranskat)abstract
- A generalized reaction scheme for the kinetic interaction of two reactants in a metabolic pathway has been examined in order to establish what minimal mechanistic patterns are required to support a damped oscillatory transient‐state kinetic behaviour of such a two‐component system when operating near a steady state. All potentially oscillating sub‐systems inherent in this scheme are listed and briefly characterized. The list includes several mechanistic patterns that may be frequently encountered in biological system (e.g. involving feedback inhibition, feed‐forward activation, substrate inhibition or product activation), but also draw attention to some hitherto unforeseen mechanisms by which the kinetic interaction of two metabolites may trigger damped oscillations. The results can be used to identify possible sources of oscillations in metabolic pathways without detailed knowledge about the explicit rate equations that apply.
|
|
