Regulation of coupling factor in field-grown sunflower: A Redox model relating coupling factor activity to the activities of other thioredoxin-dependent chloroplast enzymes

Abstract

Simultaneous, non-invasive measurements were made of the rate of photosynthetic CO2 fixation and the state of activation of the chloroplast CF1CF0-ATP synthase (CF) in field-grown sunflower (Helianthus annuus L.) during the dark-to-light transition at sunrise. CO2 fixation showed a linear response with light intensity from zero to about 500-700 μE m(-2) s(-1). However, at light intensities of only 5-22 μE m(-2) s(-1), the energetic threshold for activation of the CF was found to be significantly lowered (as compared to the pre-dawn state), presumably through reduction of the regulatory sulfhdryl groups of the γ-subunit of the CF. When these studies were extended to chamber-grown plants, it was found that as little as 5 seconds of illumination at 4 μE m(-2) s(-1) caused apparently full CF reduction. It is clear, therefore, that the catalytic activation of CF is not rate limiting to the induction of carbon assimilation under field conditions during a natural dark-to-light transition at sunrise. A model, based on the redox properties of the regulatory sulfhydryls, was developed to examine the significance of sulfhydryl midpoint potential in explaining the differences in light sensitivity and oxidation and reduction kinetics, between the CF and other thioredoxin-modulated chloroplast enzymes. Computer simulations of the light-induced regulation of three representative thioredoxin-modulated enzymes are presented.

Thioredoxin-insensitive plastid ATP synthase that performs moonlighting functions

“…2D), with a pronounced lag phase from 0 to 10 min after preillumination, followed by a approximately 10 min half time decrease in activity. The sigmoidal oxidation kinetics were previously interpreted as reflecting the equilibration of γ subunit thiols redox state with that of a larger pool of redox carriers (14). Almost identical results were obtained for atpc2, indicating that γ 2 is not required for regulation of ATP synthase.…”

“…1) indicating that the lowering of ATP synthase content or activity in gamera was due to substitution of ATPC1 by ATPC2, rather than secondary effects of altering ATPC promoter regions. We analyzed ATP synthase activity in attached leaves by exciting PSI and PSII with short (100 μs), nonsaturating LED pulses to rapidly generate pmf, and probing the resulting proton flux through the ATP complex via the decay of the ECS absorbance change at 520 nm (13,14) (Fig. 2 A-C).…”

“…After dark adaptation for tens of minutes, the γ subunit cysteine residues become oxidized, imposing a higher pmf requirement for activation, of about 100 mV, thus resulting in ATP synthase inactivation, preventing rapid hydrolysis of ATP. Redox-regulation via thioredoxin is highly sensitive to light, and acts as a "switch," rapidly transitioning between inhibition in the dark and full activity at even very low light intensities (14).…”

“…The plastid ATP synthase complex consists of nine different subunits. Four of these subunits (I, II, III 14 , and IV, also called b, b', c, and a) form the integral membrane CF O subcomplex and the remaining five subunits make up the extrinsic CF 1 subcomplex (α 3 , β 3 , γ, δ, and ε) that contains the catalytic sites of ATP synthase (1,2). The CF O subcomplex drives the synthesis of ATP at the CF 1 subcomplex via the rotational-catalysis/binding-change mechanism (3)(4)(5).…”

Light- and Metabolism-related Regulation of the Chloroplast ATP Synthase Has Distinct Mechanisms and Functions

“…As discussed previously (15,33), the decay of the ECS under these conditions is a good measure of the redox state of the ␥ subunit, i.e. the extent of the rapid phase is large when the ␥ subunit is reduced and slow when oxidized.…”