Assembly of the major light-harvesting chlorophyll-a/b complex: Thermodynamics and kinetics of neoxanthin binding.

The major light-harvesting chlorophyll-a/b complex in most higher plants contains three carotenoids, lutein, neoxanthin, and violaxanthin. How these pigments are assembled into the complex during its biogenesis is largely unknown. Here we show that neoxanthin but not lutein can dissociate from the fully assembled complex. Its equilibrium binding constant in a detergent system (0.1% n-dodecyl-beta-D-maltoside) was determined to be > or = 10(6) m(-1). Neoxanthin insertion into light-harvesting chlorophyll-a/b complex prefolded from overexpressed apoprotein (Lhcb1*2 from Pisum sativum) in the presence of chlorophylls a, b, and lutein as the sole carotenoid is kinetically controlled by an activation energy barrier of approximately 120 kJ mol(-1). This is the first thermodynamic and kinetic description of a binding equilibrium between a non-covalently bound pigment of the photosynthetic apparatus and its protein complex. Dissociation of neoxanthin from the major light-harvesting chlorophyll-a/b complex upon temperature increase is discussed in terms of providing a readily available substrate pool for synthesizing abscisic acid as part of a heat and drought stress response.