The chemical mechanism by which ATP is synthesized is not yet understood in detail. However, considerable evidence now supports a mechanism, first proposed by Paul Boyer, in which the principal energy-requiring step is the release of bound ATP from the enzyme (Boyer 1993, 1997). It has also been proposed that during catalysis a large portion of the CF₁ complex rotates about a bearing consisting of the γ subunit (Noji et al. 1997). The γ subunit may act as a camshaft does, rotating alternately against the α and α subunits (see Web Topic 12.4). The energy of the conformational movements is then translated into phosphoanhydride bond energy (Junge et al. 1997).

The thylakoid ATP synthase has two segments: CF₀ is a transmembrane segment, and CF₁ is a hydrophilic segment at the stomal surface (see textbook Figure 7.30). CF₀ translocates protons from the chloroplast lumen to the catalytic part of the enzyme (see textbook Figure 7.31) and CF₁ cataylses the conversion of ADP and inorganic phosphate to ATP.

The chloroplast ATP synthase has nine different subunits. CF₁ is made of two large subunits, α and β, plus three smaller subunits, γ, δ, and ε. Each CF₁ molecule has three copies of the α and β subunits and one copy of the γ, δ and ε subunits. The α and β subunits bind ADP and phosphate and catalyize the phosphorylation of ADP into ATP.

Studies on the mitochondrial ATP synthase have increased our understanding of how the ATP synthases work. As proposed by Paul Boyer in 1997, ATP synthesis takes place by a binding change mechanism (Web Figure 7.9.A). According to this model, the energy of the proton gradient is used to release a tightly bound form of ATP from a catalytic binding side of the enzyme. As shown in Web Figure 7.9.A, CF₁ has three nucleotide binding sites, L, T and O. ADP and inorganic phosphate are postulated to first bind to the O site, and then move to the L site. The T site binds ATP. As protons move from the lumen to the stromal region through the CF₀ channel, energy is released and the γ subunit of CF₁ rotates. The rotation causes conformation changes in the three nucleotide binding sites, which interconvert, thus changing the afiity of the sites for the nucleotides. As the T site converts into a O site, ATP is released, and another cycle starts. Direct observation of the rotation of the γ subunit of CF₁ has provided strong experimental support for the binding change mechanism (Yasuda et al. 2001).

Web Figure 7.9.A   The model of Boyer's binding change mechanism. The α and β subunits configure three nucleotide binding sites: O, which provides the early binding site for ADP and inorganic phosphate, L, to which the ADP and inorganic phosphate bind after migrating from O, and T, which tightly binds ATP. Energy ensuing from the movement of protons from the chloroplast lumen to the stroma drives the rotation of the γ subunit of CF₁, and the interconversion of the binding sites and the release of an ATP molecule. (From Malkin and Niyogi 2000, after Cross and Duncan 1996.)