Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) is one of the most abundant enzymes on Earth.  In carbon-fixing organisms, it consumes CO2 to carboxylate the sugar ribulose-1,5-bisphosphate (Ru1,5BP), and forms two molecules of 3-phosphoglycerate (3PG) as a product.  The functional form of rubisco from the bacterium Rhodospirillum rubrum is a noncovalent homodimer composed of two 51 kDa monomers.  Each subunit requires a magnesium ion (Mg2+) as a prosthetic group in the active site.Rubisco is often used as a model for protein folding studies.  On their own, rubisco monomers typically cannot fold to completion and instead are trapped in a form known as a "kinetic intermediate."  These kinetically trapped monomers cannot dimerize correctly and are prone to aggregation.Scientists interested in studying protein folding dynamics labeled the rubisco kinetic intermediate with a set of fluorophores known as a FRET (fluorescence resonance energy transfer) pair, with one fluorophore at the N-terminus and the other at the C-terminus.  A FRET signal occurs when the emission spectrum of one fluorophore (the donor) overlaps with the excitation spectrum of the other (the acceptor).  When the two fluorophores are sufficiently close to each other, the donor can transfer energy from the light it absorbs directly to the acceptor, causing the acceptor to emit light after the donor absorbs light.The GroEL/ES complex is a chaperone protein that binds and releases misfolded proteins, hydrolyzing ATP each time the protein is released.  A misfolded protein may undergo multiple rounds of binding and release before adopting its correct conformation, or it may never fold correctly and instead be targeted for destruction (Figure 1).Figure 1  Representation of GroEL/ES-mediated protein foldingResearchers diluted the labeled kinetic intermediate into a refolding buffer in the presence or absence of 200 nM GroEL/ES and then monitored the FRET signal over time.  They observed that under the conditions used, rubisco folding was first-order with respect to [unfolded rubisco] when GroEL/ES was present.  The results are shown in Figure 2.Figure 2  FRET signal of labeled rubisco with and without GroELLin Z, Rye HS. Expansion and compression of a protein folding intermediate by GroEL. Mol Cell. 2004;16(1):23-34. Question 27Based on the molecular weight of a rubisco monomer, approximately how many amino acids are present in a rubisco dimer?A.460B.690C.920D.5100