Passage
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 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 when 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. The results are shown in Figure 2.
Figure 2 FRET signal of labeled rubisco with and without GroEL
Lin Z, Rye HS. Expansion and compression of a protein folding intermediate by GroEL. Mol Cell. 2004;16(1) :23-34.
-Which of the following is true concerning magnesium in rubisco?The rubisco-Mg²⁺ interaction has a small K_d.Mg²⁺ is a cofactor of rubisco.Mg²⁺ activates rubisco by binding at an allosteric site.

Question

Passage
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 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 when 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. The results are shown in Figure 2.

Figure 2 FRET signal of labeled rubisco with and without GroEL
Lin Z, Rye HS. Expansion and compression of a protein folding intermediate by GroEL. Mol Cell. 2004;16(1) :23-34.
-Which of the following is true concerning magnesium in rubisco?The rubisco-Mg²⁺ interaction has a small K_d.Mg²⁺ is a cofactor of rubisco.Mg²⁺ activates rubisco by binding at an allosteric site.

Quizplus · Accepted Answer

11ecba2d_0e21_98d6_a3bf_9960cf55b33b_MD0008_00 Some proteins, including some enzymes, can only function with the help of non-amino acid groups called cofactors. These groups may be organic (often called coenzymes) or inorganic, and they may be tightly or loosely bound to the proteins they help. Tightly bound cofactors are called prosthetic groups. The passage states that a magnesium ion is required on each subunit for rubisco activity. Therefore, Mg²⁺ is a cofactor of rubisco (Number II). The passage also states that Mg²⁺ is a prosthetic group, meaning that it is bound tightly to rubisco. The dissociation constant K_d measures the tendency of a protein-ligand complex (ie, rubisco and magnesium) to separate into its components. A small K_d indicates a low tendency to dissociate and therefore indicates the tight binding expected of a prosthetic group (Number I). (Number III) Allosteric effectors bind proteins at positions other than the active site and either increase (activators) or decrease (inhibitors) their activity. The passage says that magnesium ions bind rubisco at the active site; so although magnesium activates rubisco, it does not do so allosterically. Educational objective: Some proteins require cofactors (non-amino acid groups) to function. Cofactors may be organic (coenzymes) or inorganic, and may bind their proteins tightly or loosely. Tightly bound cofactors are called prosthetic groups.

Passage Ribulose-1,5-Bisphosphate Carboxylase/oxygenase (Rubisco) Is One of the Most Abundant Enzymes