Chapter 2 The Chemical Basis Of Life

Site on bacterial chromosome actually contains 2

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site on bacterial chromosome actually contains 2 separate genes, GroEL & GroES that encode 2 separate proteins, GroEL & GroES 1. Under EM, purified GroEL protein appeared as cylindrical assembly consisting of 2 disks 2. Each disk was composed of 7 subunits arranged symmetrically around central axis D. Several years later – study on pea plants hinted at existence of similar assembly-promoting protein in plant chloroplasts 1. Rubisco is large chloroplast protein that catalyzes reaction in which CO 2 molecules taken up from atmosphere are covalently linked to organic molecules during photosynthesis 2. Rubisco comprises 16 subunits: 8 small subunits (molecular mass of 14,000 daltons) & 8 large subunits (55,000 daltons) 61
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3. Large Rubisco subunits, synthesized in chloroplast, are not present in independent state, but associated with huge protein assembly consisting of identical 60,000 dalton (60 kDa) subunits 4. Researchers considered possibility that complex formed by large Rubisco subunit & the 60-kDa- polypeptide was an intermediate in the assembly of a complete Rubisco molecule E. Separate study on mammalian cells – revealed existence of proteins that appeared to assist assembly of multisubunit proteins 1. Like Rubisco, antibody molecules consist of a complex of 2 different types of subunits, smaller light chains & larger heavy chains 2. Like large Rubisco subunits, heavy chains of antibody complex become associated with another protein not found in the final complex 3. This protein, which associates with newly synthesized heavy chains, but not with heavy chains that are already bound to light chains, was named binding protein (BiP) 4. BiP was subsequently found to have a molecular mass of 70,000 daltons (70 kDa) F. 1986 – 2 lines of investigation (heat-shock response & proteins that promote protein assembly) came together 1. Shown that very prominent heat shock response protein, heat-shock protein 70 (hsp70) due to its molecular mass, was identical to BiP, the protein implicated in antibody molecule assembly III. Even before heat-shock response was known, protein structure was known to be sensitive to temperature A. A small rise in temperature could cause delicate proteins to begin to unfold 1. Unfolding exposes hydrophobic residues previously buried in protein core 2. Hydrophobic residue patches on protein surfaces attract each other like fat droplets 3. Thus, when cell is heat shocked, soluble proteins are denatured & form aggregates B. 1985 report showed that after temperature elevation, newly synthesized hsp70 molecules enter cell nucleus & bind to nuclear protein aggregates 1. They then act like molecular crowbars to promote disaggregation 2. Due to their role in assisting protein assembly by preventing undesirable interactions, hsp70 & related proteins were named molecular chaperones IV. Soon demonstrated that bacterial heat-shock protein GroEL & Rubisco assembly proteins in plants are homologous proteins A. The 2 proteins share same amino acids at nearly half of >500 residues in their respective molecules 1.
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