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Enzymes are proteins that bind tightly to specific molecules called substrates .
Most enzymes are allosteric proteins that can exist in two conformations that differ in catalytic activity. Proteasomes act on proteins that get marked for destruction by the attachment of a small protein called ubiquitin . During protein synthesis, the newly assembled peptidyl tRNA in the A site is translocated to the P-site as the ribosome moves exactly three nucleotides along the mRNA molecule. This step is driven by the hydrolysis of a GTP molecule. Because of this, heat shock proteins are produced in large quantities to correct the deleterious effects of the temperature increase.