All of the statements you provided about the binding of enzymes and substrates contain elements of truth, but some are more accurate or complete than others. Here's a breakdown:
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"When substrate molecules bind to the active site of the enzyme, the enzyme undergoes a slight change in shape." - This statement is correct. It refers to the "induced fit" model, where the binding of the substrate induces a conformational change in the enzyme, optimizing the fit and enhancing catalytic activity.
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"Substrate molecules bind to the active site of the enzyme only by weak bonds, such as hydrogen bonds or hydrophobic attraction." - This statement is somewhat true but may be misleading. While it is accurate that substrates primarily bind through weak interactions (like hydrogen bonds, ionic interactions, and hydrophobic forces), some enzyme-substrate interactions can involve stronger covalent bonds during certain reactions.
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"Substrate molecules fit into the active site of an enzyme like a key fits into a lock." - This analogy is partially correct, representing the "lock and key" model of enzyme action. However, it oversimplifies the reality. The modern understanding utilizes the "induced fit" model, indicating that the enzyme and substrate can adjust their shapes to enhance binding, diverging from a rigid key/lock analogy.
In summary, the first statement about the enzyme's change in shape upon substrate binding is the most broadly accurate representation of the enzyme-substrate interaction.