From the image, the letter that represents the enzyme is B.
Enzymes are substances, made up of proteins, which are capable of speeding up the metabolic reactions in the human body.
There is a theory that explains the mechanism of action of enzymes. This is called the enzyme lock and key theory.
This theory states that enzymes bind to substrates temporarily forming enzyme-substrate complex that appears like the fitting of a lock and key.
This simply means that the same way a particular key can only fit into a particular lock is the same way a particular substrate must combine with a particular enzyme for a reaction to occur.
From the diagram,
B represents the enzyme with an active site which must fit into a substrate
A represents the substrate.
Therefore, the the letter that represents the enzyme is B.
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