Why can carbon form very large molecues

Because each carbon is identical, they all have four valence electrons, so they can easily bond with other carbon atoms to form long chains or rings.

In fact, a carbon atom can bond with another carbon atom two or three times to make double and triple covalent bonds between two carbon atoms

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products