Enzymes are essential for life, as they cause chemical reactions to occur more rapidly, which as you will learn, allows rapid transfer of energy.  Some enzymes can also be regulated.  In other words, they can be made to be more (by activators) or less effective (inhibitors), depending on the presence of specific metabolites produced from other chemical reactions that reflect the energy demand of a cell.  Thus, depending on the enzyme they can have binding sites for substrate(s), product(s), activators and inhibitors.  The enzyme to the left is a rather simple enzyme - carbonic anhydrase - the enzyme that speeds the reaction involving the transfer of bicarbonate (main proton buffer of blood) and a proton (H+) to water and carbon dioxide (CO2).  This reaction is also readily reversible under the right conditions (concentrations) of substrates and products.  Of course, in the reverse direction, what was a product is now a substrate, and vice-versa.  The green and blue dots reflect these substrate and product binding sites. As enzymes are proteins, the amino acid sequences can form different secondary and tertiary structures, and these are denoted in the illustration by the the different line configurations.  The simple line reflects amino acids in a primary (amino acid to amino acid) peptide bond sequence.  The other line structures represent more complex configurations, where the charge nature of certain side changes of difference amino acids interact with each and force a deviation in structural alignment.  It is the charge structure of proteins, and enzymes are a great example, that allow the protein to interact with its cellular or extracellular environment in a unique way.

Recommended sequence of topics:

Enzyme catalysis

Types of enzymes

Enzyme nomenclature