6.3 Enzymatic Biochemistry and Processing
Starches are broken down by enzymes known as amylases; our saliva contains amylase, so this is how starches begin to be broken down in our body. Amylases have also been isolated and used to depolymerize starch for making alcohol, i.e., yeast for bread making and for alcohol manufacturing. Chemically, the amylase breaks the carbon-oxygen linkage on the chains (α-1,4-glucosidic bond and the α-1,6-glucosidic bond), which is known as hydrolysis. Once the glucose is formed, then fermentation can take place to break the glucose down into alcohols and CO2. The amylases were isolated and the hydrolysis of glucose began to be understood in the 1800s.
However, recall that cellulose linkages are β-1.4-glucosidic bonds. These bonds are much more difficult to break, and due to cellulose crystallinity, breaking cellulose down into glucose is even more difficult. It was only during WWII that enzymatic hydrolysis of cellulose was discovered. Instead of enzymes called amylases, the enzymes that degrade cellulose are called cellulases.
Cellulases are not a single enzyme. There are two main approaches to biological cellulose depolymerization: complexed and non-complexed systems. Each cellulase enzyme is composed of three main parts, and there are multiple synergies between enzymes.