Re: What type of enzyme is needed to break down common grass

Of course humans can eat grass and will obtain some nutrients.  Just as we 
eat lettuce or other greens.  However, grass is not very nutritious, 
although it would be a good source of fiber in the diet.  And it is this 
fiber that I believe your question is addressing.

Plants produce a number of different structural polymers used to maintain 
the architecture of the cell and the whole organism.  Chief among these are 
the pectins, hemicelluloses, celluloses, and lignins.  It is these 
compounds that the human body does not digest well.  Carbohydrates are 
polymers of simple sugars forming polysaccharide (polysugar) chains.  Our 
bodies break down certain polysaccharides using specific digestive enzymes.  
Starches are broken down using enzymes called amylases.  Starch is a 
polymer made of glucose molecules strung together in a chain with chemical 
bonds between them.  These bonds are in a configuration called alpha(1 to 
4).  These alpha linkages can be broken by amylase.  Thus, the glucose is 
released and can be enter glycolysis and cellular metabolism.

Cellulose is a glucose polymer like starch.  However, the sugars are linked 
by beta linkages.  These beta linkages cannot be broken down by amylase.  
In fact, none of the digestive enzymes produced by animals can break these 
beta linkages.  Thus, cellulose is indigestible and the glucose it contains 
cannot be released as food for the animal.  Although animals are unable to 
digest cellulose, some bacteria can.  These bacteria produce an enzyme 
called cellulase which can release the glucose molecules in cellulose 
chains.  Animals known as ruminants have more than one stomach.  In one of 
these stomachs the animals maintain a dense bacterial culture.  It is here 
that cellulose and other fibrous plant materials are digested.  This 
partially digested material moves to another stomach where it is further 
broken down along with some of the microbes by the action of the animals' 
digestive enzymes (proteases, amylases, etc).  The "ectosymbiosis" with 
bacteria is what allows cows to survive on a diet of grass.  Humans do not 
maintain a suitable microbial community because of the harsh conditions of 
our one stomach and thus cannot digest grass well.

The story for lignin, another plant structual component, is similar.  
Lignin is not a polysaccharide, but rather a polymer made up of phenolic 
groups.  Its chemical structure looks like hexagonal chicken wire.  Fungi 
produce ligninases that can degrade the lignin.  Again these organisms need 
to be in some symbiotic relationship with the host animal.  Termites that 
subsist on wood high in lignin maintain suitable organisms.  Humans do not.

Cheers,

Mark Schneegurt
University of Notre Dame

PS On the definition of biochemistry.  It is the study of the chemistry of 
living systems.  So, any chemical reaction, from glucose breakdown to lipid 
biosynthesis, that occurs within or because of a living organism falls into 
the realm of biochemistry.  Biochemists often study the details of how 
enzymes catalyze chemical reactions or define intermediates in synthetic or 
catabolic pathways.