Re: Enzymes

Why is mannose toxic to honeybees?
What do hexokinase and phosphomannose isomerase do?
What do hexokinase and phosphomannose isomerase have to do with each other?


Before I answer these questions in reverse order, I feel that I should explain
what mannose is, and how it differs from other sugars.  Mannose is a mono-
saccharide, which is a simple sugar composed of only a single subunit: itself.
Mannose is a hexose, which means that it is a sugar with only six(6) carbons
( C6H12O6 ).  The most common hexoses are glucose and fructose, which are the
sugars found in most sweet things.  Below are the chemical structures of
glucose and mannose to show how similar they are:



If you noticed, the only difference is that one of the hydroxyl (-OH) side
chains is above the ring instead of below it!  Because the only difference
between these compounds is structural, they are called epimers (fructose
is an isomer of mannose).  This is actually a huge difference to the enzymes
which convert sugar into energy. Animals can only produce energy from glucose
and fructose, so other hexoses, like mannose, have to be converted into
glucose or fructose to be useable.

For most enzymes to work with sugars, they need phosphate (-PO4) groups on the
sugars to act as "handles".  So, before mannose can be converted, hexokinase
adds a phosphate group to mannose - making it phosphomannose.  Now, another
enzyme, phosphomannose isomerase, can convert phosphomannose into phospho-
fructose, so that other enzymes in the body can convert phosphofructose into
energy.  So hexokinase phosphorylates mannose, so that phosphomannose isomerase
can convert it into phosphofructose, which the body can use for energy.

A very small number of people and, as you pointed out, most bees do not have one
or more of the enzymes required to properly use mannose.  Since it can't be used,
the mannose just sits around in the cells and accumulates, until it reaches toxic
levels and kills the cells.  At low levels, mannose can be cleared from the system
faster than it accumulates, but at slightly higher levels, it is poison to any
animal that cannot convert it into fructose.

I hope this answers your questions.  As a postscript:  mannose (Man) is a very
important sugar for living things for reasons other than energy, so it is found
in most foods, especially from plants.  Most organisms can also convert other
sugars into mannose, so it is not essential to most diets.