Page 23 - TBT4135

23
believed that algal alginates are a result of a biosynthesis similar to that of
bacterial alginates.
1.2.4. Content and distribution of M and G in alginates
Several different epimerases act together to give a variety (in principle an
indefinite number) of alginates, where the content of M and G may vary from
below 20% G to more than 70% (e.g. outer cortex of L. hyperborea ).
The sequence of M and G can also vary. The epimerases are processive
enzymes which first bind to the mannuronan chain and then work their way
along the chain before they are released. Depending on the type of enzyme
fundamentally different sequences are formed. For instance, the enzyme
AlgE4 tends to produce polyalternating sequences:
..MMMMMMMMMMMMMMMM.. → .MMMG MGMGMGMGMG MMMMMM....
Another enzyme (AlgE6) forms long G-blocks:
..MMMMMMMMMMMMMMMM.. → ..MMMGGGGGGGGGGGGGGMMM....
Since many enzymes probably work together simultaneously, with different
rates and specificities, and the starting point may be partly statistic (enzymes
bind randomly to begin with), the epimerized alginates become complex
mixtures displaying compositional heterogeneity. It is therefore very unlikely
that two long alginate chains are identical. This certainly contrasts many
regular polysaccharides (e.g. xanthan or agarose), not to mention proteins.
Alginates with different sequences and G-contents may have widely different
properties. To correlate these properties to the chemical structure we
therefore need parameters describing both G-content and sequences more
accurately than only percentages. The alginate field has therefore retained a
sequence terminology originally developed for synthetic copolymers, namely
fractions, or frequencies.
The fraction of M and G residues is defined as:
FG = nG
nG + nM
FM = n M
nG + nM
= 1 − FG