POLYSACCHARIDES
Polymers of monoses linked with glycosidic bond;
they are not sweet;
insoluble in water and in common solvents (they form suspensions);
linear (cellulose) or branched (starch, pectins);
homopolysaccharides: by hydrolysis they give only a monosaccharide (starch,
cellulose, galactane, xilane, etc.);
heteropolysaccharides: by hydrolysis they give diverse molecules (pectins,
emicellulose, resins…);
they are very important in our diet;
almost all have a crystalline structure.
CLASSIFICATION
Structural
They constitute stiff, mechanical plant structures.
Storage
They are metabolic reserve (starch, glycogen).
Waste
They derive from fungi and bacteria.
CELLULOSE
Together with hemicellulose (xylans, mannans, galactans), pectins, lignin, etc. it
constitutes not digestable dietary fiber;
it is a polymer with a structural function;
it is constituted by chains having about 3000 of molecules of glucose (MW about
500.000) bonded with linkage β-1,4 which gives linearity;
the various filaments are parallel to the fibres axis, forming microfibrils that give
stiffness, resistance and insolubility;
extensin, a complex glycoprotein, is linked to the fibrils acting as cementing
together with pectins and hemicellulose.
STARCH
it gives the main part of the daily calories to our diet;
it is found in nature as supply polysaccharide of many vegetables (in tubers and
seeds);
by partial hydrolysis it gives almost only maltose;
by total hydrolysis (chemical or enzymatic) it affords glucose;
granules have characteristic forms and dimensions (2-100 µm) for each specie;
the majority of starchs is constituted by 20-30% amylose and 70-80% amylopectin.
differences among starchs depend on:
-ratio between amylose and amylopectin fractions;
-polymerization level of amylose;
-amylopectin branch.
Amylose
Its molecular size is variable depending on the specie: e.g. 1000-2000 glucose
molecules compose amylose from wheat starch and 4500 that from potatoes
starch (MW=105-106);
linkage α-1,4 ⇒ linear helix structure with 6 molecules in each coil;
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
it is soluble in water;
with I2 its suspensions are blue (25 residues: λmax=550nm).
O
Amylopectin
the polymer is made up of 106 molecules of glucose as α-glucopyranose (MW=
2*108);
linkage α-1,4 with branches (about 1 for 25 units of glucose) α-1,6 and rarely 1,3;
with I2 its suspensions are red-brown (100 residues: λmax=700nm).
Structural model as bunch
O
O
O
O
O
branches 1,6
O
O
O
O
O
O
O
O
O
O
O
O
O
O
ENZYMATIC HYDROLYSIS of STARCH
α-amylase
It is a specific enzyme, it hydrolyses α−1,4-linkages separating
large branched molecules (dextrins);
it is found in vegetables and in almost all animal organisms (men
included);
2 isoenzymatic forms exist: one produced by salivary glands
(ptyalin in saliva) and one produced by pancreas.
β-amylase
It attacks amylose from the non reducing ending releasing
maltose units; its activity stops at 1,6 branches;
it is found only in bacteria and in few animals.
liquefaction
saccharification
maltose
α-amylase
55-60°
°C
STARCH
105-110°
°C
dextrins
Glucoamylase
and
pullulanase
glucose
fructose
glucoseisomerase
There are three stages in the conversion of starch:
1) gelatinisation, involving the dissolution of the starch granules to form a viscous suspension;
2) liquefaction, involving the partial hydrolysis of the starch, with concomitant loss in viscosity;
3) saccharification, involving the production of glucose and maltose by further hydrolysis.
Dextrose Equivalent:
represents the percentage
hydrolysis of the glycosidic
linkages present.
Pure glucose has a DE of 100,
pure maltose has a DE of 50,
starch has a DE of effectively zero.