This document summarizes the biosynthesis of two disaccharides: sucrose and lactose. Sucrose is synthesized in the cytosol from triose phosphates generated in the Calvin cycle in chloroplasts. Triose phosphates are converted to sucrose through a series of enzymatic reactions involving fructose and glucose intermediates. Lactose is synthesized within the lumen of the Golgi apparatus in mammary gland cells. Glucose and galactose are converted to their nucleotide sugar forms and transported into the Golgi lumen where the lactose synthase enzyme complex catalyzes the formation of lactose from glucose and galactose.

1. BIOSYNTHESIS
OF
DISACCHARIDES
Submitted by : Kailash
M.Sc Biotechnology (P)
Submitted to: Dr. JS Duhan
Deptt. Of Biotechnology
CDLU (Sirsa)

2. SUCROSE
● Sucrose is a common, naturally occurring carbohydrate found in many
plants and plant parts. Saccharose is an obsolete name for sugars in
general, especially sucrose.
● Formula: C12H22O11
● Density: 1.59 g/cm³
● Melting point: 186 °C
● The molecule is a disaccharide combination of the monosaccharides
glucose and fructose with the formula C12H22O11.

3. Biosynthesis Of Sucrose
Location: The synthesis of sucrose is competing process that occurs in the
Cytosol.
● Triose phosphate generated by the calvin cycle acts as precursor for the
synthesis of Sucrose.
● When the cytosolic Pi is high, chloroplast triose phosphate
(Glyceraldehyde 3-phosphate+ DHAP) is exported to the cytosol and
sucrose is synthesized.
● The Phosphate Translocator catalyzes the movement of Triose
phosphate in the cytosol.

4. CALVIN CYCLE
Triose Phosphates
Pi Translocator
CHLOROPLAST
Triose Phosphates (Glyceraldehyde 3-phosphate + DHAP)
Cytosol

5. Sucrose Synthesis
1. Triose phosphates formed by calvin cycle are now converted into
Fructose-1,6-Biphosphate by enzyme Aldolase.

6. 2. Fructose-1,6-Biphosphate is converted into Fructose-6-Phosphate by
enzyme Fructose-1,6-Biphosphatase.

7. 3. Fructose-6-Phosphate is converted into Glucose-6-Phosphate by enzyme
Phospho Hexose Isomerase.

8. 4. Glucose-6-Phosphate is converted into Glucose-1-Phosphate by enzyme
Phospho Glucomutase.

9. 5. Glucose-1-Phosphate combines with UTP in the presence of enzyme UDP
Glucose Pyrophosphorylase to form UDP-Glucose.

10. 6. UDP-Glucose combines with Fructose-6-Phosphate in the presence of enzyme
Sucrose-6-Phosphate Synthase to form Sucrose-6-phosphate.

11. 7. Phosphate group is cleaved by enzyme Sucrose Phosphatase and Sucrose is
released.

13. Biosynthesis of Sucrose

14. Lactose
● Lactose is a disaccharide. It is a sugar composed of galactose and glucose subunits.
Lactose makes up around 2–8% of milk (by weight). The name comes from lac (gen.
lactis), the Latin word for milk, plus the suffix -ose used to name sugars. The
compound is a white, water-soluble, non-hygroscopic solid with a mildly sweet taste.
It is used in the food industry.
● Molecular formula: C12O11H12
● Density: 1.52 g/cm³
● Melting point: 202.8 °C

15. Biosynthesis of Lactose
● Location: Lactose is synthesized within the lumen of Golgi apparatus in
secretory cells in lactating mammary glands.
● lumen of golgi complex contains enzyme Lactose Synthase responsible
for the synthesis of lactose.
● Lactose synthatase is a complex of Galactoside Transferase (GT) + α-
Lacto albumin.
● The synthesis of one molecule of lactose occurs through two molecules
of glucose.

16. Lactose synthesis
1. Glucose-1-Phosphate is first converted into UDP-Glucose by enzyme UDP-
Glucose Phosphorylase.
UDP-Glucose Phosphorylase

17. 2. UDP-Glucose is converted to UDP-Galactose by enzyme UDP-Galactose-4-
epimerase.
UDP-Galactose-4-epimerase.

18. 3. UDP-Galactose is transported to Lumen of golgi complex by UDP-
Galactose transporter (UGT), an antiporter which uses facilitated transport
to move UDP galactose into the Golgi and exports UMP.
4. Glucose enters into Golgi by the help of GLUT-1 Transporter.
5. UDP-Galactose and Glucose become substrates for the lactose synthase
enzyme complex, comprised of the enzymatic subunit, galactosyltransferase
with its regulatory subunit, α-lacto albumin.
6. Lactose synthase creates Lactose through bonding galactose from UDP to
glucose through a glycosidic bond.
7. Although GT is found in many tissues in the body, α-lacto albumin is only
found on the inner surface of the Golgi in the mammary glands, limiting
lactose production to the mammaries.

21. THANK YOU