This presentation briefly describes the structure and function of Carbohydrates. A detailed explanation of the mechanism of catabolism of monosaccharides, disaccharides and polysaccharides in bacteria is provided.

1. CATABOLISM OF CARBOHYDRATES
Ms. P KIRUBHA PAULDAS
Assistant Professor
Department of Microbiology
S.I.C.E.S Degree College, Ambarnath.
Swayam MOOC : ACADEMIC WRITING
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This work is licensed under a Creative Commons Attribution
4.0 International License.

2. CONTENTS
1. Introduction to carbohydrates
2. Basic structure of mono, di and polysaccharides
3. Utilization of monosaccharides - Fructose, Galactose
4. Breakdown of oligosaccharides - Lactose, Maltose,
Sucrose, Cellobiose.
5. Breakdown of polysaccharides – Glycogen, Starch,
Cellulose

3.  Cx(H2O)y
 70-80% human energy needs
 Polyalcohols with aldehyde or ketone functional group
 Many chiral compounds
 A carbon is chiral if it has four different groups
 Chiral compounds have the same composition but are not
superimposable
 Display in Fisher projection
CH2OH
H OH
CHO
CH2OH
OH H
CHO
D-glyceraldehyde L-glyceraldehyde
ENANTIOMERS

5. H O
H
OHH
HOH
OHH
OHH
OHH
O
OH
OH
OH
CH2
OH
O H O
CH2
OH
OH
OH
OH
OH
Anomeric carbon

9. 3.2.3 Utilization of monosaccharides
[Fructose, Galactose]

10. Fructose
1. Excellent substrate for the growth of many bacteria
2. E.coli employs a PEP-phosphotransferase system[E1+E2+E3] to
uptake fructose
3. Product appearing in cell is fructose-1-phosphate
4. 1-phosphofructokinase is also required for utilization of fructose
5. This enzyme synthesis is induced by fructose

12. Galactose
Galactose induces synthesis of :
1. Galactokinase
2. Glucose:galactose-1-phosphate uridylyltransferase
3. UDP-glucose epimerase

13. 3.2.2 Breakdown of oligosaccharides
[Lactose,Maltose,Sucrose,Cellobiose]

14. Lactose
1. Lactose permease
2. β-galactosidase
3. β -galactoside acetyltransferase (acetylates non-metabolizable
structural analogue)

15. Maltose,Sucrose,Cellobiose
1. Intracellular breakdown is initiated by a phosphorylytic cleavage
2. Phosphorylases are economical and energy of glycosidic link is saved and
ATP is not required
3. Sucrose phosphorylase was first discovered in Pseudomonas saccharophila
4. Maltose and cellobiose phosphorylases occur in starch and cellulose
decomposers.

16. 3.2.1 Breakdown of polysaccharides
[ Glycogen, Starch, Cellulose]

17. Glycogen
In a wide range of organisms, excess glucose is converted to
polymeric forms for storage—glycogen in vertebrates and many
microorganisms.
In vertebrates, glycogen is found primarily in the liver and
skeletal muscle.
The glycogen in muscle is there to provide a quick source of
energy for either aerobic or anaerobic metabolism. Muscle
glycogen can be exhausted in less than an hour during vigorous
activity. Liver glycogen serves as a reservoir of glucose for other
tissues when dietary glucose is not available
glycogen to glucose 6-phosphate (glycogenolysis)
glucose 6-phosphate to pyruvate (glycolysis)
pyruvate to glucose (gluconeogenesis)
glucose to glycogen (glycogenesis)

18. Glycogen Breakdown Is Catalyzed by Glycogen Phosphorylase
In skeletal muscle and liver, the glucose units of the outer branches of glycogen enter
the glycolytic pathway through the action of three enzymes:
• glycogen phosphorylase
• glycogen debranching enzyme
• phosphoglucomutase
Glycogen phosphorylase catalyzes the reaction in which an (1→4) glycosidic
linkage between two glucose residues at a non-reducing end of glycogen undergoes
attack by inorganic phosphate (Pi), removing the terminal glucose residue as D-glucose-
1-phosphate. Pyridoxal phosphate is an essential cofactor in the glycogen
phosphorylase reaction; its phosphate group acts as a general acid catalyst, promoting
attack by Pi on the glycosidic bond.
Glycogen phosphorylase acts repetitively on the non-reducing ends of glycogen
branches until it reaches a point four glucose residues away from an (1→6) branch
point, where its action stops. Further degradation by glycogen phosphorylase can occur
only after the debranching enzyme, formally known as oligo (α1 → 6) to (α 1 → 4)
glucantransferase, catalyzes two successive reactions that transfer branches. Once
these branches are transferred and the glucosyl residue at C-6 is hydrolyzed, glycogen
phosphorylase activity can continue.

19. Glucose 1-Phosphate can enter Glycolysis or, in Liver, Replenish Blood Glucose
Glucose 1-phosphate, the end product of the glycogen phosphorylase
reaction, is converted to glucose 6-phosphate by phosphoglucomutase, which
catalyzes the reversible reaction
Initially phosphorylated at a Ser residue, the enzyme donates a phosphoryl group to C-
6 of the substrate, then accepts a phosphoryl group from C-1.
The glucose 6-phosphate formed from glycogen in skeletal muscle can enter glycolysis
and serve as an energy source to support muscle contraction.

22. Starch

24. Cellulose

28. 1. White, D., Drummond, J. and Fuqua, C. (2012). The physiology
and biochemistry of prokaryotes. New York: Oxford University
Press.
2. Gottschalk, G. (2009). Bacterial metabolism. New York:
Springer.
3. Lehninger, A., Nelson, D. and Cox, M. (2013). Principles of
biochemistry. New York: W.H. Freeman.