Lecture 1: Carbohydrates 1

Question 0

What are carbohydrates?
Consists of?
Structural components?

Answer 0

They are called saccharides or sugars 
-name end with -ose eg sucrose 
Consists of: 
-carbon
-hydrogen and oxygen (hydrate)
-sometimes nitrogen (amino sugars) 
Structural components: 
-hydroxide groups (-OH)
-aldehyde (aldose) or ketone (ketose)

Question 1

Why are carbohydrates important?

Answer 1

They are Building blocks for:
-lipids
-non-essential amino acids
-RNA and DNA
-components of glycoproteins and glycolipids (cellular structures and recognition systems)
Energy:
-oxidation of glucose leads to production of ATP

Question 2

Monosaccharides

Types?

Answer 2

Simplistic carbohydrates 
-cannot be hydrolysed into simpler forms 
Trioses (3C) 
Tetroses (4C) 
Pentoses (5C)
Hexoses (6C) 
Heptoses (7C)
Octoses (8C)

Question 3

What is stereoisomerism?

What are its different forms?

Answer 3

-Same chemical formula different structural configuration
Forms:
1. D and L isomerism
2. Pyranose and furanose ring structures
3. Alpha and beta anomers
4. Epimers
5. Aldose/ ketose isomerism

Question 4

D and L isomerism

Answer 4

Orientation of -H and -OH groups around carbon atom adjacent to terminal primary alcohol carbons
Look at slide 9 for image

Question 5

Optical isomerism

Answer 5

Sugars are optically active

• bending a beam of polarised light to right = (+)
• bending to left = (-)
• therefore sugar isomers can be D (+) or D(-)

Question 6

Epimers

Answer 6

Determined by orientation of -H and -OH groups around carbon atoms 2,3 and 4 on glucose.
Slide 12

Question 7

Pyranose/ furanose rings

Answer 7

-Physiologically, sugars exist as ring structures
-formed by aldehyde or keto group and neighbouring hydroxyl group
6 carbon ring= pyran
5 carbon ring = furan- he’s fuming coz he’s only got 5 C
99% of glucose in pyranose form
Slide 14

Question 8

Aldose/ ketose isomerism

Eg fructose and glucose

Answer 8

Refer to slides 14 and 15

Question 9

Alpha/ beta anomers

Answer 9

Anomeric carbon is the carbon in aldehyde or keto group of a monosaccharide
Slide 16 and 17
Ie OH below plane of ring = a-anomer
OH above plane of ring is =b-anomer

Question 10

What are the common forms of carbohydrates?

Answer 10

Monosaccharides-single sugar
Disaccharides- 2 sugar residues
Oligosaccharides - 3 to 11 sugar residues
Polysaccharides -many sugar residues

Question 11

Linking saccharides. How are they linked?

Answer 11

Saccharides are linked by glycosidic bonds
Glycosidic bonds:
-formed by -OH groups of anomeric carbon, and a second compound (eg monosaccharide, amine)
-if second group is -OH = O-glycosidic bond
-if second group is amine = N-glycosidic bond
-linkages are either a or b depending on anomeric isomer
-enzymes highly specific for each glycosidic bone

Question 12

Disaccharides

Answer 12

Two monosaccharides linked by glycosidic bond
Slide 21
Digestion: acid hydrolysis and sucrase

Question 13

What are the 3 key features of carbohydrates?

Answer 13

1. Sugars are right-handed (D-isomers)
2. Sugars (monosaccharides) form specific glycosidic bonds (a (alpha) or b (beta) between anomeric carbons and OH grouped generally on the next sugar to form disaccharides, oligo-saccharides and polysaccharides
3. Specific enzymes are needed to break these glycosidic bonds

Question 14

Glucose homeostasis

Answer 14

-Glucose is quantitatively the most important carbohydrate in animals
-plasma concentration is tightly controlled
-normal plasma glucose levels
Human- 4.5mM
Ruminants eg sheep 3-4mM
Hypoglycaemia: half normal values. Fuel deprived for brains RBC
Hyperglycaemia: extended elevation >10mM as in uncontrolled diabetes can lead to glycosylation of proteins

Question 15

What are the 4 major processes of glucose homeostasis?

Answer 15

1. Dietary carbohydrate intake ie substrate availability
2. Main hormones
   Insulin, glucagon, adrenalin
3. Tissue interrelationships
   Eg role of liver, muscle, fat and brain
4. Metabolic activity:
   Enzyme pathways; enzyme regulation

Question 16

GLUT transporters

Answer 16

All glucose uptake via cells is via
GLUT transporters (facilitated passive diffusion)
-all cells contain specific GLUT proteins
-GLUT proteins vary in their kinetic properties

Question 17

Tissue location, kinetic properties and insulin-responsiveness of GLUT transporters

Answer 17

Refer to slide 35 and 36

Question 18

GLUT 1 & 3 transporters

Answer 18

Proteins in plasma membranes that facilitate transport of glucose into cells.
Glut 1:
Tissue location: brain, placenta, kidney and colon
Km: 1mM
Insulin responsive: no
Glut 3: many tissues particularly brain, placenta and kidney
Km: <1mM
Insulin responsive= no
Normal plasma glucose at homeostasis = 5mM
Therefore both these Glut transporters will be saturated at homeostatic glucose
0 order during glucose homeostasis

Question 19

Glucose dependence of tissues. Ie what tissues are most dependant on glucose

Answer 19

Erythrocytes (GLUT 1):
-no mitochondria therefore glucose dependant for energy. Cannot use fats/ amino acids as they require mitochondria.
Brain/nervous tissue (GLUT 1 & 3):
-entirely glucose dependant. Aerobic metabolism
-human adaption: ketones in extreme starvation
Type 2 (white) muscle (GLUT 4):
-limited oxygen supply and few mitochondria.
Glycogen supplies glucose during anaerobic metabolism
Liver (GLUT 2)
-central to glucose homeostasis: glucose sink.
Metabolism not glucose dependant

Question 40

Glut 2 and 4 transporters. Tell me there tissue location, Km and wether there insulin responsive.
And glut 5

Answer 40

Glut 2:
Liver, B-islets cells if pancreas, kidney and small intestine
Km: 12mM
Insulin response: no
-non-saturating at homeostatic glucose
Transporters are first order during glucose homeostasis

Glut 4: muscle, heart, adipose tissue (white and brown)
Km: 5mM
Insulin responsive: yes
-glut 4 transporters are first order during glucose homeostasis but insulin acts to recruit more GLut 4 proteins to increase rate.

Glut 5: small intestine
Km: NC
Insulin response: no