Lecture 1: Carbohydrates 1 Flashcards
What are carbohydrates?
Consists of?
Structural components?
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)
Why are carbohydrates important?
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
Monosaccharides
Types?
Simplistic carbohydrates -cannot be hydrolysed into simpler forms Trioses (3C) Tetroses (4C) Pentoses (5C) Hexoses (6C) Heptoses (7C) Octoses (8C)
What is stereoisomerism?
What are its different forms?
-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
D and L isomerism
Orientation of -H and -OH groups around carbon atom adjacent to terminal primary alcohol carbons
Look at slide 9 for image
Optical isomerism
Sugars are optically active
- bending a beam of polarised light to right = (+)
- bending to left = (-)
- therefore sugar isomers can be D (+) or D(-)
Epimers
Determined by orientation of -H and -OH groups around carbon atoms 2,3 and 4 on glucose.
Slide 12
Pyranose/ furanose rings
-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
Aldose/ ketose isomerism
Eg fructose and glucose
Refer to slides 14 and 15
Alpha/ beta anomers
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
What are the common forms of carbohydrates?
Monosaccharides-single sugar
Disaccharides- 2 sugar residues
Oligosaccharides - 3 to 11 sugar residues
Polysaccharides -many sugar residues
Linking saccharides. How are they linked?
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
Disaccharides
Two monosaccharides linked by glycosidic bond
Slide 21
Digestion: acid hydrolysis and sucrase
What are the 3 key features of carbohydrates?
- Sugars are right-handed (D-isomers)
- 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
- Specific enzymes are needed to break these glycosidic bonds
Glucose homeostasis
-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
What are the 4 major processes of glucose homeostasis?
- Dietary carbohydrate intake ie substrate availability
- Main hormones
Insulin, glucagon, adrenalin - Tissue interrelationships
Eg role of liver, muscle, fat and brain - Metabolic activity:
Enzyme pathways; enzyme regulation
GLUT transporters
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
Tissue location, kinetic properties and insulin-responsiveness of GLUT transporters
Refer to slide 35 and 36
GLUT 1 & 3 transporters
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
Glucose dependence of tissues. Ie what tissues are most dependant on glucose
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
Glut 2 and 4 transporters. Tell me there tissue location, Km and wether there insulin responsive.
And glut 5
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
