Lecture 5- Structure of CHO Flashcards
Monosaccharides:
• Glucose is most common
• Occurs naturally
• Can’t be hydrolyzed into a smaller unit
• Considered a “reducing sugars” when there is a free anomeric carbon
Anomeric carbons in the carbon with the carbonyl group
Disaccharides:
• Sucrose is most common
• 2 monosaccharides joined by an acetyl bond (glycosidic bond)
Glycosidic bond determines whether enzyme can break down the CHO
Oligo/Polysaccharides:
• Oligosaccharides ○ 3-6 monosaccharides • Polysaccharides ○ Homo (same monosaccharide) or heteropolysacchairdes (different monosaccharides) ○ i.e. glycogen, starch, cellulose
All CHO have a H:O rato of 2:1
Triose
(3 Carbon) - metabolites of glucose
Pentose
(5 Carbon) - components of DNA and RNA
Hexose
(6 Carbon)- nutritionally the most important
○ Location of carbonyl group determines if CHO is an aldehyde of ketone
§ Aldehyde (aldose) if carbonyl group is terminal
Ketone (ketose) if carbonyl group is not terminal
Diagram
Stereoisomerism:
• 2 CHO with same molecular formula, but differ in configuration around chiral atoms (mirror images)
• Chiral carbon- has 4 different constituents
• 2 forms (D or L), determined by OH group on the highest chiral carbon
○ OH on right- D sugar
§ D sugars are nutritionally more important because digestive enzymes are specific for D sugars
OH on left- L sugar
Diagram
Number of stereoisomers for a molecule
2^n (n= # of chiral atoms)
Cyclization of CHO
• In solution, CHO cycle between cyclical and linear forms
• In cyclical form, near chiral carbon is formed (hemiacetal or hemiketal)
○ Can be alpha or beta configurations
§ Beta more common
§ Digestive enzymes can’t break down beta configurations (specific for alpha)
• Hemiacetal- monosaccharide + aldehyde
Hemiketal- monosaccharide + ketone
Diagram
Converting From Fischer to Haworth:
• Non acetyl/ ketal (Carbon 6) always points up • OH groups ○ Right in Fischer, below in Haworth ○ Left in Fischer, above in Haworth • Hemiacetal (alpha or beta) ○ If alpha, OH points down If beta, OH points up
Formation of Disaccharides
- Most common oligosaccharide
- 2 monosaccharides attached by a glycosidic bond (formed between 2 OH groups)
alpha (1,4), alpha (1,6), beta (1,4), beta (1,6)
• Configuration of anomeric OH group determines whether the disaccharide is alpha or beta ○ Alpha monosaccharide-> alpha disaccharide Possible outcomes
Chart
Polysaccharide Structure
• Long strings or branches of monosaccharides (at least 6) attached by glycosidic bonds
• Homopolysaccharides are more abundant in food
•
Starch
Starch (amylose, amylopectin) is rich in plants
Polymers of D-glucose
Diagram
Glycogen
rich in animal tissue
○ Highly branched
○ Animals need to have more readily available glucose (in case they need to run away)
Branching allows for more glucose to be available to be cleaved off from different ends, more energy available
Diagram
Solubility of Fibre:
• Water holding ability
○ Ability of a fibre to hold water, or become a viscous solution
○ Gel like, slows down absorption of other nutrients
• Absorptive ability
Ability of a fibre to bind enzymes and nutrient
Insoluble Fibres:
• Remains intact through the digestive system
• Reduces transit time
○ Moves quickly through the gut, can’t absorb as much, reduces caloric intake
• Increases fecal bulk
○ Inflammation of digest tract reduced (food moves too quickly, can’t get stuck and cause inflammation)
i.e. cellulose, lignin, some hemicelluloses
Soluble Fibres:
• Forms a gel
• Delays gastric emptying, increases transit time
• Slows down nutrient absorption
i.e. pectin, gums, beta-glucans, some hemicelluloses
Cellulose:
• Both a dietary fibre (naturally occurring) and a functional fibre (naturally occurring, but is added to a product)
• Homopolysaccharide of beta-1,4, glucose units in a linear form
• Poorly fermented by bacteria
○ Humans lack metanogens (cellulose fermenting microbes)
○ Ruminants have metanogens, can break beta 1,4, bonds in cellulose
Rich in bran, legumes, nuts, peas…etc
Hemicellulose:
• Heteropolysaccharide that varies between plants
• Mixture of alpha and beta glycosidic linkages
• Can contain both pentoses and hexoses
○ Xylose is the most common monosaccharide in hemicellulose
• Can be branched or linear
• Solubility and fermentability depends on sugar composition
Found in bran, whole grains, nuts, some vegetables and fruits
Pectin:
• Both a dietary and functional fibre
• Part of cell wall of plants
• Has a backbone of unbranched alpha 1,4-linked D galacturonic acid
• Stable at low pH
• Highly fermented by gut bacteria
○ Considered to be a good bulking agent in animal feeds
Rich in fruits (apples, oranges, lemons)
Resistant Starch:
• 4 main types, termed Rs1-4 (found in different foods)
• Typically found in plant cell walls
Resistant to amylase activity (can’t break down)
Health Benefits of Fibre:
• Maintains function and health of gut
• Insoluble fibre decreases constipation
○ Stimulates muscle contraction to break down waste
○ Decreases risk of bacterial infections
• Soluble fibred increase satiety
○ Delays gastric emptying, makes you feel full
○ Reduced diabetes, cardiovascular disease, obesity
§ Soluble fibres bind to cholesterol and glucose, less cholesterol/glucose absorbed, puts less strain on system, reduces risk of cardiovascular disease or diabetes
Feel full, won’t eat as much
