Week 3 Carbohydrate Metabolism Part A Flashcards
Key carbohydrates supplied in diet
Monosaccharides
Disaccharides
Polysaccharides
Naturally occurring food monosaccharides
Glucose- main CHO in blood
0-3% in veg, 1-8% in fresh fruit, dried fruit up to 35%
Not tolerated by diabetics
Fructose- only common ketose
0-3% in veg, 1-10% in fresh fruit, dried fruit up to 35%
Sweetest natural sugar
Galactose = rare in food free from ( instant coffee 2%) found in milk products after lactose hydrolysis
Ribose- commonest pentose
Found in meat,fish and poultry ( flavoured precursor)
Disaccharides
Sucrose
Glucose + fructose= majority of free sugars in foods
Maltose
Glucose + Glucose
Produced in brewing process
Metabolised to ethanol by yeast
Lactose
Glucose + galactose
Found in milk products after lactose hydrolysis
Polysaccharides starch
Starch contains 2 polysaccharides amylose and amylopectin
Amylose(15-30%) long, largely unbranched, chains of D-Glucose connected y a1-4 linkages
Amylopectin (70-85%) also composed of D-glose residues with (a1-4) linked chains like amylose with (a1-6) linkages
GLYCOGEN
Mammalian storage polysaccharide of glucose
NON STARCH POLYSACCHARIDES (NSPS) - CELLULOSE
Cellulose is the main polysaccharide found in plant cell walls
•Major component of dietary fibre
•Individual chains bou
Individual chains bound to each other through hydrogen bonding to form cellulose microfibrils
•Provide strength to cell walls in all directions, adjacent layers of microfibrils laid down in different directions
β-1,4-D-glucose polymer
OTHER NSPS IN FOOD
Alginate (brown seaweed)
•Linear copolymer of mannuronate and guluronate
monomers, used as a thickener/gelling agent
•Arabinoxylan (bread)
•Xylose polymer backbone with arabinose substituents
•β-glucan (oats and barley)
•Soluble glucose polymer with β-1,3 and β-1,4-glycosidic
bonds, which helps to reduce cholesterol
•Glucomannan (konjac)
•Soluble polymer of glucose and mannose, with galactose substituents, useful for treating constipation
•Pectin (jam, fruit)
CARBOHYDRATE METABOLISM IS ALMOST EXCLUSIVELY GLUCOSE METABOLISM - SOME QUICK PHYSIOLOGY
NSPs
= not easily digested or absorbed. May be fermented by gut microbiome
Glucose
Fructose
Sucrose
Lactose
Maltose
Amylose
Amylopectin
DIFFERENT WAYS OF REPRESENTING D-GLUCOSE
Fisher projection
(last OH on the right D enantiomer)
Haworth projection
(OH on anomeric carbon
below ring = α,
above ring = β)
Chair conformation - OH can be axial (vertical) or equatorial (at an angle)
Take away
All carbohydrate is broken down and absorbed as monosaccharides
•Glucose
•Galactose
•Fructose
•All this drains into the hepatic portal vein – straight into the liver
•In the liver Fructose and Galactose can be utilised or converted into glucose for export out into the systemic circulation for use by all other tissues (e.g. muscle)
•THEREFORE: Carbohydrate metabolism is really Glucose metabolism