intro to carbs Flashcards
What is the most abundant molecule in nature?
carbohydrates
function of carbs in humans
- significant fraction of dietary calories
- major energy source
- storage form of energy (glycogen)
- cell membrane components that mediate some forms of intercellular communication and retain water
structural component of many organisms
- cell walls of bacteria
- exoskeleton of insects (chitin)
- fibrous cellulose of plant cell wall (indigestible)
stoichiometric formula
(CH2O)n
consists of carbon and water in ratio of C:H:O= 1:2:1
monosaccharides
- 1 unit
- simple sugars
- glucose and fructose
disaccharides
-2 monosaccharide units
oligosaccharides
3-10 monosaccharide units
polysaccharides
more than 10 monosaccharide units
monosaccharide classification
- according to the number of carbons they contain
2. according to the type of there most oxidized functional group
isomers
compounds that have the same chemical formula but different structures
gluctose, fructose, mannose, and galactose are all structural isomers
epimers
compounds that differ in the configuration around only one specific carbon atom
glucose and galactose are C-4 epimers
glucose and mannose are C-2
enantiomers
pairs of structures that are mirror image to each other
D-glucose and L-glucose
D-glucose
found in nature
L-glucose
cannot be metabolized
monosaccharides: cyclization
> 99% of monosaccharides with 5 or more carbons exist as a ring, in which the aldehyde or kept group has reacted with an alcohol group of the same sugar
creates an anemic carbon (isomer) that can exist in either alpha or beta form
pyranose
6 membered ring with 5 carbons and one oxygen
Furanose
5 membered ring with 4 carbons and one oxygen
enzyme specificity of anomeric monosaccharides
they are specific to one of the anemic forms
Glycosidic bonds
bonds that link sugars
Beta gycosidic
beta(1-4)
alpha glycosidic
alpha(1-4)
alpha (1-6)
lactose
glucose + galactose
Beta (1-4)
Sucrose
glucose + fructose
alpha (1-2)
Maltose
glucose + glucose
alpha (1-4)
When can the ring on a cyclic mono-disaccharide open?
if the OH group on the anemic carbon of a cyclized sugar is NOT linked to another compound by a glycosidic bond
reducing agent
the sugar can act as a reducing agent once the ring opens
What can be a reducing sugar?
all monosaccharides but not all disaccharides
positive urine test for reducing sugars
indicative of underlying pathology
glycogen
highly branched polymer of glucose, alpha 1-4 and alpha 1-6 linkages
major energy storage in animal (liver and muscle cells)
amylose
polymer of glucose in plants
unbranched; alpha 1-4 glycosidic linkages
Amylopectin
polymer of glucose in plants
branched; alpha 1-4 and alpha 1-6 glycosidic linkages
a little less branched than glycogen
cellulose
unbranched glucose polymer in plants
beta 1-4 glycosidic linkages
humans can’t digest
complex carbs
carbs attached to non-carb structures such as
- purines and pyramidines
- aromatic rings (steroids)
- proteins (glycoproteins)
- lipids (glycolipids)
types of bonds by which a sugar may be attached to a non-carb group
- NH2 group = N-glycosidic link (N-linked)
- OH group = O-glycosidic link (O-linked)
Where do carbs get digested?
- mouth
- intestinal lumen
- mucosal lining of the upper jejunum
What digestive enzymes break down carbs?
glycoside hydrolases (glycosidases), use a molecule of water
Mouth enzymes
salivary alpha-amylase
intestinal lumen enzymes
pancreatic alpha-amylase
enzyme of the mucosal lining of the upper jejunum
isomaltase, galactosidase, trehalase
what two enzymes act the same but are secreted from different tissues?
salivary alpha-amylase and pancreatic alpha-amylase
role of salivary alpha amylase
breaks large insoluble carbs into smaller soluble ones
specificity of salivary alpha amylase
hydrolysis of alpha 1-4 bonds ONLY
substrates of salivary alpha amylase
any carb with alpha 1-4 bonds (starch, glycogen, maltose)
salivary alpha amylase products
short branched and unbranched oligosaccharides (dextrins)
salivary alpha amylase pH optimum
- 0
- inactivated by the acidic pH in stomach within 20 minutes
pancreatic alpha amylase role
continues to break down carb molecules into smaller soluble ones
pancreatic alpha amylase specificity
hydrolysis of alpha 1-4 bonds ONLY
pancreatic alpha amylase substrates
any carb with alpha 1-4 bonds (starch, glycogen, maltose, and smaller dextrin)
pancreatic alpha amylase product
shorter branched and unbranched oligosaccharides (dextrin) and disaccharides
pancreatic alpha amylase pH optimum
7.0
pancreatic alpha amylase and pathology
plasma levels of either pancreatic amylase or total amylase are used as a diagnostic marker for pancreatitis, should not be found in plasma
final digestions in the mucosal cells
all enzymes are transmembrane proteins of the brush border on luminal surface of the intestinal mucosal cells
sucrase/isomaltase (SI)
1 protein-2 subunits with enzymatic activities
- cleaves alpha 1-2 bonds in sucrose
- alpha 1-6 bonds (branches) in isolates
maltase-glucoamylase (MGA)
1 protein-2 enzymatic activities
- cleaves alpha 1-4 bonds in maltose/maltotriose (maltase activity)
- alpha 1-4 bonds in dextrins (glucoamylase)
lactase
beta 1-4 bonds in lactose (milk sugar)
high expression in infants and gradual decrease with age
trehalase
alpha 1-1 bonds in therapies (mushrooms and fungi)
What kind of sugars get absorbed?
ONLY MONOSACCHARIDES. in the upper jejunum and duodenum
