Carbs: Structure, Digestion and Absorption Flashcards
What are the three major sugars that can be used as fuel?
Glucose, Fructose and Galactose
**only one we really need is glucose
What does adding fructose to things do for us?
It adds sweetness to what we eat
What are simple carbohydrates?
They contain 1 or 2 monomers of glucose, fuctose and galactose
Monosaccharides contain 1
Disaccharides contain 2
What are complex carbohydrates?
Contain 100s to 1000s of monomers
Starches (from plants)
Glycogen (from animal)
Fibers (from plant)
What is sucrose?
Disaccharide composed of glucose and fructose
What is lactose?
In milk and is a combo of galactose and glucose
**galactose must be first in order for us to break it down
What is maltose?
Used in beer
composed of 2 glucose
What is trehalose?
Found is mushrooms and crab legs
composed of 2 glucose
What is starch?
How plants store glucose
Composed of only glucose
Amylose (single strand of glucose) and Amylopectin (strands of glucose that branch off each other)
What is glycogen?
Storage of glucose in humans (liver and muscle)
Has many branches of glucose (more than amylopectin)
What are fibers?
Composed of cellulose in plants for storage
Its glucose but our bodies are not able to break it down and use it for energy
Do our carbohydrates use an L-configuration or a D-configuration?
every carb in humans is D-configuration
How can you tell if a molecule is glucose when its drawn?
It will have the -OH in the UP position at the 3rd carbon
middle finger
How can you tell a molecule is galactose when drawn?
it will have an -OH in the up position in the 3 and 4 spot
How can you tell if a molecule is fructose when drawn?
It will have a keytone and only be 5 member ring
What is an aldose sugar?
Its a sugar that contains an aldehyde group
Glucose and glactose
What is a ketose sugar?
A sugar that has a keytone group
Fructose
Do fisher structures exist in nature?
No, they do not. When put into solution they will always form the hawthrone projection and make a ring structure
What is the anomeric carbon?
Its the carbon in hawthron projection that becomes chiral after the -OH of the 5 carbon attaches to it in forming the ring
Which carbon is known as the reducing end and which is the nonreducing end?
Carbon 1 is the reducing end
Carbon 6 is the nonreducing end
How do you determine if a sugar is alpha or beta?
If they -OH is on the same side of the 6 carbon is it beta
If the -OH is on the opposite side of carbon 6 then it is alpha
What is the glycosidic bond?
The linkage between the monosaccharide units of disaccharides and polysaccharides
Formed by the removal of a molecule of water (condensation rxn) between two hydroxyl groups
What are the common glycosidic bonds?
- Hydroxyl group of monosac anomeric carbon reacts with another monosac’s hydroxyl of anomeric carbon
- Hydroyxl of one monosacc’s anomeric carbon reacts to another monosacc’s hydroyxl carbon 4
- Hydroyxl of one monosacc anomeric carbon binds to another monosaccs hydroyxl carbon 6
Why does it matter what the glycosidic bond does?
It matters because enzymes recognize the bond which allows them to metabolize them
What is the carbohydrate structure of sucrose?
Glc (a1–>B2)Fru
The anomeric carbon of glucose is bound to the anomeric carbon of fuctose (which is the 2 carbon)
**its non-reducing because both anomeric carbons are in the bond
What is the carbohydrate structure of trehalose?
Glc(a1–>a1)Glc
The anomeric carbon of glucose is bound to the anomeric carbon of another glucose
**this is non-reducing since both anomeric carbons are being used
What is the carbohydrate structure of Maltose?
Glc(a1–>4)Glc
The anomeric carbon of glucose is bound to the 4 carbon of another glucose
**this is reducing because there is an open anomeric carbon
What is the carbohydrate structure of Lactose?
Gal(B1–>4)Glc
The anomeric carbon of galactose is bound to the 4 carbon of a glucose
**this is reducing since there is an unreacted anomeric carbon
**GALACTOSE MUST BE FIRST (if not we can not break it down)
What are the two types of starch and the carbohydrate structure?
Amylose- polymer of glucose with a-1–>4 glycosidic bonds (straight chain)
Amylopectin - polymer of glucose but heavily branched. It is a combo of a-1–>4 and a-1–>6 glycosidic bonds
What is the carbohydrate structure of glycogen?
Same as amylopectin - polymer of glucose but heavily branched. It is a combo of a-1–>4 and a-1–>6 glycosidic bonds
**HAS MORE BRANCHES (this allows us to have quick access to it so we can make glucose)
What other roles do carbohydrates play?
- All organisms use carbs (in ribose form) as the backbone of their nucleic acids
- Plant cell walls are constructed of cellulose which provides rigidity to the plant
- Inertebrates exoskeletons are composed of chitin, a modified sugar
How are carbs involved in regulation of activity of bio systems?
They perform glycosylation which is adding sugars to proteins
- *this can activate or deactivate the protein
- **A1C levels are tracking glycosylation (looks at amount of glycosylation on hemoglobin back 3 months (RBC only lives 120 days))
How is glycosylation used to figure out someones A1C?
You look at someones hemogolbin going back 3 months (due to the fact RBC only live 120 days)
You look at how much RBC are glycosylated, sugar added to the hemoglobin
The more exposure they have to sugar the more they will be glycosylated which is a sign of diabetes
What can be absorbed from the GI tract?
ONLY MONOSACCHARIDES
**digestion breaks down the complex sugars into monosaccharides
What is used to digest starch?
a-amylase Salivary amylase (ptyalin) 20% Pancreatic amylase breaks down 80%
How does salivary amylase (ptyalin) digest starch?
It works in the mouth and breaks down 20% of the starch we take in
It gets destroyed in the stomach due to the low pH
only a small amount of acid digestion of starches occurs in the stomach
What is the process of how pancreatic amylase works?
As food fills the upper levels of the intestinal tract, the hormone cholecystokinin is released which signals the pancreatic exocrine cells to release pancreatic amylase into the intestinal tract
This digests 80% of the starch
How does a-amylase hydrolyze the starch?
It breaks starch a-1,4 linkages randomly
NEVER PRODUCES MONOSACCHARAIDES
What are the products created by a-amylase?
Maltose (disaccharide - a-1,4 linkage)
Isomaltose (disaccharide - a-1,6 linkage)
Maltotriose (trisaccharide - a-1,4 linkage)
a-dextrins (branched oligosaccharide with both a-1,4 and a-1,6 linkages)
What is responsible for further breaking down what was created by brush border enzymes?
Brush boarder enzymes break them down into glucose which can then be absorbed
What are glucosidases?
They are enzymes found in luminal intestine (brush boarder enzymes)
a-glucosidases break a-1,4 and a-1,6 linkages
b-glucosidases break b-1,4 and 1,6 linkages
**they break the glycosidic bond
Why are their villi and microvilli where the brush boarder enzymes are?
it leads to an increase in surface area so more can be converted to glucose
What are the four main brush border glucosidases?
- sucrase-isomaltase
- lactase
- Glucoamylase
- Trehalase
Which of the brush border enzymes are inducible?
all of them except lactase
** this means that if you eat a high amount of one of the other certain types of food your body will produce more glucosidases to break it down
What is the function of sucrase-isomaltase?
Isomaltase breaks down a-1,6 and a-1,4 linkages of disacchardies
100% of a-1,6 linkage (isomaltose)
80% a-1,4 linkage (maltose)
Gives glucose
Sucrase breaks down a,b-1,2 linkages of sucrose to give glucose and dructose
100% of sucrose digestion
What is the function of lactase?
Digests disaccharide lactose (found in milk)
100% digestion of B-1,4 linkage
**remember it must be a galactose linked to a glucose in that order or we can not digest it
What is the function of glucoamylase?
It hydrolyzes a-1,4 linkages in di-,tri- and oligosaccharides
It breaks down mainly trisaccharide and a-dextrins to give glucose and isomaltose (which gets broken down by isomaltase)
It also breaks down 20% of dietary a-1,4 linkage (maltose)
What is the function of trehalase?
100% breakdown of a,a-1,1 linkage of trehalose
**treahalose is a glucose glucose 1,1 linkage
What are a-glucosidase inhibitors?
They are competitive inhibitors of a-glucosidase enzymes found in intestinal brush border membrane
They lower blood glucose lvl by preventing breakdown of dietary starches into absorbable monosaccharides
What are examples of a-Glucosidase inhibitors?
Acarbose (Precose)
Miglitol (Glyset)
What are the primary ways that sugar can be absorbed?
- simple diffusion (minimal)
- Sodium dependent transporters (SGLT1)
- Facilitated (sodium independent) transporters (GLUT 1-5)
What is GLUT 1?
High affinity for glucose
Found in all areas of blood (RBC, blood barriers)
Allows preferential uptake during hypoglycemia and is usually over expressed in tumor
What is GLUT 2?
Has lowest affinity for glucose (high Km)
Found in liver and Pancreatic B-cells
Has a high-capacity to bring glucose in but will open work if there is VERY HIGH levels of glucose in the system
What is GLUT 3?
Has lowest Km (HIGHEST affinity for glucose)
Found in the brain
Major transporter for glucose in CNS and preferential uptake during hypoglycemia by brain
What is GLUT 4?
5 mM Km for glucose
Found in adipose, skeletal and heart muscle
INSULIN MUST BE PRESENT in order for glut 4 to work
once insulin is released it binds to glut 4 and allows uptake of glucose into adipose and muscle
What is GLUT 5?
High Km ONLY FOR FRUCTOSE
Found in surface of intestinal mucosa and sperm
ONLY TRANSPORTS FRUCTOSE
What is SGLT-1?
Uses secondary active transport to bring a sodium as well as glucose or fructose into the cell
Found in luminal surface of intestinal mucosa
What is SGLT-2?
Found in the kidney
Its responsible for the re-absorption of glucose from urine
**drug target of jardiance, invokana)
How long does it take for glucose levels to peak you eat a meal and what does it depend on (what type of food lead to a higher spike)
It reaches its peak around 60-120 minutes after the meal
They return in after about 2 hours
The higher the glycemic index of the food the higher the glucose will rise
What is the glycemic index?
A value assigned to food based on how slowly or how quickly those foods cause increases in blood glucose
**higher number means higher GI which raises it higher
How does the curve on a graph differ for a food with a high GI compared to one with a lower GI?
The one with a high GI will cause a very high spike in plasma glucose and not last long
One with a low GI will cause a small shift in GI and stay in the system longer so you feel more full for longer
What are the two types of fiber?
Soluble (small)
Insoluble (large)
What is dietary fiber made of?
Indigestible sugar
The linkages we are not able to break (hydrolyze)
What is the purpose of fiber?
It assists with GI motility and mantenance of the GI tract
25-38g of fiber per day or 14g per 1000 cal
What is the fate of undigested carbohydrates in the GI tract?
Our intestinal flora and fauna can digest some of it
This produces methane and hydrogen gas as well as lactic acid
**this gas is what causes bloating and GI issues and the lactic acid can cause diarrhea thro osmotic effect
What is Carbohydrate intolerance?
A disease when someone can not digest specific carbohydrates
- *this leads the carbs to be digested by bacteria which leads to GI disturbances proportional to amount of specific sugar present
- *most common is lactose intolerance
