Biochem-Carbohydrates

Glycosidases (glycoside hydrolases) are ________ that are specific for the structure and configuration of the glycosyl residue to be removed.

endoglycosidases

Glucose and galactose are _____ epimers, while glucose and mannose are _____ epimers.

glucose and galactose: C-4 glucose and mannose: C-2

Glucose forms a ______ ring, while fructose forms a ______ ring.

glucose: pyran "f"ructose: "f"uran

_______ bonds are formed between monosaccharides to form disaccharides.

glycosidic bonds!

Glycosidic bond formation is catalyzed by _________ using nucleotide sugars (UDP-sugar)

glycosyltransferases during glycosidic bond formation, an endergonic reaction is coupled with an exergonic reaction and UDP is released for energy

Glucose, fructose, galactose, and mannose are all ________.

isomers isomers are compounds with the same chemical formula but have different structures

What can the bacterial fermentation of undigested carbs lead to?

it can lead to increases in CO2 and H2 gas this can cause bloating, abdominal cramps, and flatulence

Almost all sugars in the body will be in _______ form, while <1% of sugars will be in _______ form.

most sugars will be in CYCLIC (RING) form <1% of sugars will be in OPEN-CHAIN (ACYCLIC) form

Digestion of dietary carbohydrates occur in either the ______ or _______.

mouth or intestine

Where are the disaccharidases attached?

on the mucosal lining of the upper jejunum disaccharidases break disaccharides into monosaccharides examples of disaccharidases: isomaltase, maltase, lactase, sucrase

What are enantiomers?

pairs of structures that are mirror images, and designated as D- and L- sugars

If the -OH group on the anomeric carbon is not involved in glycosidic bond formation, the ring can open, and the sugar can act as a _______ agent and are thus called ______ sugars.

reducing agent called reducing sugars

Glucose entry into most cells is concentration-driven and independent of _________.

sodium

Fructose (as well as glucose and galactose) is absorbed into the mucosal cell via what?

sodium-independent monosaccaride transporter (GLUT 5)

What are the sites of absorption for monosaccharides?

the duodenum and upper jejunum

How are monosaccharides classified?

they are classified according to the number of carbon atoms they contain

When does pancreatic a-amylase take over the digestion?

when the acidic content reaches the small intestine and the bicarbonate neutralizes the content, allowing the pancreatic a-amylase to start digesting the dextrin into monosaccharides and disaccharides

When is salivary a-amylase denatured?

when the content reaches the stomach, because the acidic environment denatures the amylase thus the digestion done by salivary amylase STOPS in the stomach

We cannot digest cellulose at all. Why is that?

Because cellulose has ONLY b1-4 linkages (we don't have enzymes to break b1-4 linkages!!) specifically we don't have B-endoglucosidases which can digest B1-4 linkages

Glycogen is partially digested in our body. Why is that?

Because glycogen has BOTH a1-4 and a1-6 linkages (salivary amylase can only break a1-4 but not a1-6 linkages)

What glucose transporter(s) are present in hepatocytes and the beta islet cells of pancreas?

GLUT2 it is a low affinity transporter and allows glucose to enter in when glucose concentration is high (after a meal)

What glucose transporter(s) is present in adipose tissue and muscle and responds to glucose concentration in peripheral blood?

GLUT4 the rate of glucose transport in these two tissues is increased by insulin (insulin stimulates the movement of additional GLUT4 transporters to the membrane)

Lactose is made from what two monosaccharides? What linkage do they form?

Galactose and Glucose B1-4

Sucrose is made from what two monosaccharides? What linkage do they form?

Glucose and Fructose a1-2

Maltose is made from what two monosaccharides? What linkage do they form?

Glucose and Glucose a1-4

__________ hydrolyze/break glycosidic bonds

Glycosidases or glycoside hydrolases

Are galactose and mannose epimers?

NO

Our body has enzymes to break _________ linkages but not ______ and _______ linkages.

Our body has enzymes to break a1-4 linkages (salivary amylase) but NOT b1-4 and a1-6 linkages

What are the enzymes that can interconvert D- and L-isomers?

Racemases

Glucose or galactose are transported into the mucosal cell via ________ active transport.

SECONDARY it is secondary because it does not use ATP directly but it is dependent upon the Na+ concentration in the intestinal epithelial cell THUS it is dependent on Na+/K+ pump

True of False: there is no carbohydrate digestion in the stomach.

True! the final digestion process occurs primarily at the mucosal lining of the UPPER JEJUNUM (where disaccharidases are attached)

Cyclization creates an ______ carbon, generating alpha and beta configurations of the sugar.

anomeric (carbonyl carbon)

What are epimers?

carbohydrate isomers that differ in configuration around ONLY ONE specific carbon atom examples: 1) glucose and galactose (C-4 epimers) 2) glucose and mannose (C-2 epimers)

What is normal glucose concentration in peripheral blood?

4-6 mM (70-130 mg/dL)

Glucose enters B-cells through the glucose transporter GLUT2 when the glucose exceeds ______ mM.

5 mM

Glucose and Fructose both have ____ carbons.

6

What are some other substances known to stimulate insulin release?

1) amino acids from ingested proteins 2) acetylcholine released from vagus nerve endings (parasympathetic NS) 3) gastrointestinal hormones released by enteroendocrine cells of intestinal mucosa and GIP 4) 3 amino acids (alanine, glycine, and arginine) act similarly to glucose by altering the B-cell's membrane potential

What can cause the abnormal degradation of disaccharides? What can this lead to?

1) defects in specific disaccharidases in the intestinal mucosa This can lead to: -undigested carbs (osmotically active) entering the large intestine-->diarrhea -bacterial fermentation of undigested carbs -lactose intolerance

What are the steps of insulin release from a pancreatic B-cell?

1) glucose enters B-cells via GLUT2 2) glucose undergoes glycolysis/respiratory cycle producing ATP molecules by oxidation 3) ATP-dependent K+ close which depolarizes the membrane 4) the depolarization causes voltage-gated Ca+ channels to open and calcium flows into the cells 5) increased calcium level in the cell activates phospholipase C 6) phospholipase C cleaves PIP2 into IP3 and DAG 7) IP3 binds to ER and allows Ca+ release from ER (further raising intracellular Ca+) 8) Increased Ca+ causes release of insulin which are stored in secretory vesicles

Most of the sugars in the human body are D- or L- sugars?

D-sugars!!!

What are some important disaccharides? polysaccharides?

DISACCHARIDES: 1) lactose 2) sucrose 3) maltose POLYSACCHARIDES: 1) glycogen (animal source) 2) starch (plant source) 3) cellulose (plant source)

All three monosaccharides (glucose, galactose, and fructose) exist the mucosal cell and enter the portal circulation via what?

GLUT 2

What glucose transporter(s) mediate basal glucose uptake in most tissues including the brain, nerves, and red blood cells?

GLUT1 and GLUT3 these have high affinities for glucose and ensure glucose entry during periods of relative hypoglycemia. thus, at normal glucose concentration, GLUT1 and GLUT3 are at Vmax!

90% of Africans and Asians are ___________.

Lactose intolerant (because they are lactase-deficient) they cannot digest lactose bc they do not have lactase (the enzyme that digests lactose) this can cause bloating, abdominal cramps, flatulence, and diarrhea after the consumption of milk and milk products

The cyclic alpha and beta anomers of sugar in solution are in equal amounts and can be spontaneous interconverted via what mechanism?

MUTAROTATION

Glucose and galactose are transported from the lumen of the small intestine into the mucosal cell via what?

active carrier mediated sodium-dependent glucose transporter-1 (SGLT-1) look at picture in powerpoint (absorption of monosaccharides slide)

For an aldose, the anomeric carbon is on carbon # ____. For a ketose, the anomeric carbon is on carbon # ____.

aldose: C1 ketose: C2

Why are carbohydrates considered osmotically active?

because they have OH groups (these OH groups draws water from the intestinal mucosa into the lumen and leads to diarrhea)

Pancreatic a-amylase digests the ________.

dextrin Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch or glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α-(1→4) or α-(1→6) glycosidic bonds.

The rate of glucose transport in adipose and muscle tissue is increased by _________.

insulin and exercise in skeletal muscle

Acetylcholine triggers insulin release through _______, whereas the last? acts through the mechanism of adenylate cyclase.

phospholipase C

The major dietary polysaccharides are of ______ and ____ origin.

plant (starch composed of amylose and amylopectin) and animal (glycogen)