ME03 - Carbohydrates of Physiologic Significance Flashcards
Biomedical Importance of Carbohydrates
(In plants) synthesized from carbon dioxide and
water by photosynthesis and stored as starch.
o (In animals), synthesize carbohydrates from amino acids
_ Most are derived ultimately from plants
_ Acts as a storage form of energy in the body
_ Serves as cell membrane components that mediate some forms of intercellular communication
_ Serves as a structural component of many organisms, including the cell walls of bacteria, the exoskeleton of many insects, and the fibrous cellulose of plants.
Most important carbohydrate
Glucose
How is Glucose formed
Hydrolysis of Dietary Starch and Disaccharides; Converted to glucose through GLYCOLYSIS
Precursor for synthesis of all the other carbohydrates in the body
Glycogen _ for storage
_ Ribose and Deoxyribose _ in nucleic acids, building blocks
_ Galactose _ synthesis of lactose in milk, glycolipids, in combination with protein in glycoproteins & proteoglycans
Disease associated with CARBOHYDRATE METABOLISM
Diabetes Mellitus (most predominant disease in our country)
Galactosemia
Glycogen Storage Diseases o Lactose Intolerance
study of the roles of sugars in health and disease
Glycobiology
entire complement of sugars of an organism, whether free or present in more complex molecules
Glycome
analogous term to genomics and proteomics; the comprehensive study of glycomes
Glycomics
Carbohydrate Digestion
( High Carb Meal) Starch, Sucrose, Lactose - {Salivary amylase, Pancreatic amylase} – Maltose, Isomaltose, Sucrose, Lactose – (Brush border dissacharides) – Glc, Gal, Fru – (Na+/Glc symporter, active transport) – Intestine then bloodstream
Explain Hyperglycemia in well-fed state
Hyperglycemia _ inc blood glucose; transient hyperglycemia is good. We all become hyperglycemic first 2 hrs after a meal. Persistent hyperglycemia in diabetes is bad because it goes into the blood but it cannot go inside the cell
Classification of Carbohydrates
Monosaccharides | Dissacharides | Oligosaccharides | Polysaccharides
Simple sugars; sugars that cannot be hydrolyzed into simpler carbohydrates
Monosaccharides Example: Aldoses (with aldehyde group) or Ketoses (with Ketone group)
_ - OH group attached to glucose molecule
_ Aldehyde or ketone group has been reduced to
an alcohol group
Polyhydric alcohols (sugar alcohols or polyols)
_ Poorly absorbed in the body, Occurs naturally in foods
How is monosaccharides formed to create larger structures
Linked by glycosidic bonds
Condensation products of two monosaccharide units
Dissacharides Example; lactose (Glu + Gal), maltose (Glu + Glu) , isomaltose, sucrose (Glu + Fru),
trehalose
Condensation products of three to ten Monosaccharides
Oligosaccharides
More than ten monosaccharide units
Polysaccharides | Sometimes classified as Hexosans or Pentosans
May be linear or branched polymers
polysaccharides in food that are not digested by human enzymes and are the major component of dietary fiber
Nonstarch Polysaccharides
Examples of Non-starch Polysaccharides
Cellulose - from plant cell walls (glucose polymer) Inulin - the storage carbohydrates in some plants (fructose polymer)
What are the types of Isomerism`
Straight Chain, Cyclic Structure, Chair
structural formula (aldohexose)
Straight - Chain
a hemiacetal formed by reaction between the aldehyde group and a hydroxyl group
Cyclic Structure
molecule is viewed from the side and above the plane of the ring; the bonds nearest to the viewer are bold and thickened, and the hydroxyl groups are above or below the plane of the ring
Haworth projection
Difference between Chair vs Cyclic Formula
Chair confirmation _ explains formula but not different
reactions
Cyclic formula_ better idea, idea how it interacts with heme, water etc., gives 3D idea
compounds that have the same chemical formula but have different structures
Isomers
Most of the naturally occurring monosaccharides
D sugars
Determines whether the sugar belongs to the D or L series.
The orientation of - H and _OH groups around the carbon atom adjacent to the terminal primary alcohol carbon o -OHontherightside:Disomer o -OHontheleftside:Lisomer
A beam of plane-polarized light is passed through a solution of an optical isomer.
Optical Activity
oright, dextrorotatory (+)
oleft, levorotatory (_)
The direction of rotation of polarized light is independent of the stereochemistry of the sugar, True or False
True, so it may be designated D (-), D (+), L (-), or L(+)
Naturally occurring form of fructose
D (-) isomer
_ a six-membered ring
_ For glucose in solution, more than 99% is in
the pyranose form
Pyran
a five membered ring
Furan
What is the ring structure of an aldose
hemiacetal (combination of aldehyde and an alcohol group)
What is the ring structure of ketose
Hemiketal
How is Crystalline Glucose converted to a-d-glucopyranose and B-glucopyranose
Crystalline glucose is _-d-glucopyranose. The cyclic structure is retained in the solution, but isomerism occurs about position 1, the carbonyl or anomeric carbon atom, to give a mixture of _-d- glucopyranose (38%) and _-glucopyranose (62%)
Isomers differing as a result of variations in configuration of the _OH and _H on carbon atoms 2, 3, and 4 of glucose
Epimers Most important epimers of glucose:
o Mannose - Epimerized at carbon 2 Most important epimers of glucose:
o Mannose - Epimerized at carbon 2 o Galactose - Epimerized at carbon 4o Galactose - Epimerized at carbon 4
How is Fructose has the same molecular formula as glucose but differs in its structure
Aldose-ketose Isomerism since there is a potential keto group in position 2, whereas in glucose there is a potential aldehyde group in position 1
formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, and a second compound that may or may not (in the case of an aglycone) be another monosaccharide
Glycosides
Why is it that If the second group is a hydroxyl, the O-glycosidic bond is an acetal
because it results from a reaction between a hemiacetal group (formed from an aldehyde and an _OH group) and another _ OH group
If the hemiacetal portion is glucose, the resulting compound is a ____________
Glucoside
If the hemiacetal portion is galactose, the resulting compound is a ____________
Galactoside
Importance of glycosides in medicine
because of their action on the heart (cardiac glycosides) all contain steroids as the aglycone.
A molecule in which a sugar is bound to another functional group via a glycosidic bond
Glycosides
Types of glycosidic bond
O-Glycosidic bond; N-glycosidic bond
Glycoside that is an inhibitor of the Na+-K+ ATPase of cell membranes.
Ouabain
Example of glycoside that acts an an antibiotic
Streptomycin
Sugars in which one hydroxyl group has been replaced by hydrogen
_ Lack an Oxygen Atom
Deoxy sugars
Examples of deoxy sugars
_ Deoxyribose - derived from the sugar ribose by loss of an oxygen atom
_ l-fucose - Cosmetics, pharmaceuticals, dietary supplements
_ 2-deoxyglucose - Competitively inhibits G6 PO4
Components of Glycoproteins, Gangliosides, & Glycosaminoglycans
Amino Sugars (Hexosamines)
Examples of Amino Sugars (Hexosamines)
_ d-glucosamine - Constituent of hyaluronic
acid (water holding/filler)
_ d-galactosamine - Chondrosamine
(cartilage) a constituent of chondroitin
_ d-mannosamine
Clinical Importance of hexosamines
antibiotics (eg, erythromycin) contain amino sugars, which are important for their antibiotic activity
How are glycosidic bonds named?
Glycosidic bonds between sugars are named according to the numbers of the connected carbons and with regard to the position of the anomeric hydroxyl group of the sugar involved in the bond
_ If the anomeric hydroxyl is in the _ configuration, the linkage is an _-bond
_ .If it is in the _ configuration, the linkage is a _-bond.
How is LACTOSE synthesized
Lactose is synthesized by forming a glycosidic bond between carbon 1 of _-galactose and carbon 4 of glucose. The linkage is, therefore, a _(1_4) glycosidic bond
sugars composed of two monosaccharide residues linked by a glycoside bond
Dissaccharides
What is an invert sugar
Hydrolysis of sucrose yields a mixture of glucose and fructose called “invert sugar” because fructose is strongly levorotatory and changes (inverts) the weaker dextrorotatory action of sucrose
What are the Structures of important disaccharides:
The _ and _ refer to the configuration at the anomeric carbon atom
What happens when the anomeric carbon of the second residue takes part in the formation of the glycosidic bond, as in sucrose,
the residue becomes a glycoside known as a furanoside or a pyranoside
What happens as the disaccharide no longer has an anomeric carbon with a free potential aldehyde or ketone group
it no longer exhibits reducing properties
How is B-fructofuranose residue made up
results from turning the _-fructofuranose molecule through 180 degrees and inverting it.
What are the important disaccharides
Maltose | Sucrose | Lactose
The most important dietary carbohydrate in cereals, potatoes, legumes, and other vegetables
Starch
A-glucosidic chain of Starch
glucosan or glucan
2 main constituents of Starch
Amylose, Amylopectin
Composition of Amylopectin
o 24 to 30 glucose residues
o _1 _ 4 linkages in the chains
o _1 _ 6 linkages at the branch points
_ measure of its digestibility
_ based on the extent to which it raises the blood
concentration of glucose
Glycemic Index ranges from 1 (or 100) to 0 for those that are not hydrolysed at all
_ 55 or less = Low (good)
_ 56- 69 = Medium
_ 70 or higher = High (bad)
_ Animal starch
_ Storage polysaccharide
_ More highly branched structure than amylopectin _ Muscle glycogen
_ Contain up to 60,000 glucose residues
Glycogen
_ polysaccharide of fructose found in tubers and roots of dahlias, artichokes, and dandelions
_ readily soluble in water
_ used to determine the glomerular filtration rate _ no nutritional value
Inulin
Intermediates in the hydrolysis of starch
Dextrins
Chief constituent of plant cell walls
_ Insoluble
_ Consists of _-d-glucopyranose units linked by _1 _ 4 bonds
Cellulose
_ an important source of bulk in the diet
_ major component of dietary fiber
Cellulose
structural polysaccharide in the exoskeleton of crustaceans and insects
Chitin
How many glycosidic bonds Chitin have?
4, N-acetyl-d-glucosamine units joined by _1 _ 4 glycosidic bonds
_ occurs in fruits
_ a polymer of galacturonic acid linked -1 4
Pectin
_ Aka mucopolysaccharides
_ Contains amino sugars and uronic acids
Glycosaminoglycans
_ attached to a protein molecule
_ substance of connective tissue
_ Hold water occupy space
Proteoglycan
Why is Proteoglycan holding water and occupy space
hold large quantities of water and occupy space, thus cushioning or lubricating other structures, because of the large number of _OH groups and negative charges on the molecule which, by repulsion, keep the carbohydrate chains apart
Example of Proteoglycan
hyaluronic acid, chondroitin sulfate, and heparin
_ Proteins containing branched or unbranched
oligosaccharide chains
_ occur in cell membranes
_ Glycosylation
_ 5% of the weight of cell membranes
Glycoproteins
_ aka Mucoproteins
Serum albumin is a glycoprotein. True or False
TRUE
are N- or O- acyl derivatives of
neuraminic acid
Sialic acids
A nine-carbon sugar derived from mannosamine (an epimer of glucosamine) and pyruvate
Neuraminic acid
Sialic acids are constituents of both _________ and ________
Glycoproteins and Gangliosides
has 130 amino acid residues and spans the lipid membrane, with polypeptide regions outside both the external and internal (cytoplasmic) surfaces
Glycophorin
attached to the amino terminal portion of glycoprotein outside the external surface
Carbohydrates
Carbohydrates are also present in apo-protein B of plasma lipoproteins. True or False
TRUE
Importance of Salivary a-amylase in the digestion of carbohydrates
During mastication, it acts briefly on dietary starch and glycogen, hydrolyzing random _(1_4) bonds
Because branched amylopectin and glycogen also contain _(1_6) bonds, which _-amylase cannot hydrolyze, the digest resulting from its action contains a mixture of short,branched and unbranched oligosaccharides known as dextrins
Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary _-amylase.
Importance of Salivary pancreatic a-amylase in the digestion of carbohydrates
When the acidic stomach contents reach the small intestine, they are neutralized by bicarbonate secreted by the pancreas, and pancreatic _-amylase continues the process of starch digestion.
Importance of Salivary intestinal disaccharides in the digestion of carbohydrates
The final digestive processes occur primarily at the mucosal lining of the upper jejunum and include the action of several disaccharidases
Intestinal absorption of monosaccharides in the Digestion of Carbohydrates
o The duodenum and upper jejunum absorb the bulk of the monosaccharide products of Digestion
o However, different sugars have different mechanisms of absorption
o galactose and glucose are transported into the mucosal cells by an active, energy- dependent process that requires a concurrent uptake of sodium ions, and the transport protein is the sodium-dependent glucose cotransporter 1 (SGLT-1)
o Fructose utilizes an energy- and sodium- independent monosaccharide transporter (GLUT-5) for its absorption.
o All three monosaccharides are transported from the intestinal mucosal cell into the portal circulation by yet another transporter, GLUT-2
Abnormal degradation of disaccharides
o Because only monosaccharides are absorbed, any deficiency (genetic or acquired) in a specific disaccharidase activity of the intestinal mucosa causes the passage of undigested carbohydrate into the large intestine
o As a consequence of the presence of this osmotically active material, water is drawn from the mucosa into the large intestine, causing osmotic diarrhea
o This is reinforced by the bacterial fermentation of the remaining carbohydrate to two- and three-carbon compounds (which are also osmotically active) plus large volumes of CO2 and H2 gas, causing abdominal cramps, diarrhea, and flatulence.
brush border
enzymes are rapidly lost in normal individuals with severe diarrhea
Digestive enzyme deficiency
represents a reduction in the amount of enzyme produced
It is thought to be caused by small variations in the DNA sequence of a region on chromosome 2 that controls expression of the gene for lactase, also on chromosome 2.
Lactose intolerance
This autosomal recessive disorder results in an intolerance of ingested sucrose
Congenital sucrase-isomaltase deficiency
Identification of a specific enzyme deficiency can be obtained by : ________
performing oral tolerance tests with the individual disaccharides.
reliable test for determining the amount of ingested
carbohydrate not absorbed by the body, but which is metabolized instead by the intestinal flora
Measurement ofhydrogen gas in the breath
