Chapter 10: Carbohydrates
structure of dihydroxyacetone
(a ketose)
Structure of D-Glyceraldehyde
(an aldose)
Structure of L-Glyceraldehyde
(an aldose)
Structure of D-Deoxyribose
*see on slide 7
Describe cellulose
-a structural component of plants, is made of chains of glucose -a unbranched homopolymer of glucose units linked by β-1, 4-glycocidic bonds.
What are monosaccharides?
-aldehydes or ketones that contain two or more alcohol groups. -they are also the simplest carbohydrates
Describe pyranose formation:
The open-chain form of glucose cyclizes when the C-5 hydroxyl group attacks carbon atom C-1 of the aldehyde group to form an intramolecular hemiacetal. Two anomeric forms, designated α and β, can result.
What is glycogen?
The polysaccharide glycogen is the storage form of glucose in animal cells.
Proteoglycans:
The protein is attached to a particular type of polysaccharide called a glycosaminoglycan. By weight, proteoglycans are mainly carbohydrate. Proteoglycans play structural roles or act as lubricants.
In the case of the ketose fructose, the intramolecular hemiketal,
a five-carbon ring (five-membered), is called a furanose because of its resemblance to furan.
Describe chitin
a glycosaminoglycan found in insect wings and the exoskeleton of insects, crustaceans, and arachnids, is one of the most abundant carbohydrates in the world.
What is amylose
a linear polymer of glucose units linked by α-1, 4-glycosidic bonds.
In the case of glucose, the resulting intramolecular hemiacetal,
a six-carbon ring, is called a pyranose because of its resemblance to pyran (six membered)
The β linkage of cellulose yields
a straight chain capable of interacting with other cellulose molecules to from strong fibrils.
The blood type O produces no
active glycosyltransferase.
Monosaccharides are joined to ______ and ______ through ______ bonds
alcohols, amines; glycosidic
Aldoses contain an ________, whereas ketoses, such as fructose, contain a
aldehyde; ketone
What are two storage forms of glucose?
glycogen and starch
Proteoglycans are proteins attached to __________ , which make up
glycosaminoglycans; 95% of the proteoglycan by weight.
The VNTR region is highly _______, forcing the molecule into
glycosylated; an extended conformation.
The human ABO blood groups reflect the specificity of
glycosyltransferases.
If all of the monosaccharides in the polysaccharide are the same, the polysaccharide is called a
homopolymer.
In A,
N-acetylgalactosamine is added to the O by a specific glycosyltransferase.
In B,
galactose is added by another transferase.
Common monosaccharides
1) D-Ribose 2) D-Deoxyribose 3) D-Glucose 4) D-Mannose 5) D-Galactose 6) D-Fructose
There are three main classes of glycoproteins:
1) Glycoproteins 2) Proteoglycans 3) Mucins or mucoproteins
( look at images of these on slides) 1. N-linked: 2. O-linked:
1. GlcNAC 2. GalNAC
1. Formation of a cyclic hemiacetal creates....... 2. The α form means 3. The β form means
1. another diastereoisomeric form called an anomer. 2. the hydroxyl at C-1 is below the plane of the ring. 3. that the hydroxyl at C-1 is above the plane of the ring.
Describe the O-glycosidic bond:
A bond formed between the anomeric carbon atom and a hydroxyl group of another molecule is called an O-glycosidic bond, and the product is called a glycoside.
Describe the N-glycosidic bond
A bond formed between the anomeric carbon atom and an amine is called an N-glycosidic bond.
The formation of a mannose 6-phosphate marker:
A glycoprotein destined for delivery to lysosomes acquires a phosphate marker in the Golgi complex in a two-step process. First, N-acetylgalactosamine phosphotransferase adds a phospho-N-acetylglucosamine unit to the 6-OH group of a mannose residue; then, N-acetylgalactosaminidase removes the added sugar to generate a mannose 6-phosphate residue in the core oligosaccharide.
Explain how specific enzymes are responsible for oligosaccharide assembly:
A large class of enzymes, glycosyltransferases, catalyze the formation of glycosidic bonds.
Describe N-linked oligosaccharides:
A pentasaccharide core is common to all N-linked oligosaccharides and serves as the foundation for a wide variety of N-linked oligosaccharides
What do organic molecules contain?
Carbon, hydrogen, and oxygen
Explain how the lack of glycosylation can result in pathological conditions?
Certain types of muscular dystrophy can be traced to improper glycosylation of dystroglycan, a membrane protein that links the extracellular matrix with the cytoskeleton. These pathological conditions reveal the importance of proper modification of proteins by carbohydrates and their derivatives. An especially clear example of the role of glycosylation is provided by I-cell disease (also called mucolipidosis II), a lysosomal storage disease. Lysosomes are organelles that degrade and recycle damaged cellular components or material brought into the cell by endocytosis. In I-cell disease, lysosomal enzymes are secreted into the blood rather than directed to the lysosome. The defect results from a mutation that disrupts the oligosaccharide signal on the enzymes that normally directs the enzymes to the lysosome. Normally, a carbohydrate marker directs certain digestive enzymes from the Golgi complex to lysosomes where they function. In patients with I-cell disease, lysosomes contain large inclusions of undigested glycosaminoglycans and glycolipids, the "I" in the name of the disease. These inclusions are present because the enzymes normally responsible for the degradation of glycosaminoglycans are missing from affected lysosomes. Remarkably, the enzymes are present at very high levels in the blood and urine. Thus, active enzymes are synthesized, but, in the absence of appropriate glycosylation, they are exported instead of being sequestered in lysosomes. In other words, in I-cell disease, a whole series of enzymes is incorrectly addressed and delivered to the wrong location. Normally, these enzymes contain a mannose 6-phosphate residue as a component of an N-oligosaccharide that serves as the marker directing the enzymes from the Golgi complex to lysosomes. In I-cell disease, however, the attached mannose lacks a phosphate. I-cell patients are deficient in the N-acetylgalactosamine phosphotransferase catalyzing the first step in the addition of the phosphoryl group; the consequence is the mistargeting of eight essential enzymes (Figure 10.25). I-cell disease causes the patient to suffer severe psychomotor retardation and skeletal deformities, similar to those in Hurler disease.
Pyranose and Furanose Rings Can Assumer Different
Conformations
What allows monitoring the long-term control of blood glucose levels in diabetics?
Determining the amount of HbA1c in the blood
Two types of stereoisomers
Enantiomers and Diastereoisomers
What cleave common disaccharides?
Enzymes on the outer surface of intestinal epithelium
Describe the hormone erythropoietin
Erythropoietin is a glycoprotein secreted by the kidney into the blood that stimulates the production of red blood cells.
Describe Hurler disease
Formerly called gargoylism, Hurler disease is a mucopolysaccharidosis having symptoms that include wide nostrils, a depressed nasal bridge, thick lips and earlobes, and irregular teeth. In Hurler disease, glycosaminoglycans cannot be degraded. The excess of these molecules is stored in the soft tissue of the facial regions, resulting in the characteristic facial features.
Ring structures of fructose:
Fructose can form both five-membered furanose and six-membered pyranose rings. In each case, both a and B anomers are possible.
Describe glucosinolates
Glucosinolates protect plants and add flavor to our diets. They are a special class of glycosides that protect some plants from herbivory.
What are isomers
Isomers have the same molecular formula but different structures
What determine polysaccharide structure and how?
Glycosidic bonds (The β-1,4 linkages favor straight chains, which are optimal for structural purposes. The α-1,4 linkages favor bent structures, which are more suitable for storage)
What of erythropoietin enhances the stability of the protein in the blood?
Glycosylation
What are polysaccharides?
Large polymeric oligosaccharides are called polysaccharides.
Mucins or mucoproteins:
Mucins, a key component of mucus, serve as lubricants. Like proteoglycans, mucins are predominantly carbohydrate. The protein is characteristically attached to the carbohydrate by N-acetylgalactosamine. N-Acetylgalactosamine is an example of an amino sugar, so named because an amino group replaces a hydroxyl group. Mucins are often lubricants.
Is carbon dioxide organic? Why or why not?
No because it doesn't contain hydrogen
All of the blood groups share the oligosaccharide foundation called
O.
What is a common source of starch?
Pasta is a common source of starch. Pasta, thin pieces of unleavened dough molded into regular shapes, is made with flour, which has a high starch content.
What make compounds very active?
Phosphates
What are glycoproteins?
Proteins with carbohydrates attached
Examples of sugars that cyclize in solution
Ribose, glucose, fructose
What are selectins?
Selectins are a class of lectins. Some selectins bind immune cells to sites of injury. Other selectins allow attachment of an embryo to the mother's uterus.
Common disaccharides:
Sucrose, lactose, and maltose are common dietary components. (The angles in the bonds to the central oxygen atoms do not denote carbon atoms.)
What facilitate the polymerization of many mucins?
The Cys-rich domains and the D domain
What designates the structures as being in the d configuration? (look at slide 7 for clarification)
The asymmetric carbon atom farthest from the aldehyde or ketone designates the structures as being in the d configuration.
Explain how cartilage acts as a shock absorber:
The cartilage of a runner's foot cushions the impact of each step that she takes
Chair and boat forms of β-d-glucopyranose:
The chair form is more stable owing to less steric hindrance because the axial positions are occupied by hydrogen atoms. Abbreviations: a, axial; e, equatorial.
Explain Furanose formation:
The open-chain form of fructose cyclizes to a five-membered ring when the C-5 hydroxyl group attacks carbon C-2 of the ketone to form an intramolecular hemiketal. Two anomers are possible, but only the α anomer is shown.
Describe glycoproteins
The protein is the largest component by weight. Glycoproteins play a variety of roles including as membrane proteins. Many glycoproteins are components of cell membranes, where they take part in processes such as cell adhesion and the binding of sperm to eggs. Other glycoproteins are formed by linking carbohydrates to soluble proteins. Many of the proteins secreted from cells are glycosylated, or modified by the attachment of carbohydrates, including most proteins present in the serum component of blood.
General form of a glycosyltransferase reaction:
The sugar to be added comes from a sugar nucleotide—in this case, UDP-glucose. The acceptor, designated X in this illustration, can be a simple monosaccharide, a complex polysaccharide, or a serine or threonine residue belonging to a protein.
Epimers
They differ at one of several asymmetric carbon atoms
How branch point in glycogen is formed:
Two chains of glucose molecules joined by α-1,4-glycosidic bonds are linked by an α-1,6-glycosidic bond to create a branch point. Such an α-1,6-glycosidic bond forms at approximately every 10 glucose units, making glycogen a highly branched molecule.
What is amylopectin?
a branched polymer, with an α-1, 6-glycosidic bond for every 30 α-1, 4-glycosidic bonds.
Glycosidic bonds between proteins and carbohydrates: A glycosidic bond links
a carbohydrate to the side chain of asparagine (N-linked) or to the side chain of serine or threonine (O-linked)
All N-linked polysaccharides consist of
a common pentasaccharide core, consisting of three mannoses, a six-carbon sugar, and two N-acetylgalactosamine units.
Sugars as cyclic forms: The chemical basis for ring formation is that
an aldehyde can react with an alcohol to form a hemiacetal, while a ketone can react with an alcohol to form a hemiketal. (Practice writing out these equations found on slide 9)
The furanose form of fructose can also exist in
anomeric forms in which the α and β forms refer to the orientation of the hydroxyl at C-2.
Lectins are a particular class of glycan-binding protein that
are important for cell signaling. The lectins on one cell recognize and bind to carbohydrates on another cell with multiple weak interactions. Such binding facilitates cell-cell interaction.
Carbohydrates may be linked to what residues of proteins?
asparagine, serine, or threonine
Which amino acids are used for the attachment of carbohydrates to proteins?
asparagine, serine, or threonine
The monosaccharide substrates for glycosyltransferases are activated by
attachment to uridine diphosphate (UDP).
In the chair form, substituents on the carbon ring atoms have two orientations:
axial and equatorial.
Why some people hate vegetables:
because of the sharp taste of isothiocynate
Viral receptors: Influenza virus targets cells by The other major viral-surface protein,
binding to sialic acid residues located at the termini of oligosaccharides present on cell-surface glycoproteins and glycolipids. These carbohydrates are bound by hemagglutinin, one of the major proteins expressed on the surface of the virus.
_______ groups are based on
blood; protein glycosylation patterns
Pyranose rings can adopt two types of conformation, called
boat and chair.
Glucose-_____ form predominated
chair
neuraminidase, is an enzyme that
cleaves oligosaccharide chains to release the viral particle at a later stage of the viral life cycle.
The α linkages of starch and glycogen form
compact hollow cylinders suitable for accessible storage.
Monosaccharides are linked to form
complex carbohydrates
Two types of isomers
constitutional isomers and stereoismers
Oligosaccharides
contain two or more monosaccharides linked by O-glycosidic bonds
Many common sugars exist in _____ forms
cyclic
Two types of diastereoisomers
epimers and anomers
In mucins, the protein component is
extensively glycosylated to serine and threonine residues beginning with N-acetylgalactosamine. A region of the protein backbone rich in serines and threonines, called variable number of tandem repeats (VNTR), is the site of glycosylation.
Proteoglycans are key components of the
extracellular matrix
As a reducing sugar, glucose can react with hemoglobin,
forming glycosylated hemoglobin (hemoglobin A1c), which is fully functional.
Fructose forms both the pyranose form, which predominates when
fructose is free in solution, and a furanose form, commonly seen in fructose derivatives.
Monosaccharides exist in many _______ forms
isomeric
Diastereoisomers
isomers that are NOT mirror images
Anomers
isomers that differ at a new asymmetric carbon formed on ring closure; OH below=alpha OH above=beta
When glucosinolates are hydrolyzed,
isothiocyanate is produced, which generates a sharp taste that discourages further eating by the insect. Certain glucosinolates may help prevent cancer in humans
Lactase and other enzymes that hydrolyze carbohydrates are present on
microvilli that project from the outer face of the plasma membrane of intestinal epithelial cells
Mucins are glycoprotein components of
mucus
Enantiomers
nonsuperimposable mirror images
A solution of glucose contains
one-third α anomer, two-thirds β anomer, and about 1% open chain.
Mucopolysaccahridoses, such as Hurler disease, are
pathological conditions that result from the inability to degrade proteoglycans.
Carbohydrates also form ester linkages to
phosphates
Cartilage is composed, in part, of the
proteoglycan aggrecan and cartilage.
What important components of cartilage?
proteoglycans
Sugars that react with oxidizing agents are called
reducing sugars, whereas those that do not are called nonreducing sugars.
The glycosaminoglycan component of aggrecan cushions joints by
releasing water on impact, and then rebinding water.
Glycosaminoglycans are composed of
repeating units of a disaccharide, one of which is a derivative of an amino sugar and one of which carries a negative charge, either as a carboxylate or sulfate.
Although mammals CANNOT digest cellulose and other plant fibers,
soluble fibers, such as polygalacturonic acid, aid in digestion
Glycan-binding proteins bind to
specific oligosaccharides on the cell surface.
In plants, glucose is stored as ______ of which there are two forms:
starch; amylose and amylopectin
Proteoglycans, composed of polysaccharides and protein have important
structural role
Glysosylation: ______ attached
sugars
Modified monosaccharides: Carbohydrates can be modified by
the addition of substituents other than hydroxyl groups. Such modified carbohydrates are often expressed on cell surfaces.
For glucose: Because the two anomeric forms are in equilibrium that passes through an open chain form,
the free open chain form reacts with oxidizing agents.
In all classes of glycoproteins, carbohydrates are attached to
the nitrogen atom in the side chain of asparagine (N-linkage) or to the oxygen atom of the side chain of serine or threonine (O-linkage).
The numbers are the standard designations for
the positions of the carbon atoms (e.g., the number 2 identifies the carbon atom in the second position).
What are stereoisomers?
they are isomers where atoms are connected in the same order but differ in spatial arrangement
What are constitutional isomers?
they differ in the order of attachment of atoms
The smallest monosaccharides are composed of _______ carbons.
three
Erythropoietin has oligosaccharides linked to
three asparagine residues and one serine residue.
Most glucose units in glycogen are lined by
α-1, 4-glycosidic bonds, with branches formed by α-1, 6-glycosidic bonds every 10 glucose units.
_______________ adopts the chair conformation because
β-d-glucopyranose ;the axial positions are occupied by hydrogens, reducing steric hindrance.