Carbohydrates Lecture Notes

CARBOHYDRATES

Lesson Objectives:

• Define common terminologies related to carbohydrates.
• Determine the principles behind the molecular structures of carbohydrates.
• Determine the different classifications of carbohydrates and their significance.
• Explain the etiology, symptoms, and treatment of carbohydrate-related diseases.

Materials:

• Pen or Pencil
• SAS References:     * Stoker, H. General, Organic and Biological Chemistry, 6th edition     * Denniston, K., Topping, J., & Quirk, D. (2016). General, Organic, and Biochemistry (9th ed.). McGraw Hill.

Productivity Tip:

• Boost energy with stretching and light cardio.
• Breathe deeply and stay hydrated.
• Write down key concepts.
• Rest after study sessions.

Lesson Preview/Review

Introduction (3 min)

• Carbohydrates are the most abundant class of bioorganic molecules on Earth.
• They constitute about 75% of the mass of dry plant materials.
• Green plants produce carbohydrates via photosynthesis, using carbon dioxide and water with sunlight as the energy source. The equation is: $CO_2 + H_2O + Sunlight \rightarrow Carbohydrates$
• Carbohydrates are relatively low in the human body.
• Plants form carbohydrates through structural arrangement, the classification, and uses of different sugars or carbohydrates.

Functions of Carbohydrates in Humans:

• Plants use carbohydrates as structural elements (cellulose) and for energy storage (starch).
• Dietary intake of plant materials is a major carbohydrate source for humans and animals.
• The average human diet should ideally be about two-thirds carbohydrate by mass.

Activity 1: What I Know Chart, part 1 (3 mins)

• Instruction: Write what you know about the topic based on the questions.
• Questions:     1. Are all carbohydrates sweet?     2. Give at least one medical use for carbohydrates.

Main Lesson

Activity 2: Content Notes (70 mins)

Carbohydrate Definition

• A carbohydrate $(C_n(H_2O)_n)$ is a polyhydroxy aldehyde, a polyhydroxy ketone, or a compound that yields polyhydroxy aldehydes or polyhydroxy ketones upon hydrolysis.
• Glucose is a polyhydroxy aldehyde, and fructose is a polyhydroxy ketone.

Carbohydrate Classification

• Carbohydrates are classified based on molecular size as monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Understanding Principles on the Molecular Structures of Carbohydrates

• Chrirality is the handedness in molecules. Sugars look and linked together can be understood in simple yet challenging ways.
• The relationship is that of mirror images. A left hand and a right hand are mirror images of each other. Objects can be divided into two classes based on their mirror images: objects with superimposable mirror images and objects with nonsuperimposable mirror images. Superimposable mirror images (Achiral) are images that coincide at all points when the images are laid upon each other. Nonsuperimposable mirror images (Chiral) are images where not all points coincide when the images are laid upon each other.
• Molecules with “handedness” exist in left-handed and right-handed forms.

Chirality

• Chirality is determined by the presence of a carbon atom with four different groups bonded to it in a tetrahedral orientation.
• The bonds to the four different groups must be single bonds.
• The handedness-generating carbon atom is called a chiral center.
• A chiral molecule has mirror images that are not superimposable.
• An achiral molecule has mirror images that are superimposable.

Requirements to Have a Chiral Molecule

1. Tetrahedral orientation with single bonds.
2. A chiral center is an atom with four different groups bonded to it in a tetrahedral orientation.
3. A chiral molecule has non-superimposable mirror images.
4. Chiral molecules have handedness.

Stereoisomerism: Enantiomers and Diastereomers

• Stereoisomers have the same molecular and structural formulas but differ in the orientation of atoms in space.
• Constitutional isomers have atoms connected differently.
• Enantiomers are stereoisomers whose molecules are non-superimposable mirror images.
• Diastereomers are stereoisomers whose molecules are not mirror images of each other.
• Cis–trans isomers are diastereomers.

Importance of Chirality

• Right-handed and left-handed forms of a molecule can elicit different responses in the body.
• Sometimes both forms are biologically active, each giving a different response.
• The body’s response to the right-handed form of epinephrine is 20 times greater than to the left-handed form.
• Monosaccharides are almost always right-handed.
• Amino acids, the building blocks for proteins, are always left-handed molecules.

Designating Handedness (D,L) Using Fischer Projection Formulas

• Enantiomers are optically active, rotating the plane of polarized light.
• An optically active compound rotates the plane of polarized light.
• A dextrorotatory compound rotates the plane of polarized light clockwise.
• A levorotatory compound rotates the plane of polarized light counterclockwise.
• The D,L system designates handedness based on the highest-numbered chiral center.
• If the –OH group of the highest chiral carbon is on the right, it’s a D-isomer; if it’s on the left, it’s an L-isomer.

Fischer Projection Formula

• A two-dimensional structural notation for showing the spatial arrangement of groups about chiral centers in molecules.

Structures and Classification of Monosaccharide

• Monosaccharides commonly have three to seven carbon atoms.
• A three-carbon monosaccharide is a triose, four-carbon is a tetrose, five-carbon is a pentose, and six-carbon is a hexose.
• Monosaccharides are classified as aldoses or ketoses based on the type of carbonyl group present.
• An aldose contains an aldehyde functional group.
• A ketose contains a ketone functional group.
• A six-carbon monosaccharide with an aldehyde functional group is an aldohexose; a five-carbon monosaccharide with a ketone functional group is a ketopentose.
• Monosaccharides are also called sugars.
• Sugar is a general designation for monosaccharides and disaccharides.
• The word saccharide comes from the Latin Saccharum, meaning sugar.

Classification Table:

Aldoses (in D Configuration)

• Hand exercise to remember sugar structures.

Ketoses (in D Configuration)

• The table shows the possible number of optical isomers based on the number of chiral centers/carbons and the location of the hydroxyl (-OH) group in each of the chiral centers/carbons. The higher the number of chiral center/carbon, the higher is the possible number of optical isomer.

The 16 Optical Isomers of Aldohexoses

• Epimers are carbohydrates that vary in one position for the placement of the -OH group.
• D-glucose and D-galactose are epimers at carbon-4.

Cyclic Monosaccharide: Haworth Projection Formula

• Haworth projection formulas are more useful for representing the three-dimensional structure of cyclic forms of monosaccharides.
• Cyclic monosaccharides result from the intramolecular reaction of the carbonyl group with a hydroxyl group, forming cyclic hemiacetals or hemiketals.
• A cyclic monosaccharide with a six-atom ring is called a pyranose (aldohexoses), and one with a five-atom ring is called a furanose (aldopentoses and ketohexoses).
• It is because their ring structures resemble the ring structures in the cyclic ethers pyran and furan, respectively.

Note: Pyran form is only for ALDOHEXOSES

• The hemiacetal carbon atom in a cyclic monosaccharide structure is called the anomeric carbon atom.
• Anomers are cyclic monosaccharides that differ only in the positions of the substituents on the anomeric (hemiacetal) carbon atom.
• The a-stereoisomer has the -OH group on the opposite side of the ring from the -CH2OH group, and the b-stereoisomer has the -OH group on the same side of the ring as the -CH2OH group.

Biochemically Important Monosaccharides

Reactions of Monosaccharide

• Five important reactions of monosaccharides are oxidation to acidic sugars, reduction to sugar alcohols, glycoside formation, phosphate ester formation, and amino sugar formation.

Oxidation to Produce Acidic Sugars

• The redox chemistry of monosaccharides is closely linked to that of the alcohol and aldehyde functional groups.

Acidic Sugars

• Aldonic Acid: Acid group on top, uses weak oxidizing agent
• Alduronic Acid: Acid group on bottom, uses enzymes
• Aldaric Acid: Acid groups both on top and bottom, uses strong oxidizing agent

Reduction to Produce Sugar Alcohols

• The carbonyl group can be reduced to a hydroxyl group using hydrogen as the reducing agent.
  Example: Reduction of D-glucose gives D-glucitol.
  D-Glucitol is also called D-sorbitol and used as moisturizing agents in foods and cosmetics.
  D-Sorbitol is also used as a sweetening agent in chewing gum as bacteria cannot use it.

Glycoside Formation

• Cyclic forms of monosaccharides (hemiacetals) react with alcohols to form acetals (glycosides).
• A glycoside is an acetal formed from a cyclic monosaccharide by replacement of the hemiacetal carbon -OH group with an -OR group.

Phosphate Ester Formation

• The hydroxyl groups of a monosaccharide can react with inorganic oxyacids to form inorganic esters such as phosphate esters.
  Phosphate esters of glucose (glucose 1-phosphate and glucose 6-phosphate) are stable in aqueous solution and play important roles in the metabolism of carbohydrates.

Amino Sugar Formation

• Replacing a hydroxyl group with an amino group produces an amino sugar. Three common amino sugars are D-glucosamine, D-galactosamine, and D-mannosamine.
• Amino sugars and their N-acetyl derivatives are important building blocks of polysaccharides found in chitin and hyaluronic acid.
• The N-acetyl derivatives of D-glucosamine and D-galactosamine are present in the biochemical markers on red blood cells.

Disaccharides

• A disaccharide is a carbohydrate in which two monosaccharides are bonded together, through a glycosidic linkage.
• A glycosidic linkage is the bond in a disaccharide resulting from the reaction which is a carbon–oxygen–carbon bond in a disaccharide.
• Many disaccharides contain both a hemiacetal carbon atom and an acetal carbon atom.
• Tabulation of Disaccharide Features:

Oligosaccharides

• Oligosaccharides contain three to ten monosaccharide units bonded together via glycosidic linkages.
• Raffinose (trisaccharide) and stachyose (tetrasaccharide) are naturally occurring oligosaccharides found in onions, cabbage, broccoli, Brussels sprouts, whole wheat, and beans

Importance of Oligosaccharides

• A person’s blood type (O, A, B, or AB) is determined by the type of oligosaccharide attached to the person’s red blood cells.
• Four monosaccharides contribute to the make-up of the oligosaccharide “marking system.”
• TRISACCHARIDE: RAFFINOSE composed of: • α-D-galactose • α-D-glucose • β-D-fructose TETRASACCHARIDE: STACHYOSE composed of: • α-D-galactose • α-D-galactose • α-D-glucose • β-D-fructose

Polysaccharides

• A polysaccharide is a polymer containing many monosaccharide units bonded together by glycosidic linkages; also called glycans.
• Polysaccharides are not sweet and have limited water solubility.
• The -OH groups present can individually become hydrated by water molecules, resulting in a thick colloidal suspension.
  Polysaccharides, such as flour and cornstarch, are often used as thickening agents in sauces, desserts, and gravy.

Parameters to distinguish polysaccharides:

1. The identity of the monosaccharide repeating unit(s) in the polymer chain.     * Homopolysaccharide/glycan - only one type of monosaccharide monomer     * Heteropolysaccharide/glycan - with more than one (usually two) type of monosaccharide monomer
2. The length of the polymer chain.
3. The type of glycosidic linkage between monomer units.
4. The degree of branching of the polymer chain.

Types of Polysaccharides

A. Storage Polysaccharide: used as an energy source in cells (starch and glycogen).

• Starch: energy storage polysaccharide in plants.
• Glycogen: energy storage polysaccharide for animals aka animal starch      * Amylose: straight-chain glucose polymer
  * accounts for 15%–20% of the starch
  * monomer units of glucose: with 300-500
       * Amylopectin: branched glucose polymer
  * accounts for the remaining 80%–85% of the starch
  *More water soluble because of increase in branching
  * contains 100,000 glucose units
  

B. Structural Polysaccharide: serves as a structural element in plant cell walls and animal exoskeletons like chitin and cellulose.

• Cellulose: structural component of plant cell walls; the most abundant naturally occurring polysaccharide.
  * The woody portions of plants have high concentrations of this fibrous, water- insoluble substance.
  * Contains 5000 glucose units
  * Nondigestible (human lacks cellulase)
• Chitin: second most abundant naturally occurring polysaccharide after cellulose; gives rigidity to the exoskeletons of crabs, lobsters, shrimp, insects, and other arthropods.
  *Also found in the cell walls of fungi.
  * Structurally identical to cellulose, except the monosaccharide present is N-acetyl-D-glucosamine (rather than glucose)

C. Acidic Polysaccharide: with a disaccharide repeating unit in which one of the disaccharide components is an amino sugar and one or both disaccharide components have a negative charge due to a sulfate group or a carboxyl group. ex.hyaluronic acid & heparin
Acidic polysaccharides are heteropolysaccharides.

HYALURONIC ACID
* contains alternating residues of N- acetyl-β-Dglucosamine (NAG) and D- Glucuronate.
* D-Glucuronate –

HEPARIN
*highly viscous hyaluronic acid solutions serve as lubricants in the fluid of joints, * are also associated with the jelly-like consistency of the vitreous humor of the eye(hyalos). * D-Glucuronate –
* small highly-sulfated polysaccharide with only 15–90 disaccharide residues per chain

Both with negative charge groups    * Highly viscous hyaluronic acid solutions
* serve as lubricants in the fluid of joints   * are also associated with the jelly-like consistency of the vitreous humor of the eye

SPECIAL GROUP: GLYCOSAMINOGLYCANS (GAGS) aka mucopolysaccharides, or negatively charged polysaccharides
Repeating disaccharide units, commonly containing one or another amino sugar as one of the monomers in the disaccharide units.

GENERAL ROLE:*mechanical support *cushioning of joints * cellular signals in cell proliferation and cell migration *inhibitors of certain enzymes.

LOCATION *found outside cells *cell surface *part of extracellular matrix * or attached to protein core to form proteoglycans.
 When glycosamnoglycans are attached to a protein molecule the compound is called proteoglycan [proteoglycans = Glycosaminoglycans + proteins]
Proteoglycans are more carbohydrate than protein with carbohydrate moities may contain carboxylic acids or sulfated sugars thus the GAG chain carry negative charge. Core protein strands are heavily modified keratin sulfate and chondroitin sulfate. The core protein strands are held in a complex with s strand of hyaluronic acid by link proteins.
 **EXAMPLES OF GLYCOSAMINOGLYCANS (GAGS): **

• chondroitin sulfate
• heparin sulfate
• keratan sulfate
• dermatan sulfate

Functions of Glycosaminoglycans (heparan sulfate): After an injury to tissue, the oligosaccharides derived from this GAG are released to:help mediate the inflammatory response ,promote activity by growth factors, chemokines and cytokines ,recruit leukocytes to the injury site ,as anticoagulant in the form of pentasaccharide sequence, HEPARIN.
NOTE: Heparin is much smaller than heparin sulfate and that is not linked to a protein core. It is also more sulfated than the average random polysaccharide sequence in heparin sulfate.

• CHONDROITIN SULFATE (CS) structural polysaccharide of ligaments, DERMATAN SULFATE (DS) structural polysaccharide in skin. closely related GAG, which is composed of glucuronic acid and N-acetylgalactosamine

• KERATAN SULFATE (KS) structural polysaccharide in nails and found primarily in the cornea of the eye and in joint cartilage for mechanical support and structural role- formed form alternating units of galactose and sulfated N-acetylgucosamine

• AGAR: Also used as food additive to chicken liquid suspensions,Linear polymer of sulfated and unsulfated galactose prepared form marine algae w/ alternating copolymer of galactose and 3,6-anhydrous-galactose Not a proteoglycan but is purely carbohydrate,When dissolves in hot water and then cooled, it forms gels)

• INULIN: readily soluble in water and is used to determine the glomerular filtration rate.
  * is a polysaccharide of fructose and hence a fructosan found in tubers and roots of dahlias, artchokes, and dandelions

• DEXTRINS are intermediates in the hydrolysis of starch.

Carbohydrate Related Diseases

1.) Diabetes

• Is a chronic (long-lasting) health condition that affects how your body turns food into energy. If you have diabetes, your body either doesn’t make enough insulin or can’t use the insulin it makes as well as it should. When there isn’t enough insulin or cells stop responding to insulin, too much blood sugar stays in your bloodstream. Symptoms of Diabetes: The Big 3 (3Ps) – These are the common ones seen in diabetic patients
  1. Polyuria – increase urination
  2. Polydipsia – increase thirst
  3. Polyphagia – increase eating due to hunger
  
  Types of Diabetes:
  
  1. Type 1 - most diagnosed in children and teenagers.
  2. Type 2 - happens when your body stops responding to the insulin your pancreas makes.      3. Gestational - this type of diabetes is a response to the hormonal changes that happen during pregnancy.

2.) Galactosemia

• Is a rare, hereditary disorder of carbohydrate metabolism that affects the body’s ability to convert galactose to glucose. Galactose is a sugar contained in milk. Symptoms of a Classic Galactosemia   How is galactosemia treated? Treatment requires the strict exclusion of lactose/galactose from the diet

Activity 3: Skill-building Activities (with answer key)

Matching type: Match column A with the definition in column B. MATCHING TYPE: Characterize the members of each of the following pairs of structure as: Enantiomer, Diasteriomer, Neither enantiomer nor diasteriomer. FISCHER PROJECTION FORMULA: Instruction: Identify the 1st sugar as D or L isomer. Then draw its mirror image. HAWORTH PROJECTION: *Instruction: Identify the structure of sugar units. ENCIRCLE the letter of your choice. *STRUCTURAL SIMILARITIES and DIFFERENCES*: *SACCHARIDE*

Activity 4: What I Know Chart, part 2 (3 mins)

• Answer the “What I Learned” column in Activity 1.

Activity 5: Check for Understanding (20 mins)

• Multiple Choice questions

Lesson Wrap-Up

Activity 6: Thinking about Learning (5 mins)

• Work Tracker
• Think about your Learning:     1. What interests you about the lesson today?     2. Do you have questions in mind that you are interested to be discussed? Please write it down.

FAQs

  Q: What are the negative effects of carbohydrates?
A: Refined carbs may increase blood triglycerides, blood sugar levels and cause insulin resistance.
Q: How can carbohydrates lead to diabetes? A: If blood glucose levels rise too rapidly, too often, the cells can eventually become faulty and not respond properly to insulin’s instructions eventually causing cells need more insulin to react eventually insulin production drops leading to the desase..
Q: Should carbohydrates be eaten before exercise? A: Ingestion of carbs before exercise for delaying fatigue and performance can be improved by ensuring these stores are high when first beginning exercise
Q: What happens when all carbs are used? A: movement and motivation both become more difficult, which can then affect performance and will cause further problems .