Chapter 15: Carbohydrates: Structures, Functions, and Health Implications

Diabetes Nurses and Healthcare Context

• Role of Diabetes Nurses: These professionals assist patients in learning to monitor medication and blood sugar levels.

• Symptom Monitoring: Nurses look for specific complications such as diabetic nerve damage and vision loss.

• Prerequisite Knowledge: Effective care requires knowledge of the endocrine system, as this system is frequently involved in obesity and other related diseases.

Fundamental Characteristics of Carbohydrates

• Composition: Carbohydrates are made from the elements carbon ($C$), hydrogen ($H$), and oxygen ($O$).

• Dietary Function: They serve as a major source of energy in the human diet.

• Etymology: They are also referred to as saccharides, a term derived from the word for "sugars."

• Classification Levels: Carbohydrates are categorized into three main types:   - Monosaccharides: The simplest form of carbohydrates.   - Disaccharides: Consist of two monosaccharide units.   - Polysaccharides: Contain many monosaccharide units.

The Cycle of Photosynthesis and Oxidation

• Production: Photosynthesis in plants produces carbohydrates like glucose ($C_{6}H_{12}O_{6}$).

• Synthesis Requirements: Plants synthesize these compounds from carbon dioxide ($CO_{2}$), water ($H_{2}O$), and energy from the sun.

• Cellular Oxidation: In living cells, glucose is oxidized to produce $CO_{2}$, $H_{2}O$, and energy.

Structural Classification of Monosaccharides

• General Structure: Monosaccharides contain several hydroxyl groups ($OH$) attached to a carbon chain consisting of three to eight carbon atoms.

• Hydroxyl Distribution: Hydroxyl groups are present on all carbons except the carbonyl carbon.

• Functional Group Classification:   - Aldoses: Monosaccharides containing an aldehyde group.   - Ketoses: Monosaccharides containing a ketone group.

• Classification by Carbon Count:   - Triose: Three carbon atoms.   - Tetrose: Four carbon atoms.   - Pentose: Five carbon atoms.   - Hexose: Six carbon atoms.

• Examples of Combined Classification:   - Aldopentose: A five-carbon saccharide with an aldehyde group.   - Ketohexose: A six-carbon saccharide with a ketone group.   - Aldohexose: A six-carbon saccharide with an aldehyde group.   - Ketopentose: A five-carbon saccharide with a ketone group.

Chirality and Stereoisomers

• Structural Isomers: Molecules with the same molecular formula but different bonding arrangements.

• Stereoisomers: Molecules where atoms are bonded in the same sequence but differ in their spatial arrangement.

• Chirality Defined: An object or molecule is chiral if its mirror image is nonsuperimposable (it cannot be completely matched by rotation or movement).   - Analogy: Left and right hands are chiral.

• Achiral Defined: An object is achiral if its mirror image is identical to the original and can be superimposed on it.

• Chiral Carbon Atoms: A carbon atom is chiral if it is bonded to four different atoms or groups.

• Enantiomers: Stereoisomers that are nonsuperimposable mirror images of each other.

• Achiral Carbon Conditions: A carbon is achiral if it has two or more identical atoms or groups bonded to it.

Fischer Projections and Stereoisomer Identification

• Fischer Projection Definition: A two-dimensional representation of a three-dimensional molecule.

• Orientation Rules:   - The most oxidized group is placed at the top.   - Vertical Lines: Represent bonds that go back/away from the viewer.   - Horizontal Lines: Represent bonds that go forward/toward the viewer.   - Intersections: Represent carbon atoms, which are usually chiral.

• Drawing for Glyceraldehyde: The carbonyl group (most oxidized) is at the top; the middle carbon is the only chiral center.

• D and L Notation System: Determined by the position of the hydroxyl ($OH$) group on the chiral carbon farthest from the carbonyl group.   - L-Isomer: The $OH$ group is on the left.   - D-Isomer: The $OH$ group is on the right.

• Erythrose Example: Mirror images are created by reversing the positions of all $OH$ groups on the horizontal lines.

Enantiomers in Biological Systems

• Biological Effects: Often, one stereoisomer possesses a different biological effect than its mirror image.

• Carvone Case Study:   - One enantiomer (from spearmint plants) smells and tastes like spearmint.   - Its mirror image (from caraway plants) has the odor and taste of caraway (rye bread).

• Sensory Response: Human senses of smell and taste are highly responsive to the chirality of molecules.

Important Monosaccharides: Glucose, Galactose, and Fructose

• Isomerism: Glucose, galactose, and fructose are all hexoses with the same molecular formula ($C_{6}H_{12}O_{6}$) and are structural isomers of each other.

• D-Glucose:   - Known as dextrose and blood sugar.   - The most common hexose.   - Found in fruits, vegetables, corn syrup, and honey.   - Functions as a building block for sucrose, lactose, cellulose, and glycogen.

• D-Galactose:   - An aldohexose obtained from the disaccharide lactose found in milk.   - Important for the cellular membranes of the brain and nervous system.   - Differs from glucose only in the arrangement of the $OH$ group on carbon 4.

• D-Fructose:   - A ketohexose.   - The sweetest of all carbohydrates; twice as sweet as sucrose (table sugar).   - One of the hydrolysis products of sucrose.   - High-fructose corn syrup (HFCS): A sweetener made by using an enzyme to break sucrose down into glucose and fructose.

Clinical Health: Blood Glucose Levels and Diabetes

• Normal Levels: In the body, glucose typically has a normal blood level of $70-90\,mg/dL$.

• Glucose Tolerance Test: Measures blood glucose levels for several hours after a subject ingests glucose.

• Hyperglycemia (Diabetes Mellitus):   - Occurs when the pancreas cannot produce sufficient insulin.   - Glucose levels can rise as high as $350\,mg/dL$ of plasma.   - Symptoms: Thirst, excessive urination, increased appetite, and weight loss.   - In older adults, it is often a consequence of excessive weight gain.

• Hypoglycemia:   - Blood glucose levels rise and then decrease rapidly to levels as low as $40\,mg/dL$.   - Can result from an overproduction of insulin by the pancreas.   - Symptoms: Dizziness, general weakness, and muscle tremors.   - Treatment: A prescribed diet consisting of several small, high-protein, low-carbohydrate meals.

Haworth Structures of Monosaccharides

• Ring Formation: Most stable forms of pentoses and hexoses are five- or six-atom rings.

• Synthesis: Haworth structures result from the reaction between a carbonyl group and a hydroxyl group within the same molecule.

• Drawing Procedure:   1. Turn the Fischer projection clockwise by $90^{\circ}$.   2. Groups on the right of the vertical Fischer chain are placed below the horizontal Haworth ring.   3. Groups on the left of the Fischer chain are placed above the Haworth ring.   4. Fold the chain into a hexagon (for hexoses) and bond the oxygen on carbon 5 to carbon 1.   5. For D-isomers, the $CH_{2}OH$ group (carbon 6) is placed above the ring.

• Anomers (Alpha and Beta):   - $\alpha$ isomer: The new $OH$ group on carbon 1 is drawn below the ring.   - $\beta$ isomer: The new $OH$ group on carbon 1 is drawn above the ring.

• Mutarotation: The process in solution where cyclic structures open and close, allowing $\alpha$-D-glucose to convert to $\beta$-D-glucose and vice versa. Only a small amount of open chain exists at any time.

• Fructose Haworth Structure: Forms a five-atom ring structure with carbon 2 at the right corner of the ring (reaction between the $OH$ on carbon 5 and the ketone on carbon 2).

Chemical Properties: Oxidation and Reduction

• Oxidation (Sugar Acids):   - Although mostly cyclic, monosaccharides in solution have small amounts of open-chain form.   - Aldehydes with adjacent hydroxyl groups can be oxidized to carboxylic acids by agents such as Benedict’s solution.   - Naming: Replace the "-ose" ending with "-onic acid."   - Reducing Sugar: A carbohydrate that reduces another substance (e.g., the open-chain form of D-glucose).   - Fructose Oxidation: Fructose (a ketohexose) can be oxidized in basic Benedict’s solution because it rearranges into glucose (an aldose) via the movement of the carbonyl group between carbon 1 and carbon 2.

• Reduction (Sugar Alcohols/Alditols):   - The carbonyl group is reduced to an alcohol group.   - Naming: Replace the "-ose" ending with "-itol."   - Examples:     - D-Glucose reduces to D-Sorbitol.     - D-Xylose reduces to D-Xylitol.     - D-Mannose reduces to D-Mannitol.   - Usage: Used as sweeteners in sugar-free products (diet drinks, sugarless gum) suitable for diabetic diets.

Structure and Linking of Disaccharides

• Formation: A disaccharide forms when two monosaccharides combine through a dehydration reaction (releasing water).

• Reaction Summary:   - $Glucose + Glucose \rightarrow Maltose + H_{2}O$   - $Glucose + Galactose \rightarrow Lactose + H_{2}O$   - $Glucose + Fructose \rightarrow Sucrose + H_{2}O$

• Maltose (Malt Sugar):   - Composed of two D-glucose molecules.   - Obtained from starch hydrolysis; used in cereals, candies, and brewing.   - Linked by an $\alpha$-1,4-glycosidic bond (between $OH$ on carbon 1 of the first unit and carbon 4 of the second).

• Lactose (Milk Sugar):   - Found in milk: $6-8\%$ of human milk, $4-5\%$ of cow's milk.   - Composed of $\beta$-D-galactose and $\alpha$- or $\beta$-D-glucose.   - Linked by a $\beta$-1,4-glycosidic bond.

• Sucrose (Table Sugar):   - Consists of $\alpha$-D-glucose and $\beta$-D-fructose.   - Linked by an $\alpha$,$\beta$-1,2-glycosidic bond (carbon 1 of glucose to carbon 2 of fructose).   - Sources: Sugar cane ($20\%$ by mass) or sugar beets ($15\%$ by mass).   - Property: Cannot form an open chain or be oxidized; it is not a reducing sugar.

Complex Polysaccharides and Starch

• Definition: Polymers of D-glucose that differ by branching and glycosidic bond types.

• Starch: Storage form of glucose in plants (rice, wheat, potatoes, beans).   - Amylose ($20\%$ of starch): Continuous unbranched chain of 250 to 4000 glucose molecules connected by $\alpha$-1,4-glycosidic bonds. Coils in a helical fashion.   - Amylopectin ($80\%$ of starch): Branched-chain polysaccharide. Contains glucose molecules connected by $\alpha$-1,4 and $\alpha$-1,6-glycosidic bonds.

• Hydrolysis of Starch: Hydrolyzes in water/acid to smaller saccharides called dextrins, then to maltose, and eventually glucose.

• Digestion: Digested by amylase (saliva) and maltase (intestines). Provides approximately $50\%$ of nutritional calories.

Animal Starch, Cellulose, and Specialized Polysaccharides

• Glycogen (Animal Starch):   - Glucose polymer stored in the liver and muscles of animals.   - Similar to amylopectin but more highly branched (branches every 10–15 glucose units).   - Joined by $\alpha$-1,4-glycosidic bonds with $\alpha$-1,6 branches.   - Hydrolyzed in cells to maintain blood glucose and provide energy between meals.

• Cellulose:   - Major structural unit of wood and plants (cell walls).   - Unbranched chains connected by $\beta$-1,4-glycosidic bonds.   - Insoluble in water as it cannot form hydrogen bonds with it.   - More resistant to hydrolysis than starches.   - Digestion: Humans cannot digest cellulose because they lack the enzyme to break down $\beta$-1,4 bonds.

• Glycosaminoglycans:   - Unbranched polysaccharides consisting of repeating disaccharide units (amino sugar + uronic acid or galactose).   - Highly polar and negatively charged.   - Function to attract water, cushion, or lubricate body structures.   - Keratan Sulfate: Found in the cornea, cartilage, bone, and animal horns. Unsulfonated deposits can cause corneal cloudiness.