10/26 Lecture

Sweet Taste Receptors and Ligand Binding

Taste Perception Pathway:
- Chemical sensation originates at taste receptor cells in the taste buds, sending neural signals to the brain for processing and interpretation.
- Sweetness perception is mediated by a distinct G-protein coupled receptor (GPCR) heterodimer complex consisting of two subunits: $\text{TAS1R2}$ and $\text{TAS1R3}$ .
- Natural sweet ligands, including monosaccharides and disaccharides such as glucose, fructose, and sucrose, require simultaneous binding across both the $\text{TAS1R2}$ and $\text{TAS1R3}$ subunits to trigger the downstream intracellular G-protein signaling cascade.
- These simple carbohydrates represent the primary evolutionary dietary triggers for sweetness sensation.
Artificial Sweeteners:
- Common artificial non-nutritive sweeteners include aspartame and sucralose.
- Unlike natural sugars, artificial sweeteners bind to only one of the two GPCR subunits to activate the sweet gustatory pathway, circumventing the requirement for dual-subunit binding.
Stereochemical Specificity in Receptor Activation:
- Receptor-ligand interactions in the gustatory system exhibit strict stereochemical fidelity.
- Aspartame consists of two chemically modified amino acids linked together (a modified dipeptide structure).
- Both amino acid residues must reside in their precise chiral configurations; inverting the chirality of either amino acid residue abolishes receptor binding, completely eliminating sweet taste perception.

Classification and Physical Characteristics of Monosaccharides

Chemical Definition of Monosaccharides:
- Monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones.
- Each monosaccharide backbone contains a single carbonyl group ( $\text{C}=\text{O}$ ) situated either at an end position or internally, while the remaining carbon atoms typically bear a hydroxyl group ( $-\text{OH}$ ).
Classification by Functional Group:
- Aldose: A monosaccharide in which the carbonyl group is located at the terminal position of the carbon backbone, forming an aldehyde functional group.
- Ketose: A monosaccharide in which the carbonyl group is located at an internal position along the carbon backbone, forming a ketone functional group.
Classification by Carbon Chain Length:
- Triose: A three-carbon sugar ( $\text{C}_3\text{H}_6\text{O}_3$ ).
- Tetrose: A four-carbon sugar ( $\text{C}_4\text{H}_8\text{O}_4$ ).
- Pentose: A five-carbon sugar ( $\text{C}_5\text{H}_{10}\text{O}_5$ ), such as ribose and deoxyribose found in nucleic acid backbones.
- Hexose: A six-carbon sugar ( $\text{C}_6\text{H}_{12}\text{O}_6$ ), which represents the upper carbon limit typically encountered in primary biochemical pathways.
Physical Properties:
- Aqueous solutions of monosaccharides are optically clear and colorless.
- Evaporation of the water solvent drives monosaccharides to organize into clear, transparent crystalline solids.

Chirality and Stereochemical Terminology

Chiral Centers in Aldoses versus Ketoses:
- A chiral carbon (stereocenter) is bonded to four distinct chemical substituents.
- Aldoses: Chirality emerges beginning at three carbons (aldotrioses):
- In glyceraldehyde ( $\text{C}_1\text{H}(=\text{O})-\text{C}_2\text{H}(\text{OH})-\text{C}_3\text{H}_2\text{OH}$ ), $\text{C}_1$ is achiral due to its double bond to oxygen.
- $\text{C}_3$ is achiral because it is covalently bonded to two identical hydrogen atoms.
- $\text{C}_2$ is bonded to four distinct groups ( $-\text{H}$ , $-\text{OH}$ , $-\text{CHO}$ , and $-\text{CH}_2\text{OH}$ ), making it a chiral stereocenter.
- Ketoses: Chirality emerges beginning at four carbons (ketotetroses):
- In a three-carbon ketose (dihydroxyacetone: $\text{C}_1\text{H}_2\text{OH}-\text{C}_2(=\text{O})-\text{C}_3\text{H}_2\text{OH}$ ), $\text{C}_1$ and $\text{C}_3$ each possess two identical hydrogens, and $\text{C}_2$ has a double bond to oxygen; hence, zero chiral carbons exist in ketotrioses.
- Ketoses with four or more carbons possess at least one chiral center.
Hierarchy of Isomeric Relationships:
- Isomers / Stereoisomers: Molecules sharing identical elemental compositions and identical molecular weights, but possessing different spatial arrangements of their atoms.
- Enantiomers: Stereoisomers that are non-superimposable mirror images of each other. Due to the high number of possible internal stereocenter permutations, forming an exact non-superimposable mirror image across all chiral centers is relatively rare across large carbohydrate families.
- Diastereoisomers: Stereoisomers that share the same chemical formula and connectivity but are not mirror images of one another. Diastereoisomers exhibit distinct physical and chemical properties, including differing aqueous solubilities.
- Epimers: A specific subset of diastereoisomers that differ in stereochemical configuration at one, and only one, chiral carbon center.
- Anomers: Diastereoisomers of cyclic carbohydrates that differ in stereochemical configuration exclusively at the newly formed chiral carbon created upon ring closure (the anomeric carbon).

D- and L-Stereoisomer Configuration System

Standard Fischer Projection Conventions:
- Fischer projections represent carbohydrates as vertical carbon backbones with horizontal bonds projecting outward and vertical bonds projecting backward.
Algorithmic Determination of $\text{D}$ versus $\text{L}$ Stereoisomers:
- Step 1: Number the carbon backbone sequentially from top to bottom, initiating from the terminal carbon closest to the carbonyl functional group:
- In aldoses, the aldehyde carbon is designated $\text{C}_1$ .
- In ketoses, numbering begins at the end that grants the ketone carbonyl carbon the lowest possible integer value (typically $\text{C}_2$ ).
- Step 2: Identify all chiral carbons along the chain.
- Step 3: Locate the highest-numbered chiral carbon (the stereocenter furthest from the carbonyl group):
- For a standard aldohexose, $\text{C}_6$ has two hydrogens and is achiral; therefore, $\text{C}_5$ serves as the reference chiral center.
- For an aldotriose, $\text{C}_3$ is achiral; therefore, $\text{C}_2$ serves as the reference chiral center.
- Step 4: Evaluate the spatial orientation of the hydroxyl group ( $-\text{OH}$ ) on this highest-numbered chiral carbon:
- If the $-\text{OH}$ group is oriented to the right, the molecule is the $\text{D}$ -enantiomer.
- If the $-\text{OH}$ group is oriented to the left, the molecule is the $\text{L}$ -enantiomer.
Biological Stereocenter Preference:
- Biological systems maintain strict stereochemical specificity: natural cellular enzymes possess stereospecific active sites that selectively synthesize and catabolize $\text{D}$ -sugars.
- This physiological preference for $\text{D}$ -carbohydrates contrasts directly with the stereochemical preference for $\text{L}$ -amino acids in protein biochemistry.

Diastereoisomers, Epimers, and Anomers

Diagnostic Criterion for Epimers:
- Two monosaccharides can be classified as epimers if, and only if, their spatial configuration differs at exactly a single chiral carbon. Differing at zero centers means they are identical; differing at two or more centers means they are non-epimeric diastereoisomers.
Comparative Case Studies:
- $\text{D}$ -Glucose versus $\text{D}$ -Mannose:
- Carbon 1: Both contain terminal aldehyde groups ( $-\text{CHO}$ ).
- Carbon 2: $-\text{OH}$ points left in $\text{D}$ -mannose; $-\text{OH}$ points right in $\text{D}$ -glucose (Configuration differs).
- Carbon 3: $-\text{OH}$ points left in both.
- Carbon 4: $-\text{OH}$ points right in both.
- Carbon 5: $-\text{OH}$ points right in both.
- Carbon 6: Both contain achiral $-\text{CH}_2\text{OH}$ .
- Conclusion: $\text{D}$ -glucose and $\text{D}$ -mannose are epimers at $\text{C}_2$ .
- $\text{D}$ -Glucose versus $\text{D}$ -Galactose:
- Carbon 1: Identical ( $-\text{CHO}$ ).
- Carbon 2: Identical ( $-\text{OH}$ on right).
- Carbon 3: Identical ( $-\text{OH}$ on left).
- Carbon 4: $-\text{OH}$ points right in $\text{D}$ -glucose; $-\text{OH}$ points left in $\text{D}$ -galactose (Configuration differs).
- Carbon 5: Identical ( $-\text{OH}$ on right).
- Carbon 6: Identical ( $-\text{CH}_2\text{OH}$ ).
- Conclusion: $\text{D}$ -galactose is an epimer of $\text{D}$ -glucose at $\text{C}_4$ .
- $\text{D}$ -Mannose versus $\text{D}$ -Galactose:
- Carbon 2 differs ( $\text{D}$ -mannose is left, $\text{D}$ -galactose is right).
- Carbon 4 differs ( $\text{D}$ -mannose is right, $\text{D}$ -galactose is left).
- Conclusion: Because they differ at two chiral centers ( $\text{C}_2$ and $\text{C}_4$ ), $\text{D}$ -mannose and $\text{D}$ -galactose are not epimers.
Pedagogical Utility of Epimeric Relationships:
- Mastering the primary structure of $\text{D}$ -glucose provides a structural template: $\text{D}$ -galactose is drawn simply by inverting the $-\text{H}$ and $-\text{OH}$ substituents at $\text{C}_4$ , and $\text{D}$ -mannose by inverting the substituents at $\text{C}_2$ .

Structural and Physiological Profiles of Key Monosaccharides

Glucose ( $\text{Glc}$ ):
- Structure: A six-carbon aldohexose.
- Metabolism: Acts as the primary metabolic fuel preferred by most animal cells, and serves as the obligatory, primary carbohydrate fuel source utilized by the mammalian brain under standard physiological conditions.
- Dietary Sources: Hydrolysis of plant starches (amylose and amylopectin), breakdown of intracellular muscle and liver glycogen reserves, and enzymatic digestion of common dietary disaccharides (lactose, sucrose, and maltose).
Fructose ( $\text{Fru}$ ):
- Structure: A six-carbon ketohexose with the carbonyl group at $\text{C}_2$ .
- Gustatory Potency: Stimulates taste receptors to generate a sweetness signal perceived by the central nervous system as significantly more intense than sucrose (table sugar) or glucose; hence, it is widely utilized commercially in high-fructose corn syrup.
- Metabolism: Primarily extracted and metabolized in the liver. Diets disproportionately elevated in fructose overwhelm standard hepatic catabolism, driving hepatic lipid accumulation (fatty liver deposits) and metabolic dysfunction.
Galactose ( $\text{Gal}$ ):
- Structure: A six-carbon aldohexose ( $\text{C}_4$ -epimer of glucose).
- Biological Roles: Abundant cell-surface component covalently linked to proteins (glycoproteins) and lipids (glycolipids); primary structural constituent of the disaccharide lactose ( $\text{glucose} + \text{galactose}$ ) present in mammalian milk.
- Galactosemia: A severe metabolic disorder caused by congenital genetic mutations in the enzymes responsible for converting galactose into downstream glycolytic intermediates.
- Inability to metabolize galactose leads to systemic, toxic galactose accumulation in neonatal tissues.
- A primary, rapidly diagnosable clinical hallmark of galactosemia in newborn infants is the formation of bilateral cataracts in the eyes.
- Clinical Management: Total dietary elimination of lactose and galactose sources halts toxicity and promotes normal developmental recovery.

Cyclization of Aldoses into Haworth Projections

Thermodynamics and Dynamics of Ring Formation:
- In aqueous physiological solutions, monosaccharides containing five or six carbons undergo an intramolecular condensation reaction, establishing a dynamic equilibrium between linear (Fischer) and cyclic (Haworth) structures.
Mechanism of Aldohexose Cyclization (Six-Membered Pyranose Rings):
- Step 1: Nucleophilic Attack: The nucleophilic oxygen atom of the hydroxyl group attached to the second-to-last carbon ( $\text{C}_5$ in a hexose) attacks the electrophilic carbonyl carbon ( $\text{C}_1$ ).
- Step 2: Ether Bridge Formation: The oxygen of the $\text{C}_5$ hydroxyl group becomes integrated into the covalent ring framework as an ether bridge connecting $\text{C}_5$ and $\text{C}_1$ .
- Step 3: Conversion to Anomeric Center:
- Prior to cyclization, $\text{C}_1$ is a planar, trigonal $sp^2$ -hybridized carbonyl carbon possessing zero chirality.
- Upon nucleophilic attack, $\text{C}_1$ adopts tetrahedral $sp^3$ geometry, becoming a newly created chiral center designated the anomeric carbon.
- The original carbonyl oxygen is protonated to generate a new hydroxyl group ( $-\text{OH}$ ) residing directly on $\text{C}_1$ .
- Step 4: Orientation of $\text{C}_6$ :
- Carbon 6 ( $-\text{CH}_2\text{OH}$ ) is not included within the closed ring structure and projects outward.
- For all $\text{D}$ -sugars, $\text{C}_6$ is placed above the plane of the ring.
- For $\text{L}$ -sugars, $\text{C}_6$ is oriented below the plane of the ring.
Rules for Fischer to Haworth Group Transposition:
- For non-anomeric internal stereocenters ( $\text{C}_2$ , $\text{C}_3$ , $\text{C}_4$ in hexoses):
- Substituents located on the right side of the vertical Fischer chain are placed below the ring (pointing down) in the Haworth projection.
- Substituents located on the left side of the vertical Fischer chain are placed above the ring (pointing up) in the Haworth projection.
- Example with $\text{D}$ -glucose:
- $\text{C}_2$ $-\text{OH}$ is on the right in Fischer $\rightarrow$ points down in Haworth.
- $\text{C}_3$ $-\text{OH}$ is on the left in Fischer $\rightarrow$ points up in Haworth.
- $\text{C}_4$ $-\text{OH}$ is on the right in Fischer $\rightarrow$ points down in Haworth.
Anomer Nomenclature ( $\alpha$ vs. $\beta$ Configurations):
- Because the planar $\text{C}_1$ carbonyl group rotates freely prior to attack, the incoming nucleophile can trap the newly formed anomeric $-\text{OH}$ group on either face of the ring.
- Universal Definition across $\text{D}$ - and $\text{L}$ -Sugars:
- $\alpha$ -Anomer: The newly formed anomeric $-\text{OH}$ group and the reference $\text{C}_6$ ( $-\text{CH}_2\text{OH}$ ) project on opposite faces (trans) of the carbohydrate ring plane.
- $\beta$ -Anomer: The newly formed anomeric $-\text{OH}$ group and the reference $\text{C}_6$ ( $-\text{CH}_2\text{OH}$ ) project on the same face (cis) of the carbohydrate ring plane.
- Application to $\text{D}$ -Aldohexoses (where $\text{C}_6$ points UP):
- $\alpha$ - $\text{D}$ -anomer: The $\text{C}_1$ $-\text{OH}$ points down (opposite side of $\text{C}_6$ ).
- $\beta$ - $\text{D}$ -anomer: The $\text{C}_1$ $-\text{OH}$ points up (same side as $\text{C}_6$ ).
- Application to $\text{L}$ -Aldohexoses (where $\text{C}_6$ points DOWN):
- $\alpha$ - $\text{L}$ -anomer: The $\text{C}_1$ $-\text{OH}$ points up (opposite side of $\text{C}_6$ ).
- $\beta$ - $\text{L}$ -anomer: The $\text{C}_1$ $-\text{OH}$ points down (same side as $\text{C}_6$ ).

Cyclization of Ketoses

Mechanism of Ketohexose Cyclization (Five-Membered Furanose Rings):
- In a six-carbon ketose such as $\text{D}$ -fructose, the hydroxyl group residing on the second-to-last carbon ( $\text{C}_5$ ) attacks the internal ketone carbonyl carbon ( $\text{C}_2$ ).
- The ring structure is bounded between $\text{C}_2$ and $\text{C}_5$ , forming a five-membered furanose ring containing four carbons and one ether oxygen.
- Exocyclic Carbons: Both $\text{C}_1$ ( $-\text{CH}_2\text{OH}$ ) and $\text{C}_6$ ( $-\text{CH}_2\text{OH}$ ) are excluded from the cyclic ring framework:
- For a $\text{D}$ -ketose, $\text{C}_6$ projects above the plane of the ring.
- Anomeric Center at $\text{C}_2$ :
- Carbon 2 becomes the chiral anomeric center bearing two substituents: the exocyclic $\text{C}_1$ ( $-\text{CH}_2\text{OH}$ ) group and the newly formed anomeric hydroxyl group ( $-\text{OH}$ ).
Determining Anomers in Ketoses:
- The stereochemical classification as $\alpha$ or $\beta$ is dictated solely by the position of the newly formed $\text{C}_2$ $-\text{OH}$ relative to $\text{C}_6$ :
- $\alpha$ -Anomer: The newly formed $-\text{OH}$ at $\text{C}_2$ is oriented on the opposite side relative to $\text{C}_6$ (points down in a $\text{D}$ -sugar).
- $\beta$ -Anomer: The newly formed $-\text{OH}$ at $\text{C}_2$ is oriented on the same side relative to $\text{C}_6$ (points up in a $\text{D}$ -sugar).
- Position $\text{C}_1$ only after establishing the orientation of the anomeric $-\text{OH}$ : $\text{C}_1$ occupies the vacant site opposite the $\text{C}_2$ $-\text{OH}$ .
- Internal ring positions ( $\text{C}_3$ , $\text{C}_4$ ) adhere to standard rules: groups on the right of the Fischer chain point downward; groups on the left point upward.

Hemiacetal and Acetal Chemistry

Chemical Definitions:
- Acetal: A functional group in which a single carbon atom is covalently bonded to two separate alkoxy/ether groups ( $-\text{C}-\text{O}-\text{R}$ ).
- Hemiacetal: A functional group in which a single carbon atom is simultaneously bonded to one ether oxygen linkage ( $-\text{C}-\text{O}-\text{C}-$ ) and one free hydroxyl group ( $-\text{C}-\text{OH}$ ).
Carbohydrate Application:
- Unsubstituted cyclic monosaccharides exist as hemiacetals (or hemiketals for ketoses) because the anomeric carbon is linked to a single ring ether oxygen and retains a free anomeric $-\text{OH}$ .
- When monosaccharides condense to form disaccharides or polysaccharides via glycosidic linkages, the anomeric hemiacetal is converted into a full acetal linkage.

Questions and Academic Clarifications

Student Inquiry Regarding Structural Retention and Drawing Requirements:
- Question: Is the expectation to be capable of drawing out the full chemical structures of all carbohydrates from memory, or primarily to interpret given representations?
- Response:
- Full structural recognition and drawing proficiency are explicitly required for a foundational set of five monosaccharides:
  1. Glyceraldehyde (three-carbon aldotriose)
  2. Ribose (five-carbon aldopentose)
  3. Glucose (six-carbon aldohexose)
  4. Galactose (six-carbon aldohexose)
  5. Fructose (six-carbon ketohexose)
- Mastering the structure of $\text{D}$ -glucose is paramount, as it forms the mechanical foundation for downstream biochemical pathways (including glycolysis).
- Retaining the epimeric relationship where $\text{D}$ -galactose is simply the $\text{C}_4$ -epimer of $\text{D}$ -glucose eliminates the burden of independent memorization: constructing galactose requires only drafting glucose and reversing the stereocenters at Carbon 4.