Key Carbohydrate Structures

Why This Matters

Carbohydrates aren't just "sugars"—they're the molecules that power metabolism, build cell walls, and enable cells to recognize each other. When you're tested on carbohydrate structures, you're really being tested on your understanding of stereochemistry, bond formation, and structure-function relationships. The difference between a digestible starch and an indigestible fiber comes down to a single bond orientation. That's the level of detail biochemistry exams demand.

As you work through these structures, focus on why each structural feature matters. Can humans digest it? Does it store energy or provide structure? Is the anomeric carbon free or locked in a bond? Don't just memorize names—know what concept each structure illustrates and how small changes in configuration lead to dramatically different biological functions.

---

Monosaccharide Fundamentals

Before tackling complex carbohydrates, you need to master the building blocks. Monosaccharides are classified by their carbonyl group position (aldose vs. ketose) and carbon chain length, which determines their cyclization behavior and metabolic fate.

Monosaccharides

• Simplest carbohydrate units—cannot be hydrolyzed into smaller sugars; general formula $C_n(H_2O)_n$
• Common hexoses include glucose, fructose, and galactose—all share the formula $C_6H_{12}O_6$ but differ in stereochemistry
• Serve as metabolic fuel and building blocks for disaccharides, polysaccharides, and glycoconjugates

Aldoses and Ketoses

• Aldoses contain a terminal aldehyde group ($-CHO$) at C1—glucose and galactose are key examples
• Ketoses contain an internal ketone group ($C=O$) typically at C2—fructose is the most important dietary ketose
• Classification affects cyclization—aldoses form pyranose rings more readily; ketoses often form furanose rings

Pyranose and Furanose Ring Structures

• Pyranose rings are six-membered—formed when C5 hydroxyl attacks the carbonyl; glucose predominantly exists as glucopyranose
• Furanose rings are five-membered—formed when C4 hydroxyl attacks the carbonyl; fructose commonly forms fructofuranose
• Ring size affects stability and reactivity—pyranose forms are generally more stable in solution

---

Stereochemistry and Anomeric Configuration

The three-dimensional arrangement of atoms in carbohydrates determines everything from digestibility to biological recognition. Anomeric configuration—whether the hydroxyl is α or β—is perhaps the single most important structural feature for predicting carbohydrate function.

Anomers (α and β)

• Anomers differ at the anomeric carbon—the hemiacetal/hemiketal carbon formed during cyclization (C1 in aldoses, C2 in ketoses)
• α-anomer has the anomeric $-OH$ axial (trans to $CH_2OH$ in D-sugars); β-anomer has it equatorial (cis to $CH_2OH$)
• Determines polysaccharide digestibility—α-linkages are hydrolyzed by human enzymes; β-linkages generally are not

Fischer Projections

• Two-dimensional representation of chiral centers—vertical bonds project backward; horizontal bonds project forward
• D/L designation determined by bottom-most chiral center—D-sugars have $-OH$ on the right; most biological sugars are D-form
• Essential for comparing stereoisomers—epimers, enantiomers, and diastereomers are easily identified

Haworth Projections

• Cyclic representation showing ring conformation—depicts the hemiacetal/hemiketal ring with substituent orientations
• "Down" in Fischer = "right" in Haworth for D-sugars; anomeric $-OH$ down = α, up = β
• Standard for illustrating glycosidic bonds—shows connectivity between monosaccharide units in di- and polysaccharides

---

Glycosidic Linkages and Sugar Classification

The bonds connecting monosaccharides determine the properties of larger carbohydrates. Glycosidic bonds lock the anomeric carbon, eliminating mutarotation and defining whether a sugar can act as a reducing agent.

Glycosidic Bonds

• Covalent bonds between anomeric carbon and hydroxyl—formed via condensation reaction with loss of $H_2O$
• Named by anomer and carbons involved—α(1→4) means α-anomeric C1 bonded to C4 of next sugar
• Bond type determines digestibility—humans have α-glucosidases but lack β-glucosidases for most β-linkages

Reducing and Non-reducing Sugars

• Reducing sugars have a free anomeric carbon—can open to expose aldehyde/ketone and reduce $Cu^{2+}$ or $Ag^+$ in diagnostic tests
• Non-reducing sugars have no free anomeric carbon—sucrose is the classic example (both anomeric carbons involved in bond)
• Clinically relevant—Benedict's test for reducing sugars detects glucose in urine (diabetes screening)

Disaccharides

• Two monosaccharides joined by one glycosidic bond—sucrose, lactose, and maltose are the "big three"
• Sucrose = glucose α(1→2) fructose (non-reducing); lactose = galactose β(1→4) glucose (reducing)
• Hydrolyzed by specific enzymes—lactase cleaves lactose; sucrase cleaves sucrose; deficiencies cause intolerance

Oligosaccharides

• Short chains of 3-10 monosaccharides—found attached to proteins (glycoproteins) and lipids (glycolipids)
• Critical for cell recognition and signaling—ABO blood groups determined by oligosaccharide antigens
• N-linked vs. O-linked—attached to asparagine (N) or serine/threonine (O) in glycoproteins

---

Energy Storage Polysaccharides

Storage polysaccharides share a common strategy: pack glucose into large, osmotically inactive polymers that can be rapidly mobilized. The key structural feature is α-glycosidic linkages, which human enzymes can hydrolyze.

Starch (Amylose and Amylopectin)

• Primary plant storage carbohydrate—found in seeds, tubers, and grains; major dietary energy source
• Amylose is linear with α(1→4) bonds; amylopectin is branched with α(1→4) backbone and α(1→6) branch points
• Branching increases solubility and mobilization—amylopectin's branches provide more non-reducing ends for enzyme access

Glycogen

• Primary animal storage carbohydrate—stored in liver (blood glucose regulation) and muscle (local energy)
• Highly branched structure with α(1→4) backbone and α(1→6) branches every 8-12 residues—more branched than amylopectin
• Rapid glucose mobilization—extensive branching provides many sites for glycogen phosphorylase to act simultaneously

---

Structural Polysaccharides

Structural polysaccharides provide mechanical strength rather than storing energy. The defining feature is β-glycosidic linkages, which create extended chains capable of forming strong hydrogen-bonded networks.

Cellulose

• Most abundant organic molecule on Earth—primary component of plant cell walls
• Linear chains of β(1→4) linked glucose—chains align to form microfibrils stabilized by extensive hydrogen bonding
• Indigestible by humans—we lack cellulase; provides dietary fiber that promotes gut health

Chitin

• Second most abundant polysaccharide—found in arthropod exoskeletons, fungal cell walls, and mollusk shells
• β(1→4) linked N-acetylglucosamine ($GlcNAc$)—similar structure to cellulose but with acetylamino group at C2
• Stronger than cellulose—additional hydrogen bonding from amide groups increases mechanical strength

Glycosaminoglycans

• Long unbranched chains of repeating disaccharide units—typically contain uronic acid and amino sugar
• Highly negatively charged—sulfate and carboxyl groups attract water, creating gel-like extracellular matrix
• Examples include hyaluronic acid (joints), heparin (anticoagulant), and chondroitin sulfate (cartilage)

---

Quick Reference Table

---

Self-Check Questions

1. Both starch and cellulose are glucose polymers—what single structural difference makes starch digestible and cellulose indigestible for humans?

2. You're given an unknown disaccharide that tests positive with Benedict's reagent. What can you conclude about its structure, and which common disaccharide would test negative?

3. Compare glycogen and amylopectin: what structural feature do they share, how do they differ quantitatively, and how does this difference relate to their biological functions?

4. A Haworth projection shows the anomeric hydroxyl pointing down in a D-glucose molecule. Is this the α or β anomer, and which polysaccharide would this form if linked α(1→4)?

5. Explain why chitin is mechanically stronger than cellulose despite both having β(1→4) linkages. What structural modification accounts for this difference?