Carbohydrates

BCH 100 — Introductory Biochemistry · Dr. Radi

build Jul 17 · 19:00 · CC BY-NC-SA 4.0 · owned figures (RDKit / matplotlib / PyMOL)
Dr. Radi

By the end of this unit, you can…

  • Classify monosaccharides (aldose/ketose, D/L, epimers, anomers) and convert Fischer ↔ Haworth projections
  • Explain reducing sugars, glycosidic bonds, and the major di- and polysaccharides (maltose, lactose, sucrose, starch, glycogen, cellulose)
  • Connect sugar chemistry to the body — glycation/HbA1c, lactose intolerance, cartilage GAGs, and ABO blood typing
Dr. Radi

Today's route 🗺️

  1. Monosaccharides — Structure & Stereochemistry
  2. Fischer → Haworth — Drawing the Ring
  3. Sugar Reactions — Reducing Sugars, Browning & Phosphates
  4. Disaccharides & the Glycosidic Bond
  5. Polysaccharides — Storage, Structure & Recognition
Dr. Radi

1 · Monosaccharides — Structure & Stereochemistry

"Name and classify the simple sugars — aldose vs ketose, D vs L, and the epimers that differ by a single –OH."

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SUGAAAARRRR!

Carbohydrates are (CH₂O)ₙ — literally hydrates of carbon. The simplest ones, monosaccharides, come in two flavors by their carbonyl: an aldose carries an –CHO at the end, a ketose carries a C=O in the middle.

Glucose (aldose) and fructose (ketose) share the same formula, C₆H₁₂O₆ — same atoms, different shape, different taste.
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D or L? Look at the bottom chiral carbon

Sugars are chiral, and we anchor them to glyceraldehyde. The –OH on the last stereocenter decides it: on the right = D, on the left = L. Almost every sugar in your body is D.

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Epimers: one –OH flipped

Change the configuration at just one carbon and you get an epimer — a different sugar with a different job. Galactose is glucose flipped at C4; mannose is glucose flipped at C2. That's it. One –OH.

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When a sugar can't be handled: galactosemia

Babies get galactose from milk (it's half of lactose). If the enzyme GALT is missing, galactose can't be turned into glucose — it backs up and poisons the infant (cataracts, liver and brain damage). That's why every newborn is screened.

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2 · Fischer → Haworth — Drawing the Ring

"Close the open chain into a ring and convert a Fischer projection to a Haworth — the right→down rule, and the α/β anomers it creates."

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The Fischer projection

A Fischer projection draws the sugar as a straight vertical chain: horizontal bonds come toward you, vertical bonds go back. Number from the carbonyl end (that's C1). Glucose is an aldohexose — six carbons, aldehyde on top.

KNOW HOW TO DRAW GLUCOSE. The –OH pattern (right, left, right, right) is worth memorizing.
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Close the ring: Fischer → Haworth

In water the sugar curls up: C5's –OH attacks C1, making a six-membered ring (a hemiacetal). Converting is one rule — an –OH on the RIGHT in Fischer points DOWN in the ring. The C1 carbon is now brand-new and special: the anomeric carbon.

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α and β: the anomers

Closing the ring makes C1 a fresh stereocenter, so you get two versions. α puts the new –OH down; β puts it up. They freely interconvert in solution through the open chain — that's mutarotation.

Dr. Radi

3 · Sugar Reactions — Reducing Sugars, Browning & Phosphates

"Follow what that free anomeric carbon does — reduce copper, brown your toast, glycate your hemoglobin, and pick up a phosphate."

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Reducing sugars

If a sugar's anomeric carbon is free, the ring can pop open to a reactive aldehyde — and an aldehyde will reduce Cu²⁺, turning blue Benedict's reagent brick-red. That's a reducing sugar. Sucrose can't (both anomeric carbons are tied up) — so it's non-reducing.

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The Maillard reaction — why seared food is delicious

Heat a reducing sugar next to a protein's –NH₂ and they fuse into brown, savory melanoidins: the crust on bread, the sear on a steak. Same chemistry, no enzyme needed.

Photo: WikiForAfrica, CC BY-SA 4.0 (Wikimedia Commons)
Delicious in the pan — but the exact same reaction quietly runs on the proteins inside you (that's HbA1c).
Dr. Radi

HbA1c: sugar on your hemoglobin

Glucose sticks onto hemoglobin all on its own — no enzyme. Since a red cell lives ~120 days, the % of glycated Hb (HbA1c) reports your average blood sugar over months. An A1c of 7% works out to about 154 mg/dL — the diabetes target.

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Sugar phosphates

Bolt a phosphate onto a sugar's –OH and you make a phosphoester — like glucose-6-phosphate. The charge traps it inside the cell (no sneaking back across the membrane) and primes it for metabolism. Every sugar entering a pathway gets tagged this way.

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4 · Disaccharides & the Glycosidic Bond

"Join two sugars with a glycosidic bond and meet the big three — maltose, lactose, and sucrose."

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The glycosidic bond

Link two sugars by joining the anomeric carbon of one to an –OH of the other, kicking out a water. That's a glycosidic bond. Its flavor — α or β — is set by the anomer you started from, and it decides everything about the chain you build.

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The big three disaccharides

Maltose = Glc–Glc, α(1→4); lactose = Gal–Glc, β(1→4); sucrose = Glc–Fru, joined anomeric-to-anomeric. That last link locks both anomeric carbons — so sucrose is the one that's non-reducing.

KNOW ALL THREE: the sugars, the linkage, and does it still reduce?
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Lactose intolerance

To absorb lactose you need lactase to cut it. In most adults lactase fades after childhood — so lactose sails on to the colon, where bacteria ferment it into gas and bloating. Not an allergy: just a missing enzyme.

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5 · Polysaccharides — Storage, Structure & Recognition

"Scale up to thousands of sugars: starch and glycogen for storage, cellulose for strength, and the sugar coats that ID your cells."

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Same glucose, very different jobs

String glucose together and the linkage decides everything. Starch and glycogen use α(1→4) chains with α(1→6) branches — easy to build and break for fuel. Cellulose uses straight β(1→4), H-bonded into fibers we can't digest.

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Sugars that cushion: cartilage

Not all sugar is fuel. In cartilage, a core protein bristles with glycosaminoglycan (GAG) chains — a molecular bottlebrush. Those chains are negatively charged, so they pull in water and swell into a shock-absorbing gel that cushions your joints.

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Sugars that ID you: blood type

Your ABO blood type is a sugar. Red cells carry a short glycan ending in the H antigen; add one terminal sugar and you're set — GalNAc → type A, galactose → type B, neither → type O. Transfuse the wrong one and the immune system attacks that sugar.

Dr. Radi

Can you…?

  • ☐ classify monosaccharides (aldose/ketose, D/L, epimers, anomers) and convert Fischer ↔ Haworth projections?
  • ☐ explain reducing sugars, glycosidic bonds, and the major di- and polysaccharides (maltose, lactose, sucrose, starch, glycogen, cellulose)?
  • ☐ connect sugar chemistry to the body — glycation/HbA1c, lactose intolerance, cartilage GAGs, and ABO blood typing?

If any box stays empty, the practice site has a drill for it. 🧪

Dr. Radi