sutton e3 slop

What is the definition of a carbohydrate? Polyhydroxy aldehydes or ketones

or molecules that yield them on hydrolysis; empirical formula (CH2O)n

How are carbohydrates classified by carbonyl group? Aldose: carbonyl at C-1 (aldehyde). Ketose: carbonyl at C-2 (ketone).

How are carbohydrates classified by carbon number? Triose (3C)

tetrose (4C)

How are carbohydrates classified by size? Monosaccharide (1 unit)

disaccharide (2 units)

What is the D/L rule for sugars? Find the highest-numbered chiral carbon (farthest from carbonyl). -OH on the right = D; -OH on the left = L.

Why are human sugars D? Human enzymes and transporters are stereospecific for D-sugars; D-glucose is the biological form

not L-glucose.

How many stereoisomers exist for a molecule with n chiral centers? 2^n. An aldohexose has 4 chiral centers → 16 isomers (8 D

8 L).

What are constitutional isomers? Same molecular formula but different connectivity (e.g.

glucose vs fructose: aldose vs ketose).

What are enantiomers? Non-superimposable mirror images; differ at every chiral center (e.g.

D-glucose vs L-glucose).

What are diastereomers? Stereoisomers that are NOT mirror images; differ at some but not all chiral centers.

What are epimers? Diastereomers that differ at exactly one chiral center.

What are glucose's two important epimers? Galactose (C-4 epimer) and mannose (C-2 epimer).

What is the relationship between galactose and mannose? They differ at two carbons (C-2 and C-4)

so they are diastereomers but NOT epimers.

What are anomers? Epimers that differ only at the anomeric carbon

formed on ring closure (e.g.

What happens when a sugar cyclizes in water? An -OH on the same chain attacks the carbonyl: aldehyde + alcohol → hemiacetal; ketone + alcohol → hemiketal.

What is the anomeric carbon? The former carbonyl carbon

which becomes a new chiral center on ring closure (C-1 in glucose

What is the difference between pyranose and furanose? Pyranose = six-membered ring (e.g.

glucose). Furanose = five-membered ring (e.g.

In a Haworth projection

what does α vs β mean? α: anomeric -OH down (trans to CH2OH). β: anomeric -OH up (cis to CH2OH).

What is mutarotation? α and β anomers interconvert through the open chain until equilibrium (~63% β

~36% α

What is a reducing sugar? A sugar with a free anomeric carbon that can open to an aldehyde and reduce Cu2+ (Benedict's test: blue → brick-red Cu2O).

Are all monosaccharides reducing? Yes

including fructose

What is sucrose's reducing status and why? Sucrose is NON-reducing because both anomeric carbons (glucose C-1 and fructose C-2) are locked in the glycosidic bond.

What is the difference between O-glycosidic and N-glycosidic bonds? O-glycosidic: sugar-sugar or sugar-Ser/Thr. N-glycosidic: sugar-base in nucleotides or sugar-Asn in N-linked glycoproteins.

What is the notation for a glycosidic bond? e.g.

Glc-α(1→4)-Glc: anomer

What is a sugar alcohol? A sugar whose carbonyl is reduced to -OH (e.g.

sorbitol

Why is xylitol used in sugar-free gum? Sugar alcohols are poorly fermented by oral streptococci

so little acid is produced and enamel demineralization is reduced.

What are amino sugars? Sugars with -OH replaced by -NH2

often N-acetylated (e.g.

What are sugar acids? Sugars with carbon oxidized to -COOH (e.g.

glucuronic acid in GAGs; sialic acid capping glycoproteins).

What are deoxy sugars? Sugars with an -OH replaced by -H (e.g.

2-deoxyribose in DNA; fucose in blood-group antigens).

What are the monosaccharides and linkage of maltose? Glucose + glucose; Glc-α(1→4)-Glc; reducing; digested by maltase.

What are the monosaccharides and linkage of lactose? Galactose + glucose; Gal-β(1→4)-Glc; reducing; digested by lactase (a β-galactosidase).

What are the monosaccharides and linkage of sucrose? Glucose + fructose; Glc-α(1↔2)β-Fru; NON-reducing; digested by sucrase.

Which disaccharide is non-reducing and why? Sucrose

because both anomeric carbons are involved in the glycosidic bond

Why is sucrose especially cariogenic? S. mutans glucosyltransferases split sucrose and use its bond energy to build sticky

insoluble glucans that form the plaque matrix; released sugars are fermented to lactic acid.

What is the Stephan curve? A graph of plaque pH after sugar exposure: pH falls below the critical ~5.5 within minutes

then recovers over 30-60 minutes as saliva buffers.

Why does frequency of sugar exposure matter more than amount? Each exposure restarts the pH drop

keeping plaque pH below the critical threshold for longer

What is the critical pH for enamel demineralization? ~5.5; below this

hydroxyapatite begins to dissolve.

What is amylose? Linear α(1→4)-linked glucose polymer from plant starch; coils into a helix.

What is amylopectin? Plant starch with α(1→4) backbone and α(1→6) branches every ~24-30 residues.

What is glycogen? Animal storage polysaccharide with α(1→4) backbone and α(1→6) branches every ~8-12 residues; more branched than amylopectin.

Why is glycogen more branched than amylopectin? More branches = more non-reducing ends = faster glucose release when needed.

What is cellulose? β(1→4)-linked glucose polymer; humans lack enzymes to cleave β(1→4) bonds

so it is indigestible fiber.

Why can humans digest starch but not cellulose? Starch is α-linked; human amylase and α-glucosidases cleave α bonds. Cellulose is β(1→4)-linked; no human enzyme cleaves that bond.

What are N-linked glycoproteins? Glycans attached to asparagine (Asn) residues; added in ER/Golgi.

What are O-linked glycoproteins? Glycans attached to serine/threonine (Ser/Thr) residues; often start with GalNAc.

What are salivary mucins? MUC5B and MUC7; heavily O-glycosylated proteins that lubricate oral tissues

form acquired pellicle

What are glycosaminoglycans (GAGs)? Long unbranched repeats of amino sugar + uronic acid

often sulfated and strongly negative; hold water. Examples: hyaluronan

What is the ABO antigen core? A shared H antigen core (...Gal + Fuc). Type A adds GalNAc; Type B adds Gal; Type O adds nothing.

What does lysozyme do? Hydrolyzes the β(1→4) bond between NAM and NAG in bacterial peptidoglycan

lysing bacteria (especially Gram-positive).

Where is salivary α-amylase active and what does it do? In the mouth; cleaves internal α(1→4) bonds of starch → maltose

maltotriose

What happens to salivary amylase in the stomach? Gastric acid (pH 1.5-3.5) denatures it

so little carbohydrate digestion occurs there.

Where does starch digestion resume? In the small-intestinal lumen

by pancreatic α-amylase after bicarbonate neutralizes chyme.

What brush-border enzymes finish carbohydrate digestion? Maltase (maltose → glucose)

sucrase (sucrose → glucose + fructose)

What form of carbohydrate is absorbed? Only monosaccharides are absorbed; all di- and oligosaccharides must be hydrolyzed first.

How is glucose absorbed at the apical membrane? SGLT1: Na+-coupled secondary active transport (glucose or galactose with 2 Na+).

How is fructose absorbed at the apical membrane? GLUT5: facilitated diffusion.

How do monosaccharides exit the enterocyte? GLUT2 at the basolateral membrane: facilitated diffusion of glucose

galactose

Where do absorbed monosaccharides go first? Hepatic portal vein → liver.

What is the basis of oral rehydration therapy (ORT)? SGLT1 co-transports glucose and Na+; water follows osmotically

making ORT more effective than water alone.

Which glucose transporter is insulin-responsive? GLUT4

in skeletal muscle and adipose tissue; insulin moves it to the membrane.

Which glucose transporter is the β-cell glucose sensor? GLUT2

in liver and pancreatic β-cells; low affinity

Which glucose transporter supplies the brain? GLUT1

in brain and red blood cells; high affinity

Which glucose transporter is fructose-specific? GLUT5

in the intestine (apical).

What is lactose intolerance? Lactase deficiency → lactose reaches the colon undigested → osmotic diarrhea and bacterial fermentation (gas

organic acids).

What are the types of lactose intolerance? Primary (age-related decline after weaning; most common)

secondary (after gut injury)

A patient develops bloating and diarrhea after milk. What enzyme is deficient and what bond is not hydrolyzed? Lactase (β-galactosidase); the Gal-β(1→4)-Glc bond of lactose is not hydrolyzed.

Why does uncontrolled diabetes increase caries risk? High blood glucose spills into saliva and gingival fluid

and reduced salivary flow decreases buffering

Why does uncontrolled diabetes cause fruity breath? Without effective insulin

cells cannot use glucose (GLUT4)

Why defer elective invasive dental treatment in a patient with glucose 520 mg/dL and ketonuria? Uncontrolled diabetes with possible ketoacidosis: impaired healing and high infection risk; refer to physician first.

A patient sips a sweetened drink slowly over two hours. Explain caries risk using the Stephan curve. Each sip restarts the pH drop

so plaque pH stays below ~5.5 for most of the two hours instead of recovering; frequency drives demineralization.

Starch and cellulose are both glucose polymers. Why can we digest only one? Starch is α-linked and human amylase/α-glucosidases cleave α bonds. Cellulose is β(1→4)-linked and no human enzyme cleaves that bond.

A freshly dissolved sample of pure α-D-glucose slowly changes optical rotation. Why? Mutarotation: the ring opens and recloses through the open chain until α and β reach equilibrium (~36% α

~63% β).

Why can sorbitol not form a glycosidic bond as the anomeric partner? It is a sugar alcohol: its carbonyl has been reduced

so it cannot cyclize and has no anomeric carbon.

What is the net yield of glycolysis per glucose? 2 ATP

2 NADH

Where does glycolysis occur? In the cytosol; no oxygen is required.

What are the three stages of glycolysis? Stage 1 (steps 1-3): trap and prime

2 ATP spent. Stage 2 (steps 4-5): cleave to two trioses. Stage 3 (steps 6-10

Which enzyme catalyzes the committed

rate-limiting step of glycolysis? Phosphofructokinase-1 (PFK-1).

What are the three irreversible steps of glycolysis and their enzymes? 1. Glucose → glucose-6-P (hexokinase/glucokinase). 2. Fructose-6-P → fructose-1

6-bisP (PFK-1). 3. PEP → pyruvate (pyruvate kinase).

What is the most potent activator of PFK-1? Fructose-2

6-bisphosphate.

What inhibits PFK-1? ATP

citrate

What activates PFK-1? AMP and fructose-2

6-bisphosphate.

How does glucagon affect PFK-1 in the liver? Glucagon → PKA phosphorylates PFK-2/FBPase-2 → fructose-2

6-bisP falls → PFK-1 off.

How does insulin affect PFK-1 in the liver? Insulin → PFK-2 active → fructose-2

6-bisP rises → PFK-1 on.

Why does glucokinase

not hexokinase

What is the difference between hexokinase and glucokinase? Hexokinase: low Km

found in most tissues

Which transporter partners with glucokinase in liver and β-cells? GLUT2 (high Km)

allowing glucose uptake to track blood glucose.

Why must pyruvate be converted to lactate when oxygen is short? Lactate dehydrogenase regenerates NAD+; without NAD+

glyceraldehyde-3-P dehydrogenase (step 6) stops and glycolysis halts.

What enzyme converts pyruvate to lactate? Lactate dehydrogenase (LDH).

What is the Pasteur effect? In the presence of oxygen

glycolysis slows because aerobic metabolism produces more ATP

How does fluoride reduce acid production by plaque bacteria? It enters bacteria as HF

releases F- inside

What is the main anticaries effect of fluoride? Promoting enamel fluorapatite formation and remineralization; enolase inhibition is an added benefit.

Why is xylitol toxic to plaque bacteria? It is taken up and phosphorylated by the PEP-driven sugar transport system

but xylitol-5-phosphate cannot enter glycolysis; it is dephosphorylated and expelled

Why do red blood cells depend on glycolysis? They have no mitochondria

so glycolysis to lactate is their only ATP source.

What is the 2

3-BPG shunt and what does it do? 1

What happens in pyruvate kinase deficiency? Red cells cannot make enough ATP; cells stiffen and are removed by the spleen

causing hemolytic anemia.

How does fructose enter glycolysis? Fructokinase → fructose-1-P → aldolase B → DHAP + glyceraldehyde → G3P. It enters below PFK-1

skipping the main control point.

How does galactose enter glycolysis? Galactokinase → galactose-1-P → UDP-galactose → epimerase (C-4 inversion) → UDP-glucose → glucose-6-P.

Why is fructose metabolized faster than glucose in the liver? It enters glycolysis below PFK-1

bypassing the main rate-limiting control step.

What are the four fates of glucose-6-phosphate? 1. Glycolysis. 2. Glycogen synthesis. 3. Pentose phosphate pathway. 4. Free glucose (liver and kidney only

via glucose-6-phosphatase).

A patient with type 1 diabetes took insulin but skipped breakfast and presents confused and sweaty with glucose 45 mg/dL. Why is the brain vulnerable? The brain runs almost entirely on glucose (~120 g/day)

stores little glycogen