Monosaccharides Classification Quiz

Questions and Answers

Monosaccharides can be classified into two types based on the functional group: __________ and ketoses.

aldoses

In aqueous solutions, glucose can form a cyclic structure known as __________.

hemiacetal

D- and L-isomers of monosaccharides are classified based on the orientation of the __________ group.

hydroxyl

The disaccharide __________ is composed of glucose and fructose linked by an α(1→2) glycosidic bond.

sucrose Signup and view all the answers

Polysaccharides can be classified as __________ if they consist of one type of monosaccharide.

homopolysaccharides Signup and view all the answers

Pyruvate is converted to ______ before entering the TCA cycle.

acetyl-CoA Signup and view all the answers

The Pyruvate Dehydrogenase Complex requires five cofactors including thiamine pyrophosphate (TPP), lipoic acid, coenzyme A, FAD, and ______.

NAD⁺ Signup and view all the answers

During the TCA cycle, citrate is synthesized from acetyl-CoA and ______.

oxaloacetate Signup and view all the answers

Isocitrate is converted to α-ketoglutarate by the enzyme ______.

isocitrate dehydrogenase Signup and view all the answers

Succinate dehydrogenase catalyzes the oxidation of succinate to ______, producing FADH₂.

fumarate Signup and view all the answers

Study Notes

Monosaccharides

Classified by the number of carbon atoms:
- Trioses (3 carbons)
- Tetroses (4 carbons)
- Pentoses (5 carbons, e.g., ribose)
- Hexoses (6 carbons, e.g., glucose, fructose)
Functional groups:
- Aldoses (contain an aldehyde group, e.g., glucose)
- Ketoses (contain a ketone group, e.g., fructose)
Exhibit chirality, allowing existence in different stereoisomers due to asymmetric carbons.

D- and L-Isomers

Classified based on the orientation of the hydroxyl group (-OH) on the farthest asymmetric carbon from the carbonyl group.
Most naturally occurring sugars are in D-form.

Cyclization of Monosaccharides

In aqueous solutions, monosaccharides like glucose and fructose can cyclize:
- Aldoses form hemiacetals, often creating six-membered rings (pyranose).
- Ketoses form hemiketals, typically resulting in five-membered rings (furanose).
Cyclization introduces an anomeric carbon with two configurations:
- Alpha (α) if the hydroxyl group on the anomeric carbon is down.
- Beta (β) if it is up.

Disaccharides

Comprised of two monosaccharides linked by a glycosidic bond:
- Sucrose (glucose + fructose): Table sugar, α(1→2) glycosidic bond.
- Lactose (galactose + glucose): Found in milk, β(1→4) glycosidic bond.
- Maltose (glucose + glucose): Product of starch digestion, α(1→4) glycosidic bond.

Polysaccharides

Long chains of monosaccharides linked by glycosidic bonds.
Types include:
- Homopolysaccharides (one type of monosaccharide)
- Heteropolysaccharides (more than one type)

Pyruvate Dehydrogenase Complex (PDC)

Converts pyruvate from glycolysis to acetyl-CoA before entering the TCA cycle.
Involves:
- Decarboxylation: Removal of carbon from pyruvate, releasing CO₂.
- Oxidation: Electrons transferred to NAD⁺, forming NADH.
- Formation of acetyl-CoA by attaching the remaining two-carbon unit to coenzyme A.
Composed of three enzymes and requires five cofactors: TPP, lipoic acid, CoA, FAD, and NAD⁺.

Regulation of PDC

Inhibited by phosphorylation when energy levels are high (high ATP, NADH, and acetyl-CoA).
Activated by dephosphorylation when energy levels are low (high ADP and pyruvate).

TCA Cycle

A series of eight reactions that oxidize acetyl-CoA to CO₂, generating energy molecules (NADH, FADH₂, GTP).
Key steps include:
- Citrate formation from acetyl-CoA and oxaloacetate (catalyzed by citrate synthase).
- Sequential transformations producing isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, and malate.

Gluconeogenesis

Energy-intensive process to synthesize glucose from two pyruvate molecules, requiring:
- 4 ATP, 2 GTP, 2 NADH.
Regulated to prevent simultaneous activation of glycolysis:
- Hormonal control by insulin (inhibits), glucagon (stimulates), and cortisol (stimulates).

Glycogenolysis

Breakdown of glycogen into glucose.
Key steps:
- Glycogen phosphorylase cleaves glucose units into glucose-1-phosphate.
- Conversion of glucose-1-phosphate to glucose-6-phosphate by phosphoglucomutase.
- In the liver, glucose-6-phosphatase converts glucose-6-phosphate to free glucose.

Regulation of Glycogen Metabolism

Insulin promotes glycogenesis and inhibits glycogenolysis; glucagon stimulates glycogenolysis and inhibits glycogenesis.
Allosteric regulation via glucose-6-phosphate and AMP for glycogen phosphorylase and synthase.

Glycogen Storage Diseases

Genetic disorders due to enzyme deficiencies affecting glycogen metabolism.
Example: von Gierke Disease, a deficiency of glucose-6-phosphatase, causing hypoglycemia and liver glycogen accumulation.

Pentose Phosphate Pathway

Involves interconversion of ribose-5-phosphate using transketolase and transaldolase, linking to glycolysis.
Produces NADPH for biosynthesis and detoxification, affecting cellular function and metabolic balance.

Clinical Relevance of the Pentose Phosphate Pathway

G6PD deficiency leads to reduced NADPH, increasing oxidative stress risk.
Tumor cells may exhibit increased PPP activity for rapid growth support.
Altered PPP activity can influence glucose metabolism and complications in diabetes.