MCAT Chemical and Physical Foundations of Biological Systems Flashcards: 5d Carbohydrates Glycoconjugates

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This deck focuses on 5d Carbohydrates Glycoconjugates, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Chemical and Physical Foundations of Biological Systems.

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Flashcard 1: What is mutarotation in aqueous solution?

Answer: Interconversion of $\alpha$ and $\beta$ anomers via the open-chain form. Mutarotation occurs as the ring opens to the linear form, allowing rotation at the anomeric carbon before re-closing.

Flashcard 2: In a Haworth projection of a D-sugar, which direction does the $CH_2OH$ group point?

Answer: Up. Haworth projections represent the cyclic form where the $CH_2OH$ is above the plane for D-sugars to match Fischer orientation.

Flashcard 3: What determines D versus L configuration for a monosaccharide in a Fischer projection?

Answer: The OH on the highest-numbered chiral carbon: right = D, left = L. In Fischer projections, the configuration at the penultimate carbon mirrors D/L-glyceraldehyde, determining the series for sugars.

Flashcard 4: What is the difference between N-linked and O-linked glycosylation in proteins?

Answer: N-linked: Asn amide N; O-linked: Ser/Thr hydroxyl O. N-linked glycosylation attaches via asparagine's nitrogen in the ER, while O-linked uses serine/threonine oxygens in the Golgi.

Flashcard 5: What are anomers, and which carbon is involved in their interconversion?

Answer: Stereoisomers differing at the anomeric carbon. Anomers arise from cyclization at the carbonyl carbon, creating a new stereocenter that allows interconversion through ring opening.

Flashcard 6: What is the anomeric carbon in an aldose versus a ketose (in cyclic form)?

Answer: Aldose: $C_1$; ketose: $C_2$. The anomeric carbon is the former carbonyl carbon that becomes chiral in the cyclic hemiacetal or hemiketal form.

Flashcard 7: What is the definition of a reducing sugar?

Answer: A sugar with a free anomeric carbon (hemiacetal/hemiketal). Reducing sugars can open their ring to expose a free aldehyde or ketone group, enabling oxidation in reactions like Fehling's test.

Flashcard 8: Identify whether sucrose is reducing or nonreducing.

Answer: Nonreducing. Sucrose lacks a free anomeric carbon due to its glycosidic bond involving both anomeric positions, preventing ring opening.

Flashcard 9: Which monosaccharide is an aldose and which is a ketose: glucose vs fructose?

Answer: Glucose is an aldose; fructose is a ketose. Glucose has an aldehyde group, classifying it as an aldose, while fructose's ketone group makes it a ketose.

Flashcard 10: What is the relationship between D/L notation and optical rotation sign ($+$ or $-$)?

Answer: No fixed relationship; D/L does not predict $+$ or $-$. D/L denotes relative configuration based on glyceraldehyde, independent of the direction of light rotation, which is an empirical property.

Flashcard 11: What is the definition of a carbohydrate in biochemical terms?

Answer: Polyhydroxy aldehyde or ketone, or a compound yielding them. Carbohydrates are classified based on their polyhydroxylated carbonyl structure, which allows for hydration and energy storage functions in biology.

Flashcard 12: What is the functional group difference between an aldose and a ketose?

Answer: Aldose: aldehyde at $C_1$; ketose: ketone (often at $C_2$). The carbonyl group's position and type define aldoses and ketoses, influencing their chemical reactivity and cyclic structures.

Flashcard 13: What distinguishes a proteoglycan from a glycoprotein?

Answer: Proteoglycan: mostly GAG carbohydrate; glycoprotein: mostly protein. Proteoglycans have extensive GAG chains dominating mass for structural roles, unlike glycoproteins with shorter oligosaccharides for signaling.

Flashcard 14: What is the key chemical feature of glycosaminoglycans (GAGs) that drives water retention?

Answer: High negative charge density (often sulfate and carboxylate groups). The anionic groups in GAGs attract cations and water via electrostatic interactions, enabling hydration in extracellular matrices.

Flashcard 15: Which statement is correct: humans can digest starch, cellulose, both, or neither?

Answer: Humans digest starch but not cellulose. Humans possess $\alpha$-amylase for starch's $\alpha$-linkages but lack cellulase for cellulose's $\beta$-linkages.

Flashcard 16: Which polymer is unbranched $\beta(1\rightarrow^4)$ glucose and forms structural fibers in plants?

Answer: Cellulose. Cellulose's linear $\beta$-linkages form hydrogen-bonded fibers providing plant cell wall rigidity, indigestible to humans.

Flashcard 17: Which term describes two sugars that differ only at one stereocenter (not anomers)?

Answer: Epimers. Epimers are diastereomers differing at a single asymmetric carbon, excluding the anomeric one, which distinguishes them from anomers.

Flashcard 18: Which polymer is primarily $\alpha(1\rightarrow^4)$ glucose with $\alpha(1\rightarrow^6)$ branches in animals?

Answer: Glycogen. Glycogen's branching structure facilitates rapid glucose release for energy in animals via enzymatic hydrolysis.

Flashcard 19: What monosaccharides and linkage define sucrose?

Answer: Glucose + fructose with an $\alpha(1\rightarrow^2)\beta$ linkage. Sucrose's unique linkage joins glucose's anomeric carbon to fructose's, making it nonreducing and invert sugar upon hydrolysis.

Flashcard 20: What disaccharide is composed of galactose + glucose with a $\beta(1\rightarrow^4)$ linkage?

Answer: Lactose. Lactose's $\beta(1\rightarrow^4)$ linkage between galactose and glucose is cleaved by lactase in the human digestive system.

Flashcard 21: What disaccharide is composed of glucose + glucose with an $\alpha(1\rightarrow^4)$ linkage?

Answer: Maltose. Maltose forms from starch breakdown, with its $\alpha(1\rightarrow^4)$ bond hydrolysable by human $\alpha$-amylase and maltase.

Flashcard 22: What disaccharide is composed of glucose + glucose with a $\beta(1\rightarrow^4)$ linkage?

Answer: Cellobiose (the repeating disaccharide unit of cellulose). Cellobiose's $\beta(1\rightarrow^4)$ linkage allows linear chaining in cellulose, which humans cannot hydrolyze due to lacking $\beta$-glucosidase.

Flashcard 23: What linkage connects monosaccharides in oligo- and polysaccharides?

Answer: Glycosidic bond (acetal/ketal linkage). Glycosidic bonds form via dehydration between an anomeric carbon and another sugar's hydroxyl, creating stable acetal linkages.

Flashcard 24: What type of bond forms when a monosaccharide cyclizes to a ring?

Answer: Intramolecular hemiacetal (aldose) or hemiketal (ketose). Cyclization involves nucleophilic attack by a hydroxyl group on the carbonyl, forming a hemiacetal or hemiketal ring structure.

Flashcard 25: For a D-sugar in Haworth form, what defines the b^1 versus b^2 anomer?

Answer: $\alpha$: anomeric OH down; $\beta$: anomeric OH up. In D-sugars, the $\alpha$ anomer has the anomeric hydroxyl trans to the $CH_2OH$, pointing down, while $\beta$ is cis, pointing up.