UWorld Biochemistry: Metabolic Reactions Quiz 2

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Questions and Answers

How does acetylation affect the interaction between histones and DNA, and what is the consequence of this interaction?

  • Acetylation strengthens salt bridges, decreasing transcription.
  • Acetylation stabilizes DNA phosphates, decreasing transcription.
  • Acetylation has no effect on salt bridges but directly inhibits DNA replication.
  • Acetylation disrupts salt bridges, increasing transcription. (correct)

Under what specific conditions is it necessary to convert concentrations to millimoles (mmol) when performing calculations?

  • Only convert to mmol if matching a volume in liters and concentration in molarity.
  • Only convert to mmol if matching a volume in mL and concentration in M (mol/L). (correct)
  • Always convert to mmol when dealing with any concentration.
  • Only convert to mmol if the molecular weight of the substance is known.

Why do cis double bonds, rather than trans double bonds, primarily contribute to increased membrane fluidity in fatty acids?

  • _Trans_ double bonds have a higher energy and cause lipids to vibrate more.
  • _Trans_ double bonds introduce kinks that disrupt lipid packing more effectively.
  • _Cis_ double bonds introduce bends that prevent tight lipid packing. (correct)
  • _Cis_ double bonds are shorter and thus allow for more space between lipids.

During prolonged starvation, what metabolic conversion primarily provides the brain with a major source of fuel?

<p>Fatty acids from triacylglycerols are converted to ketone bodies. (B)</p> Signup and view all the answers

Why is glycerol not accumulated during starvation, and what is it primarily used for?

<p>Glycerol is rapidly used for gluconeogenesis to maintain blood glucose levels. (A)</p> Signup and view all the answers

Why are glycogen stores not a significant contributor to energy during prolonged starvation?

<p>Glycogen stores are depleted within the first 24 hours of fasting. (B)</p> Signup and view all the answers

What happens to amino acids during the increased protein catabolism that occurs in starvation, and how are they used?

<p>Amino acids are broken down and used for glucose production. (C)</p> Signup and view all the answers

Phospholipids can be separated using chromatography due to variations in what properties?

<p>Charge, size, and solubility (D)</p> Signup and view all the answers

How do omega-3 fatty acids and sphingolipids contribute to cardiovascular and dermatological health, respectively?

<p>Omega-3s reduce cholesterol clustering; sphingolipids restore membrane stability. (A)</p> Signup and view all the answers

What is the difference in how lipases act on emulsified lipids with ester bonds versus those without ester bonds?

<p>Lipases emulsify all lipids but hydrolyze only those with ester bonds. (A)</p> Signup and view all the answers

How might statins, drugs that block cholesterol production, affect the levels of steroid hormones?

<p>Statins may lower steroid hormone levels by blocking cholesterol production. (A)</p> Signup and view all the answers

What characteristic of the –(CH₂)₁₄– chain makes it hydrophobic?

<p>The nonpolar nature of the carbon-hydrogen bonds. (B)</p> Signup and view all the answers

Which property of the phospholipid bilayer primarily blocks the diffusion of hydrophilic molecules across the membrane?

<p>The hydrophobic core formed by fatty acid tails. (C)</p> Signup and view all the answers

Which of the following correctly represents the synthesis pathway from isoprene to steroid hormones?

<p>Isoprene → Monoterpene → Squalene → Cholesterol → Steroid hormones (B)</p> Signup and view all the answers

Which nitrogenous bases are classified as purines?

<p>Adenine and Guanine (B)</p> Signup and view all the answers

Identify the pyrimidines from the following list of nitrogenous bases.

<p>Cytosine and Uracil (B)</p> Signup and view all the answers

How does the ring structure differ between purines and pyrimidines?

<p>Purines have 2 rings, pyrimidines have 1 ring. (B)</p> Signup and view all the answers

If you observe a molecule with a repeating 5-carbon branched pattern, what type of molecule is it likely to be?

<p>A terpene (C)</p> Signup and view all the answers

What is the relationship between isoprene and terpenes in terms of their carbon count?

<p>Isoprene has 5 carbons, but terpenes are made by linking multiple isoprenes together. (A)</p> Signup and view all the answers

What structural characteristic defines a sterol as a type of terpene?

<p>A four-ring core structure. (A)</p> Signup and view all the answers

What structural feature is essential for a molecule to be classified as a fatty acid?

<p>A long hydrocarbon chain with a terminal carboxylic acid group. (D)</p> Signup and view all the answers

How do the molecular weights of DNA monomers typically rank from highest to lowest?

<p>dGMP &gt; dAMP &gt; dTMP &gt; dCMP (A)</p> Signup and view all the answers

In gel electrophoresis, how does the weight of DNA strands affect their migration speed, and which monomer influences it the most?

<p>Heavier strands migrate slower; influenced by dGMP. (A)</p> Signup and view all the answers

Which carbon on a sugar molecule typically links to proteins?

<p>C1 (α-anomeric carbon) (A)</p> Signup and view all the answers

At which carbon atom does the β-linkage on a sugar molecule typically connect to other sugars?

<p>C4 (C)</p> Signup and view all the answers

Which carbon atom in a sugar molecule is a common site for phosphorylation?

<p>C6 (C)</p> Signup and view all the answers

What distinguishes epimers from other types of stereoisomers?

<p>Epimers differ at only one chiral center. (C)</p> Signup and view all the answers

How are epimers, enantiomers, and constitutional isomers different from each other?

<p>Epimers differ at one stereocenter, enantiomers differ at all stereocenters, constitutional isomers differ in atom connectivity. (C)</p> Signup and view all the answers

What structural characteristic defines a reducing sugar, and why is sucrose not classified as one?

<p>A reducing sugar has a free anomeric carbon; sucrose has no free anomeric carbons. (D)</p> Signup and view all the answers

What structural feature determines whether a sugar is classified as reducing?

<p>The presence of a free anomeric carbon. (B)</p> Signup and view all the answers

Which of the following disaccharides is classified as reducing?

<p>Lactose (D)</p> Signup and view all the answers

Why is sucrose classified as a non-reducing sugar?

<p>Both of its anomeric carbons are involved in a bond. (A)</p> Signup and view all the answers

What is the primary function of glycosidases?

<p>To cleave glycosidic bonds. (C)</p> Signup and view all the answers

Why are radiolabels used in studying molecular processes?

<p>To track where molecules go during cleavage. (A)</p> Signup and view all the answers

In what way does phosphorylation commonly facilitate enzymatic activity in metabolic pathways?

<p>It often sets the stage for other enzymes to act. (A)</p> Signup and view all the answers

When calculating proportions in a DNA sequence, what should be used as the basis for the calculation?

<p>Base pair counts. (C)</p> Signup and view all the answers

All monosaccharides are reducing, but why is this not true for all disaccharides or polysaccharides?

<p>Monosaccharides always have a free anomeric carbon available for oxidation. (D)</p> Signup and view all the answers

Flashcards

Salt Bridges in Chromatin

Positively charged histone residues and negatively charged DNA phosphates tighten chromatin. Acetylation disrupts these bridges and increases transcription.

Cis Double Bonds and Membrane Fluidity

Only cis double bonds in fatty acids increase membrane fluidity by introducing bends that prevent tight lipid packing.

Fatty Acids to Ketone Bodies

During prolonged starvation, fatty acids from triacylglycerols are converted to ketone bodies, which become a major fuel source for the brain.

Glycerol's Fate During Starvation

Glycerol is rapidly used for gluconeogenesis to help maintain blood glucose levels.

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Glycogen Depletion

Glycogen stores are depleted within the first 24 hours of fasting and do not contribute significantly during prolonged starvation.

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Protein Catabolism in Starvation

Protein catabolism increases during starvation, leading to amino acid breakdown used for glucose production, not energy storage.

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Phospholipid Separation

Phospholipids vary in charge, size, and solubility; use these properties to separate them using different types of chromatography.

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Omega-3s and Sphingolipids

Omega-3 fatty acids reduce cholesterol clustering, helping prevent atherosclerosis. Sphingolipids like ceramides restore membrane stability, helping with conditions like psoriasis.

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Lipid Hydrolysis

All lipids are emulsified, but only those with ester bonds (like triglycerides and phospholipids) are hydrolyzed by lipases.

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Statins and Steroid Hormones

Steroid hormones (testosterone, cortisol, etc.) are synthesized from cholesterol, so statins may lower their levels by blocking cholesterol production.

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Hydrocarbon Chain Polarity

The long –(CH₂)₁₄– chain is always hydrophobic.

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Hydrophobic Core Function

The hydrophobic core of the phospholipid bilayer, formed by fatty acid tails, prevents hydrophilic molecules from diffusing across the membrane.

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Steroid Synthesis Pathway

The synthesis order is: Isoprene → Monoterpene → Squalene → Cholesterol → Steroid hormones

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Purines

Adenine (A), Guanine (G)

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Pyrimidines

Cytosine (C), Uracil (U), Thymine (T)

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Purine vs. Pyrimidine Rings

Purines have 2 rings, pyrimidines have 1 ring

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Terpene Composition

A terpene is a molecule made from isoprene units (C₅H₈).

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Sterol Structure

Sterols are a type of terpene with a four-ring core structure (three 6-membered rings + one 5-membered ring).

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Fatty Acid Definition

Fatty acids are long hydrocarbon chains with a terminal carboxylic acid group.

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DNA Monomer Weight

The molecular weight of DNA monomers from highest to lowest: dGMP > dAMP > dTMP > dCMP

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DNA Migration in Gel Electrophoresis

In gel electrophoresis, heavier DNA strands (with more dGMP) migrate slower; lighter strands (with more dCMP) migrate faster.

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Alpha Anomeric Carbon Linkage

The α-anomeric carbon (C1) links to proteins.

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Beta Linkage Location

The β-linkage connects to other sugars at C4.

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Carbon 6 Role

C6 is a common site for phosphorylation.

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Epimer Definition

Epimers are stereoisomers that differ at only one chiral center.

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Sugar Isomer Types

If two sugars differ at one stereocenter, they are epimers; if they differ at all, they are enantiomers; if they differ in atom connectivity, they are constitutional isomers.

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Reducing Sugar Definition

A reducing sugar has a free anomeric carbon and can form new glycosidic bonds. Sucrose is not a reducing sugar because both anomeric carbons are involved in its bond.

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Monosaccharides as Reducing Sugars

All monosaccharides (like galactose and ribose) are reducing.

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Lactose as a Reducing Sugar

Lactose is reducing (one free anomeric carbon).

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Glycosidase Function

Glycosidases cleave glycosidic bonds.

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Radiolabel Use

Radiolabels track where molecules go during cleavage.

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Phosphorylation's Role

Phosphorylation often sets the stage for other enzymes to act — especially in carbohydrate, protein, and signal transduction pathways.

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Base Pair Proportion Calculation

Always calculate proportions based on base pair counts, especially when asked about A–T vs. G–C content.

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Study Notes

  • Salt bridges between positively charged histone residues and negatively charged DNA phosphates tighten chromatin.
  • Acetylation disrupts these salt bridges and increases transcription.
  • It's necessary to convert to mmol when matching a volume in mL with a concentration in M (mol/L).
  • Only cis double bonds increase membrane fluidity.
  • Cis double bonds introduce bends in the fatty acid chain that prevent tight lipid packing.
  • During prolonged starvation, fatty acids from triacylglycerols are converted to ketone bodies, becoming a major fuel source for the brain.
  • Glycerol is rapidly used for gluconeogenesis to help maintain blood glucose levels and does not build up during starvation.
  • Glycogen stores are depleted within the first 24 hours of fasting and do not contribute significantly during prolonged starvation.
  • Protein catabolism increases during starvation, leading to amino acid breakdown.
  • These amino acids are used for glucose production, not energy storage.
  • Phospholipids vary in charge, size, and solubility.
  • These properties can be used to separate phospholipids using different types of chromatography.
  • Omega-3 fatty acids reduce cholesterol clustering, aiding in the prevention of atherosclerosis.
  • Sphingolipids like ceramides restore membrane stability, helping with conditions like psoriasis.
  • All lipids are emulsified, but only those with ester bonds, such as triglycerides and phospholipids, are hydrolyzed by lipases.
  • Steroid hormones (testosterone, cortisol, etc.) are synthesized from cholesterol.
  • Statins may lower steroid hormone levels by blocking cholesterol production.
  • Long –(CH₂)₁₄– chains are always hydrophobic.
  • The hydrophobic core of the phospholipid bilayer prevents hydrophilic molecules from diffusing across the membrane.
  • The correct order of synthesis is: Isoprene → Monoterpene → Squalene → Cholesterol → Steroid hormones.
  • Purines include adenine (A) and guanine (G).
  • Pyrimidines include cytosine (C), uracil (U), and thymine (T).
  • Purines have 2 rings, while pyrimidines have 1 ring.
  • Terpenes are molecules made from isoprene units (C₅H₈).
  • Terpenes are likely to be present if a repeating 5-carbon branched pattern can be observed.
  • A single isoprene has 5 carbons.
  • Terpenes are made by linking multiple isoprenes together.
  • Sterols are a type of terpene with a four-ring core structure (three 6-membered rings + one 5-membered ring).
  • Fatty acids are long hydrocarbon chains with a terminal carboxylic acid group.
  • Molecules without a terminal carboxylic acid group are not fatty acids.
  • The molecular weight of DNA monomers from highest to lowest: dGMP > dAMP > dTMP > dCMP.
  • In gel electrophoresis, heavier DNA strands (with more dGMP) migrate slower.
  • In gel electrophoresis, lighter strands (with more dCMP) migrate faster.
  • The α-anomeric carbon (C1) links to proteins.
  • The β-linkage connects to other sugars at C4.
  • C6 is a common site for phosphorylation.
  • Epimers are stereoisomers that differ at only one chiral center.
  • If two sugars differ at one stereocenter, they are epimers.
  • If two sugars differ at all stereocenters, they are enantiomers.
  • If two sugars differ in atom connectivity, they are constitutional isomers.
  • Reducing sugars have a free anomeric carbon and can form new glycosidic bonds.
  • Sucrose is not a reducing sugar because both anomeric carbons are involved in its bond.
  • A sugar is reducing if it has a free anomeric carbon.
  • All monosaccharides (like galactose and ribose) are reducing.
  • Lactose is reducing because it has one free anomeric carbon.
  • Sucrose is a non-reducing sugar (both anomeric carbons are involved in the bond).
  • Glycosidases cleave glycosidic bonds.
  • Radiolabels track where molecules go during cleavage.
  • Phosphorylation often sets the stage for other enzymes to act, especially in carbohydrate, protein, and signal transduction pathways.
  • Always calculate proportions based on base pair counts, especially when asked about A–T vs. G–C content.

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