An amino acid has a pKa of 2.2 for its alpha-carboxyl group and a pKa of 9.2 for its alpha-amino group. At physiological pH (7.4), what is the predominant ionic form of this amino acid?
Biochemistry
Question · Easy
Km or Vmax: an aria of rivals
Short
Which of the following amino acids is most likely to be found in the interior of a globular protein in aqueous solution?
Chapter 1: Amino Acids, Peptides & Protein Structure
1.1 Amino Acids, Ionization & Isoelectric Points
Study guide
Passage: A researcher studies a mutant enzyme that converts substrate S to product P. Kinetic data collected at varying [S] with fixed enzyme concentration are shown below: [S] (mM) | Initial velocity (µM/min) 0.5 | 15 1.0 | 25 2.0 | 37 5.0 | 48 10.0 | 53 The researcher estimates Vmax ≈ 60 µM/min from a Lineweaver-Burk plot. What is the approximate Km of this enzyme?
Question · Medium
Passage (continued from above): A competitive inhibitor (I) is added at a fixed concentration. Which change in kinetic parameters is expected?
During glycolysis, glucose is converted to two molecules of pyruvate. Which of the following correctly describes the net energy yield per glucose in glycolysis under aerobic conditions?
Metabolism
A patient with Type I (insulin-dependent) diabetes develops ketoacidosis. Which metabolic pathway is primarily responsible for the accumulation of ketone bodies in this condition?
1.2 Primary, Secondary, Tertiary & Quaternary Structure
Which of the following best describes the role of ubiquitin in eukaryotic protein degradation?
Molecular Biology
In the citric acid cycle (TCA cycle), one turn around the cycle starting from acetyl-CoA produces which of the following?
Passage: A biochemist studies the electron transport chain (ETC) in isolated mitochondria. She adds rotenone, a specific inhibitor of Complex I (NADH dehydrogenase). Which of the following results would be expected? She measures oxygen consumption, NADH levels, and the proton gradient (ΔpH across the inner mitochondrial membrane).
Passage (continued): The biochemist then adds succinate (a TCA cycle intermediate) to the inhibited preparation. What effect would this have on ATP synthesis?
1.3 Non-Enzymatic Protein Function
Which bond in a polypeptide is responsible for the planar, rigid structure of the peptide backbone that restricts rotation?
The alpha helix and beta sheet are both stabilized primarily by which type of non-covalent interaction?
Passage: A molecular biologist is studying a eukaryotic gene. She notes that the pre-mRNA transcript is 12,000 nucleotides long, but the mature mRNA is only 1,800 nucleotides. She also observes that the 5' end of the mature mRNA has a 7-methylguanosine cap and the 3' end has a poly-A tail of ~200 adenosines. Which process accounts for the large size difference between the pre-mRNA and the mature mRNA?
Passage (continued): The researcher discovers that an alternative splicing event in this gene can include or exclude a specific 300-nt exon. How many distinct mRNA isoforms can be generated from this gene if this is the only alternative splicing event?
1.4 Enzyme Classification & Mechanisms
Which of the following correctly describes the structure of DNA according to the Watson-Crick model?
In eukaryotic DNA replication, which enzyme synthesizes the RNA primer needed to start each new DNA strand?
A cell in G2 phase is exposed to ionizing radiation that causes double-strand DNA breaks. Which cellular response is MOST directly responsible for preventing this cell from entering mitosis prematurely?
Cell Biology
Question · Hard
Which organelle is responsible for modifying, sorting, and packaging proteins destined for secretion or delivery to the plasma membrane?
Chapter 2: Enzyme Kinetics, Regulation & Bioenergetics
2.1 Enzyme Kinetics & Michaelis-Menten Model
A scientist uses a pulse-chase experiment with [³H]-leucine to track newly synthesized secretory proteins. After a short pulse, radioactivity is found predominantly in the rough ER. After a 30-minute chase, most radioactivity is in the Golgi. What would happen to this trafficking if GTP hydrolysis by ARF1 (ADP-ribosylation factor 1) were permanently blocked?
In Mendelian genetics, two heterozygous parents (Aa × Aa) are crossed. What fraction of their offspring is expected to be homozygous recessive (aa)?
Genetics
A man with an X-linked recessive disorder marries a woman who is a carrier. What is the probability that their son will be affected?
Hardy-Weinberg equilibrium requires which of the following conditions?
2.2 Enzyme Inhibition & Kinetics Modifications
Passage: A researcher tests a bacterial strain's ability to grow on minimal medium. Wild-type bacteria grow on minimal medium (glucose + inorganic salts), but a mutant (auxotroph) requires the addition of the amino acid histidine to grow. The researcher performs a complementation test by fusing two independent histidine-requiring mutants (mutant A and mutant B) in the same cell. If the fused cell can grow on minimal medium, what does this result indicate?
Which of the following correctly pairs a lipid class with its primary biological function?
Which of the following best describes the mechanism by which aspirin (acetylsalicylic acid) irreversibly inhibits cyclooxygenase (COX) enzymes?
Passage: A researcher studies a bacterium that has been transformed with a plasmid containing the lac operon. Normally, when lactose is absent, the lac repressor binds the operator and blocks transcription of lacZ, lacY, and lacA. When lactose (or allolactose) is present, it binds the repressor, causing allosteric release from the operator, allowing transcription. A mutation is discovered in which the lac repressor can no longer bind allolactose. What would be the effect on lac operon expression?
2.3 Allosteric Regulation & Covalent Modification
Passage (continued): In the same bacterium, a researcher introduces a second, independent mutation that deletes the operator region (O⁻ mutation). What is the expected phenotype for lac operon expression in this double-mutant strain (repressor cannot bind inducer AND operator is deleted)?
Which of the following describes the correct orientation and polarity rules for transcription of a gene?
A cell biologist discovers a protein that localizes to the inner mitochondrial membrane. The protein contains a proton channel that can uncouple the proton gradient from ATP synthesis, dissipating energy as heat. This describes which physiological protein?
The pentose phosphate pathway (PPP) is important in red blood cells primarily because it generates which molecule critical for protecting cells from oxidative damage?
2.4 Bioenergetics & Free Energy
Passage: Researchers are investigating signal transduction in a pancreatic beta cell. Normally, when blood glucose rises, glucose enters beta cells via GLUT2 transporters, is metabolized, and raises the ATP/ADP ratio. This closes ATP-sensitive K⁺ channels (K_ATP channels), depolarizing the cell membrane. Depolarization opens voltage-gated Ca²⁺ channels, and the resulting Ca²⁺ influx triggers insulin secretion. A drug is found that keeps K_ATP channels permanently open. What effect does this drug have on insulin secretion?
Physiology
Passage (continued): A second drug is found that is a sulfonylurea. Sulfonylureas bind to and close K_ATP channels regardless of intracellular ATP levels. How does this drug affect blood glucose?
In skeletal muscle, which of the following events occurs FIRST after a motor neuron action potential reaches the neuromuscular junction?
Which of the following correctly describes the mechanism of action of the sodium-potassium ATPase (Na⁺/K⁺-ATPase)?
2.5 Biological Oxidation-Reduction Reactions
In eukaryotic translation, at which ribosomal site does aminoacyl-tRNA first enter during elongation?
A patient presents with a frameshift mutation caused by the insertion of a single nucleotide early in the coding sequence of a gene. Which of the following outcomes is MOST likely?
The action potential in a neuron involves which of the following sequential ionic events?
Which of the following correctly explains why myelinated axons conduct action potentials faster than unmyelinated axons of the same diameter?
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