Molecular Biology of the Cell · Part 1 of 2 · 10 MCQs per part
Part 1 of 2
Amino Acids, Polypeptide Structure, and Protein Folding
Proteins are the workhorses of the cell — enzymes, motors, scaffolds, signals, and antibodies are all proteins. Their staggering functional diversity arises from one source: the virtually limitless ways a chain of 20 amino acids can fold into a precise three-dimensional shape.
3.1 Amino Acids — The Building Blocks
All proteins are polymers of amino acids. There are 20 standard amino acids, each with a central α-carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain (R group). The R group determines each amino acid's chemical character: nonpolar/hydrophobic (e.g., leucine, valine), polar uncharged (e.g., serine, threonine), positively charged (e.g., lysine, arginine), negatively charged (e.g., aspartate, glutamate), or aromatic (e.g., phenylalanine, tryptophan).
Key term
Amino acid
The monomer of proteins; a molecule with an amino group, a carboxyl group, a hydrogen, and a variable side chain all attached to a central α-carbon.
3.2 The Peptide Bond and Primary Structure
Amino acids are joined by peptide bonds — covalent bonds formed between the carboxyl group of one amino acid and the amino group of the next, with loss of water. The resulting chain is called a polypeptide. The sequence of amino acids in a polypeptide is its primary structure, determined by the gene encoding that protein. Even a single amino acid substitution can abolish protein function (as in sickle-cell anemia, where glutamate is replaced by valine in β-globin).
Polypeptide chains have directionality: the end retaining a free amino group is the N-terminus; the end retaining a free carboxyl group is the C-terminus. By convention, sequences are written N → C.
3.3 Secondary Structure
Local regions of a polypeptide adopt regular, repeating conformations stabilized by hydrogen bonds between backbone atoms — this is secondary structure. The two most common elements are:
Alpha-helix (α-helix): a right-handed coil in which each backbone N–H donates a hydrogen bond to the backbone C=O four residues earlier. The R groups project outward from the helix axis.
Beta-sheet (β-sheet): extended strands that align side by side, with hydrogen bonds forming between strands. Strands can be parallel (same N→C direction) or antiparallel (opposite directions). R groups alternate above and below the sheet plane.
Key term
Secondary structure
Regular, repeating local conformations of a polypeptide backbone (such as α-helices and β-sheets) stabilized by hydrogen bonds between backbone atoms.
3.4 Tertiary Structure and Protein Folding
The overall three-dimensional shape of a single polypeptide chain is its tertiary structure. It is stabilized by multiple noncovalent interactions between R groups: hydrophobic packing (nonpolar residues buried in the core), hydrogen bonds, electrostatic interactions, and van der Waals forces. Disulfide bonds — covalent S–S bonds between cysteine residues — can provide additional covalent stabilization, especially in extracellular proteins.
Protein folding is guided by the primary sequence (Anfinsen's principle: the native state is the thermodynamically most stable structure under physiological conditions). In the cell, however, folding is assisted by molecular chaperones such as Hsp70 and chaperonins (e.g., GroEL/GroES in bacteria, TRiC in eukaryotes), which prevent aggregation of unfolded or partially folded chains.
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Pause & Recall
Why do hydrophobic residues tend to end up in the interior of a folded protein rather than on its surface?
Exposing hydrophobic residues to water is thermodynamically unfavorable (hydrophobic effect). Burying them in the protein interior away from water increases the entropy of the surrounding solvent and creates a stable hydrophobic core held together by van der Waals contacts — the primary driving force for protein folding.
Practice Questions — Part 1Score: 0 / 10
1. What differentiates one amino acid from another?
All 20 amino acids share the same backbone (amino group, carboxyl group, α-carbon, hydrogen). What distinguishes them is the R group — the side chain — which varies in size, charge, polarity, and chemical reactivity, determining the protein's properties.
2. A peptide bond is formed between:
A peptide bond is a covalent amide bond formed by condensation (loss of water) between the α-carboxyl group of one amino acid and the α-amino group of the next. The R groups are not involved.
3. The primary structure of a protein is:
Primary structure is the linear sequence of amino acids linked by peptide bonds, written N→C. It is encoded by the gene and determines all higher levels of structure through the physical-chemical properties of the side chains.
4. In an α-helix, hydrogen bonds form between:
In the α-helix, each backbone N–H donates a hydrogen bond to the C=O of the residue four positions earlier in the sequence. This regular pattern winds the chain into a right-handed helix. R groups project outward and do not form the helix bonds.
5. Which of the following best describes the role of molecular chaperones?
Chaperones (e.g., Hsp70, chaperonins) bind exposed hydrophobic regions of unfolded or partially folded proteins, preventing inappropriate aggregation and providing an environment for correct folding. They do not alter the final native structure encoded in the sequence.
6. A β-sheet differs from an α-helix in that it:
A β-sheet forms by alignment of extended backbone strands, with hydrogen bonds running laterally between adjacent strands. An α-helix forms intrastrand hydrogen bonds within a single coiled chain. Both are stabilized by backbone (not R group) hydrogen bonds.
7. Disulfide bonds contribute to protein structure by:
Disulfide bonds form specifically between the sulfhydryl (–SH) groups of two cysteine residues through oxidation. They are covalent bonds and provide extra structural stability, particularly in secreted or extracellular proteins exposed to oxidizing environments.
8. Anfinsen's experiment with ribonuclease demonstrated that:
Anfinsen denatured ribonuclease and showed it refolded spontaneously upon removing denaturant, regaining full activity. This established that the native structure is determined solely by the amino acid sequence — the sequence encodes the 3D fold.
9. Which amino acid is most commonly found at the interior hydrophobic core of a globular protein?
Leucine (and other hydrophobic residues like valine, isoleucine, phenylalanine) are preferentially buried in the hydrophobic core. Lysine and aspartate are charged and prefer surface locations; serine is polar uncharged and is often surface-exposed.
10. Which level of protein structure is directly encoded by DNA?
The DNA sequence of a gene directly encodes the amino acid sequence (primary structure) of a protein through the genetic code. All higher-order structures — secondary, tertiary, quaternary — arise from the physical-chemical properties of that sequence.
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Section B · Recall Questions · Part 1
Type your answer, then click Check to reveal the sample answer.
B1
Describe the general structure of an amino acid and the role of the R group.
Sample answer: An amino acid has a central α-carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain (R group). The R group determines chemical character: nonpolar, polar, positively charged, negatively charged, or aromatic.
B2
Describe the formation of a peptide bond and what is released during the reaction.
Sample answer: A peptide bond forms between the carboxyl group (–COOH) of one amino acid and the amino group (–NH₂) of the next in a condensation reaction that releases a water molecule. The result is a –CO–NH– linkage in the polypeptide backbone.
B3
Describe the hydrogen bonding pattern that stabilizes an α-helix.
Sample answer: In an α-helix, each backbone N–H group donates a hydrogen bond to the backbone C=O four residues earlier in the sequence. This creates a regular helix with 3.6 residues per turn. R groups project outward from the helical axis.
B4
Compare parallel and antiparallel β-sheets in terms of strand orientation and hydrogen bond geometry.
Sample answer: In a parallel β-sheet, adjacent strands run in the same N→C direction; hydrogen bonds are slightly bent. In an antiparallel β-sheet, adjacent strands run in opposite directions, forming more direct, linear hydrogen bonds and generally greater stability.
B5
What types of interactions stabilize the tertiary structure of a globular protein?
Sample answer: Tertiary structure is stabilized by: (1) hydrophobic interactions (nonpolar residues buried in the core), (2) hydrogen bonds between polar R groups, (3) electrostatic interactions (salt bridges) between charged R groups, (4) van der Waals contacts in the core, and (5) disulfide bonds between cysteine residues (especially in extracellular proteins).
B6
What is the function of Hsp70 chaperones in protein folding?
Sample answer: Hsp70 binds to short hydrophobic segments exposed in nascent or unfolded proteins, preventing aggregation. In an ATP-dependent cycle, Hsp70 releases the polypeptide to allow folding attempts, then rebinds if the protein is still unfolded, giving it repeated opportunities to reach its correct native structure.
B7
Under what conditions do disulfide bonds form, and why are they more common in extracellular proteins?
Sample answer: Disulfide bonds form in oxidizing environments through oxidation of two cysteine –SH groups to form S–S. The cytoplasm is reducing (high glutathione), preventing disulfide formation. The ER lumen and extracellular space are oxidizing, favoring disulfide formation — which explains why secreted and cell-surface proteins (antibodies, insulin) use disulfide bonds for extra stability.
B8
State Anfinsen's principle (the thermodynamic hypothesis of protein folding) and explain its significance.
Sample answer: Anfinsen's principle states that the native (biologically active) conformation of a protein is the one with the lowest free energy under physiological conditions and that this information is completely determined by the amino acid sequence. Significance: the gene contains all information needed to specify a functional protein; no external "template" is needed for folding.
B9
Explain how the single amino acid change in sickle-cell hemoglobin causes the disease phenotype at the molecular level.
Sample answer: The E6V mutation (glutamate → valine at position 6 of β-globin) replaces a charged, hydrophilic side chain with a nonpolar one. In deoxy conformation, the valine fits into a hydrophobic pocket of adjacent hemoglobin molecules, causing polymerization into long fibers that distort red blood cells into a sickle shape, blocking capillaries.
B10
What is meant by the N-terminus and C-terminus of a polypeptide, and what is the conventional way to write a protein sequence?
Sample answer: The N-terminus (amino terminus) is the end of the polypeptide chain with a free α-amino group; the C-terminus (carboxyl terminus) has a free α-carboxyl group. By convention, protein sequences are always written from N-terminus to C-terminus (left to right), which also reflects the direction of synthesis on the ribosome.
Section C · Critical Thinking · Part 1
Develop analytical responses, then compare with the sample.
C1
Despite Anfinsen's principle, why is computationally predicting a protein's 3D structure from its sequence alone still considered a major challenge (or was, until recently)?
Sample answer: Although the native structure is encoded in the sequence, the number of possible conformations is astronomically large (Levinthal's paradox). Even a short 100-residue protein has ~10¹³⁰ possible conformations if sampled randomly. The energy landscape is complex and sequence-to-structure mapping rules were not well-understood. AlphaFold (2020–2021) largely solved this using deep learning, but the underlying physical-chemical problem remained hard for decades.
C2
Explain how failure of protein quality control and protein misfolding contribute to diseases such as Alzheimer's or Parkinson's.
Sample answer: In Alzheimer's disease, amyloid-β peptides misfold and aggregate into β-sheet-rich fibrils (amyloid plaques), which disrupt neuronal function. In Parkinson's, α-synuclein aggregates in Lewy bodies. These aggregations overwhelm the cell's chaperone and proteasomal degradation systems. The aggregates are toxic — they sequester other proteins, disrupt membranes, and trigger cell death pathways.
C3
Why do proteins typically have hydrophobic residues in their interior and hydrophilic residues on their surface, and what happens when this arrangement is disrupted?
Sample answer: Burying hydrophobic residues maximizes solvent entropy (hydrophobic effect) and creates a tight van der Waals-packed core. Surface hydrophilic residues interact favorably with water, stabilizing the folded state. If a buried residue is mutated to a charged one, it disrupts the hydrophobic core, destabilizes folding, and often causes misfolding, aggregation, or loss of function.
C4
Proline is often called a "helix breaker." Explain why, based on its unique chemical structure.
Sample answer: Proline's side chain forms a ring that covalently bonds back to its own backbone nitrogen. This prevents the N–H from donating a hydrogen bond — essential for α-helix formation. It also restricts φ backbone dihedral angles, imposing a rigid kink. A proline in an α-helix causes the helix to bend or terminate, hence its role as a "helix breaker."
C5
Evolutionary analysis reveals that some amino acid positions in a protein are highly conserved across species, while others are variable. What can you infer about the functional role of a conserved versus a variable position?
Sample answer: Conserved positions are under strong negative (purifying) selection — mutations there are almost always deleterious, indicating the residue is essential for function (e.g., catalytic residues, binding contacts, structural core). Variable positions tolerate substitutions, meaning different amino acids there are roughly equivalent in function. Conservation mapping onto a known structure highlights active sites and critical interfaces.
Section D · Interactive Questions · Part 1
Enter your answer and click Check for instant feedback.
D1
How many standard amino acids are used to build proteins? (number)
D2
What type of bond joins amino acids in a polypeptide chain? (two words)
D3
What secondary structure element is a right-handed coil stabilized by backbone hydrogen bonds? (two words, e.g. "alpha helix")
D4
Which amino acid forms disulfide bonds? (one word)
D5
Proteins that assist newly synthesized polypeptides to fold correctly are called _______ (one word).
Part 2 →
With a firm grasp of how polypeptides fold, we move to protein function — quaternary assemblies, enzymatic catalysis, allosteric regulation, molecular motors, and GTP-binding proteins that connect structure to cellular activity.
Part 2 of 2
Quaternary Structure, Protein Function, and Molecular Machines
3.5 Quaternary Structure
Many proteins function as assemblies of two or more polypeptide subunits — this is quaternary structure. Subunits associate through the same noncovalent interactions that stabilize tertiary structure (hydrophobic contacts, hydrogen bonds, electrostatic interactions). Hemoglobin is a classic example: a tetramer of two α and two β subunits. Quaternary assemblies allow cooperative behavior, where binding of a ligand to one subunit influences the affinity of other subunits (cooperativity).
Key term
Quaternary structure
The arrangement of two or more polypeptide subunits in a multi-subunit protein complex, held together by noncovalent interactions.
3.6 Proteins as Enzymes
Enzymes bind specific substrates at their active sites and catalyze reactions by lowering activation energy. Catalytic mechanisms include: acid-base catalysis (proton donation/acceptance), covalent catalysis (transient covalent enzyme-substrate intermediate), metal ion catalysis, and proximity/orientation effects. The catalytic triad of serine proteases (Ser-His-Asp) is a textbook example of how three properly positioned residues cooperate to catalyze peptide bond hydrolysis.
Enzyme kinetics are described by the Michaelis-Menten equation: v = Vmax[S]/(Km + [S]), where Km is the substrate concentration at half-maximal velocity and Vmax is the maximum rate. Km approximates the enzyme's affinity for its substrate.
3.7 Allosteric Regulation and Signal Transduction
Allosteric proteins change conformation when a regulatory molecule binds at a site remote from the active site. This conformational change can activate or inhibit enzyme activity. The MWC (concerted) and sequential models describe how allosteric changes propagate through oligomeric proteins. Many signaling proteins exist in two conformational states (active/inactive) toggled by regulatory inputs.
GTP-binding proteins (G proteins) are molecular switches: they are active when bound to GTP and inactive when GTP is hydrolyzed to GDP. The intrinsic GTPase activity of the G protein turns it off. GAPs (GTPase-activating proteins) accelerate GTP hydrolysis; GEFs (guanine nucleotide exchange factors) promote GDP→GTP exchange, reactivating the protein. Ras and other small GTPases relay signals from cell-surface receptors to the nucleus.
3.8 Motor Proteins
Motor proteins use energy from ATP hydrolysis to move along cytoskeletal tracks. Myosin moves along actin filaments (muscle contraction, cell division). Kinesin and dynein move cargo along microtubules: kinesin moves toward the plus end (typically toward the cell periphery); dynein moves toward the minus end (typically toward the cell center). Motor proteins convert chemical energy (ATP) into directed mechanical work.
Practice Questions — Part 2Score: 0 / 10
1. Hemoglobin is an example of quaternary structure because it:
Quaternary structure refers to the assembly of multiple polypeptide subunits. Hemoglobin is a tetramer of two α and two β subunits associated by noncovalent interactions. The α-helices within each subunit are tertiary/secondary structure.
2. Km in Michaelis-Menten kinetics approximates:
Km is the substrate concentration at which the enzyme works at half its maximum rate. A low Km means the enzyme achieves half-maximal rate at low [S] — high affinity. A high Km means low affinity (high [S] needed).
3. A GTP-binding protein (G protein) is in its active state when:
G proteins function as molecular switches: GTP-bound = active (ON); GDP-bound = inactive (OFF). GTPase activity slowly hydrolyzes GTP to GDP, turning the switch off. GEFs reactivate by promoting GDP→GTP exchange.
4. Kinesin motor protein moves cargo along microtubules toward the:
Most kinesins move toward the plus end of microtubules (typically toward the cell periphery/plasma membrane). Dynein moves toward the minus end (toward the centrosome/nucleus). Myosin moves along actin, not microtubules.
5. Cooperativity in hemoglobin oxygen binding means that:
Hemoglobin shows positive cooperativity: binding O₂ to one heme group induces a conformational change in the subunit that propagates to neighboring subunits, increasing their O₂ affinity. This produces the sigmoidal O₂ saturation curve, enabling efficient O₂ loading in the lungs and unloading in tissues.
6. In serine proteases, catalysis occurs via:
The catalytic triad Ser-His-Asp works as a relay: Asp positions His, which deprotonates Ser, making Ser a powerful nucleophile that attacks the peptide bond. A covalent acyl-enzyme intermediate forms, then water attacks to release the product and regenerate the enzyme.
7. A GTPase-activating protein (GAP) functions by:
GAPs (GTPase-activating proteins) provide a catalytic arginine finger that stimulates the intrinsic GTPase activity of the G protein, accelerating GTP→GDP hydrolysis and inactivating the switch. GEFs do the opposite — promote GDP→GTP exchange (activation).
8. Motor proteins such as myosin and kinesin use ATP hydrolysis to:
Motor proteins couple ATP hydrolysis to conformational changes that produce directed movement along actin (myosin) or microtubule (kinesin, dynein) tracks. This converts chemical energy into mechanical work — for muscle contraction, organelle transport, and chromosome segregation.
9. Which statement about allosteric regulation is correct?
Allosteric regulation involves a conformational change induced by binding of a regulatory molecule at a site distinct from the active site. It can activate or inhibit the enzyme and is a key mechanism in metabolic pathway control. Most allosteric enzymes are oligomeric.
10. Dynein differs from kinesin in that dynein moves cargo toward the:
Cytoplasmic dynein is a minus-end-directed motor protein that moves cargo toward the microtubule organizing center (MTOC/centrosome) — typically toward the nucleus. Kinesin is generally plus-end directed. This directionality enables cells to sort organelles and vesicles to different compartments.
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Section B · Recall Questions · Part 2
Type your answer, then click Check to reveal the sample answer.
B1
Define quaternary structure and give one biological example.
Sample answer: Quaternary structure is the arrangement of two or more polypeptide subunits into a functional complex, held together by noncovalent interactions. Example: hemoglobin (α₂β₂ tetramer), collagen (triple helix), or ATP synthase (multi-subunit rotary motor).
B2
Explain the meaning of Km and Vmax in Michaelis-Menten enzyme kinetics.
Sample answer: Vmax is the maximum reaction velocity achieved when all enzyme active sites are saturated with substrate. Km is the substrate concentration at which velocity equals Vmax/2; it is an inverse measure of enzyme-substrate affinity (low Km = high affinity).
B3
Compare the roles of GEFs and GAPs in regulating G protein activity.
Sample answer: GEFs (guanine nucleotide exchange factors) catalyze the release of GDP and binding of GTP, activating the G protein. GAPs (GTPase-activating proteins) accelerate the intrinsic GTPase activity, promoting GTP hydrolysis to GDP and inactivating the G protein. Together they control the duration of G protein signaling.
B4
Describe the mechanochemical cycle by which myosin II generates force during muscle contraction.
Sample answer: (1) Myosin head bound to actin releases actin when ATP binds. (2) ATP hydrolysis cocks the head to a high-energy state. (3) Myosin rebinds actin weakly, then strongly. (4) Power stroke: release of Pi triggers a lever-arm swing, generating ~5 nm movement. (5) ADP is released, and the cycle repeats. Net result: actin filaments slide past myosin.
B5
Explain why hemoglobin's O₂ saturation curve is sigmoidal, and what biological advantage this provides.
Sample answer: The sigmoidal curve reflects positive cooperativity: the first O₂ binds weakly, but each successive binding increases affinity of remaining sites. This enables efficient loading at the high pO₂ of the lungs and steep unloading at the low pO₂ of tissues — more O₂ delivery per cycle than a non-cooperative carrier such as myoglobin would provide.
B6
Describe an example of allosteric activation (positive allosteric modulation) in a metabolic context.
Sample answer: Phosphofructokinase-1 (PFK-1), the key regulatory enzyme of glycolysis, is allosterically activated by AMP and ADP (signals of low energy) and inhibited by ATP and citrate. When AMP binds the allosteric site, PFK-1's affinity for fructose-6-phosphate increases, speeding glycolysis to generate more ATP.
B7
Describe how kinesin transports vesicles within a cell and why directionality matters.
Sample answer: Kinesin walks hand-over-hand along microtubule tracks toward the plus end using repeated ATP hydrolysis cycles. In most cells, microtubule plus ends point outward (to the periphery). Kinesin transports secretory vesicles, mitochondria, and other cargo from the cell center to the periphery. Directionality ensures organelles and vesicles reach the correct destination.
B8
What happens to misfolded proteins that cannot be rescued by chaperones?
Sample answer: Misfolded proteins that cannot be re-folded are targeted for degradation by the ubiquitin-proteasome system. E3 ubiquitin ligases recognize misfolded proteins and attach ubiquitin chains; the 26S proteasome recognizes polyubiquitinated proteins and degrades them into short peptides that are recycled as amino acids.
B9
What determines the substrate specificity of an enzyme?
Sample answer: Substrate specificity is determined by the shape, charge distribution, and chemical properties of the active site. The active site is complementary to the transition state of the substrate. Only substrates with the correct size, shape, and functional groups can bind tightly enough for catalysis — other molecules bind too weakly to be converted efficiently.
B10
Why are mutations that impair the GTPase activity of Ras commonly found in cancer cells?
Sample answer: Ras is a small GTPase that, when active (GTP-bound), stimulates cell proliferation via the MAP kinase pathway. Mutations at Gly12 or Gln61 impair GTP hydrolysis, locking Ras in the active GTP-bound state. This constitutively activates proliferation signals, driving uncontrolled cell division — a hallmark of cancer. Mutant Ras acts as a dominant oncogene.
Section C · Critical Thinking · Part 2
Develop analytical responses, then compare with the sample.
C1
Compare the oxygen-binding curves of hemoglobin and myoglobin. Why would a myoglobin-like O₂ carrier be less efficient for O₂ transport in the circulatory system?
Sample answer: Myoglobin binds O₂ with a hyperbolic curve and very high affinity (Kd ~1 torr) — it would be nearly saturated even at tissue pO₂, releasing little O₂. Hemoglobin's sigmoidal cooperativity allows nearly full loading at lung pO₂ (~100 torr) and steep unloading at tissue pO₂ (~20–40 torr). Cooperativity effectively turns hemoglobin into a sensitive, switchable O₂ carrier rather than an O₂ storage protein like myoglobin.
C2
Imatinib (Gleevec) is a cancer drug that inhibits the BCR-ABL kinase, which is constitutively active in chronic myelogenous leukemia. Explain how a constitutively active kinase drives cancer and why imatinib is selective.
Sample answer: BCR-ABL results from a chromosomal translocation fusing the BCR gene to ABL1, producing a kinase that is always active regardless of normal signals. This continuously phosphorylates substrates that promote cell survival and proliferation, preventing apoptosis and driving leukemia. Imatinib fits tightly into the ATP-binding pocket of BCR-ABL in its inactive conformation, blocking substrate phosphorylation. Its selectivity arises from unique features of BCR-ABL's inactive state not shared by most other kinases.
C3
How does a cell ensure that kinesin transports cargo to the cell periphery while dynein brings other cargo back toward the nucleus, without the two motors simply canceling each other out on the same track?
Sample answer: Cargo sorting is achieved by differential loading of motor proteins and their regulators. Adaptor proteins on specific vesicles or organelles recruit either kinesin or dynein selectively. Tug-of-war models suggest both motors can be present simultaneously but regulatory inputs (phosphorylation, small GTPases) bias which motor "wins." Scaffold proteins like JIP1 coordinate kinesin and dynein activity on the same cargo, ensuring net transport in one direction under appropriate signals.
C4
HIV protease inhibitors are a cornerstone of antiretroviral therapy. Explain the principles that make them effective and specific.
Sample answer: HIV protease cleaves viral polyproteins into functional components needed for virion maturation. Inhibitors were designed to mimic the peptide transition state using non-hydrolyzable peptidomimetic scaffolds that fit tightly in the active site. They are specific because HIV protease's homodimeric active site has a distinct shape and substrate preference from human aspartyl proteases. Structural analysis of the enzyme guided rational drug design, producing drugs with nanomolar affinities.
C5
What advantages do large multi-protein complexes ("protein machines") offer over individual proteins acting independently?
Sample answer: Multi-protein complexes offer: (1) Substrate channeling — reaction intermediates are passed directly between active sites without diffusing away, increasing efficiency and preventing side reactions. (2) Coordinated regulation — a single regulatory signal can control multiple activities simultaneously. (3) Structural scaffolding — components are held in precise geometry for complex tasks (e.g., the ribosome positions mRNA, tRNA, and peptidyl transferase center exactly). (4) Division of labor — different subunits can specialize in different tasks, building more sophisticated molecular machines than any single protein could achieve.
Section D · Interactive Questions · Part 2
Enter your answer and click Check for instant feedback.
D1
Hemoglobin is a tetramer — how many polypeptide subunits does it contain? (number)
D2
What nucleotide does a G protein bind when it is in the active (ON) state? (abbreviation)
D3
Which motor protein walks toward the minus end of microtubules? (one word)
D4
The substrate concentration at which enzyme velocity is half of Vmax is called _______ (symbol, two letters+subscript).
D5
What motor protein is responsible for muscle contraction by moving along actin filaments? (one word)