Structural & Motor Proteins
The three principal cytoskeletal filaments, from thinnest to thickest.
Actin filaments (~7 nm), intermediate filaments (~10 nm), microtubules (~25 nm).
Enzymes speed reactions by lowering this barrier without changing the equilibrium point.
The activation energy (ΔG‡) / free energy of the transition state.
In the Michaelis-Menten equation, V0 equals this fraction of Vmax when [S] = Km.
One-half (50%) of Vmax.
Hemoglobin's sigmoidal O₂ curve comes from this property that myoglobin lacks.
Cooperativity (O₂ binding at one site raises affinity of the others); needs the tetramer's quaternary structure.
Why is hemoglobin more suitable than myoglobin for transporting O₂ from lungs to tissues?
A) Higher O₂ affinity than myoglobin
B) Quaternary structure allows cooperative binding and release
C) Higher affinity for carbonic anhydrase
D) Only exists in the R state
B) Quaternary structure allows cooperative binding and release
Collagen's triple helix has this residue at every third position, in the repeating triplet Gly-Pro-Hyp.
Glycine — its tiny side chain is the only one that fits the crowded helix axis.
A holoenzyme equals the apoenzyme plus this.
Its required cofactor(s) — metal ion and/or coenzyme.
An inactive enzyme precursor activated by specific proteolytic cleavage is called this.
A zymogen (e.g., trypsinogen → trypsin; chymotrypsinogen).
This allosteric molecule binds and stabilizes the T state of hemoglobin, promoting O₂ release in tissues.
2,3-BPG (2,3-bisphosphoglycerate).
Which filament is built from a fibrous (elongated) protein?
A) Actin filament
B) Intermediate filament
C) Microtubule
D) All are built from fibrous protein
B) Intermediate filament
These two motor proteins: one walks actin in muscle contraction, the other walks microtubules to haul cargo.
Myosin (on actin) and kinesin (on microtubules); both use ATP hydrolysis.
The catalytic triad Ser-His-Asp in trypsin uses primarily this catalytic strategy.
Covalent catalysis (a transient covalent bond forms between the active-site serine and the substrate).
On a Lineweaver-Burk (double-reciprocal) plot, the x- and y-intercepts equal these two values.
x-intercept = –1/Km; y-intercept = 1/Vmax.
The stimulation of O₂ release by rising CO₂ and H⁺ (falling pH) in active tissues is named this.
The Bohr effect.
The catalytic triad (His, Ser, Asp) in trypsin primarily uses which strategy?
A) General acid/base
B) Metal ion
C) Catalysis by approximation
D) Covalent catalysis
D) Covalent catalysis
This vitamin converts Pro→Hyp in collagen; its deficiency causes weak fibers and this disease.
Vitamin C; deficiency causes scurvy (bleeding gums, poor wound healing).
An enzyme that joins two molecules using ATP hydrolysis belongs to this EC class.
Ligases (EC 6).
An allosteric enzyme gives this shaped kinetic curve; its inhibitor stabilizes the T state and its substrate the R state.
A sigmoidal curve (reflecting cooperativity between subunits).
Sickle-cell anemia arises from this specific amino-acid change in the β-chain.
Glu6→Val — creating a hydrophobic patch that makes deoxy-HbS polymerize.
Glyphosate binds only the EPSP synthase enzyme-substrate complex and alters both Km and Vmax. It is a:
A) Competitive inhibitor
B) Noncompetitive inhibitor
C) Uncompetitive inhibitor
D) Irreversible inhibitor
C) Uncompetitive inhibitor
In the coiled-coil heptad repeat (positions a–g), residues at these two positions are non-polar and form the hydrophobic strip.
Positions a and d (1 and 4) — non-polar residues line up on one face because an α-helix turns every 3.6 residues.
In lysozyme, Asp52 and Glu35 use these two catalytic strategies respectively.
Asp52 = covalent catalysis (nucleophile); Glu35 = general acid/base catalysis (proton donor/acceptor).
Competitive, noncompetitive, and uncompetitive inhibitors each affect Km and Vmax in this way.
Competitive: raises Km, Vmax unchanged.
Noncompetitive: Km unchanged, lowers Vmax.
Uncompetitive: lowers both Km and Vmax.
Lipinski's rule flags poor oral absorption; name any two of its four thresholds.
MW > 500; H-bond donors > 5; H-bond acceptors > 10; logP > 5 (any two).
At [S]=10 mM v=2.2×10³; at [S]=1 M and 10 M, v=4.4×10³ µmol/min. Km and Vmax are:
A) 1 mM; 6.0×10³
B) 10 mM; 4.4×10³
C) 1 mM; 4.4×10³
D) 10 mM; 6.0×10³
B) 10 mM; 4.4×10³