1) Which of the following statements is consistent with the structure of the Kcs
ID: 181195 • Letter: 1
Question
1) Which of the following statements is consistent with the structure of the KcsA K+ channel?
a) This protein is a homotetramer composed of hydrophobic alpha helices that form a central channel that is hydrophilic.
b) This protein is a homotetramer composed of hydrophobic alpha helices that form a central channel that is hydrophilic at the openings and hydrophobic in the middle. c) This protein is a heterotetramer composed of hydrophobic alpha helices that form a central channel that is hydrophilic.
d) This protein is a heterotetramer composed of hydrophobic alpha helices that form a central channel that is hydrophilic at the openings and hydrophobic in the middle.
2) In this structure, the K+ ions are partly stabilized by several ______ that point toward and _____ the cations.
a)backbone carbonyl oxygens; donate electron density to
b) side chains; donate electron density to
c)backbone carbonyl oxygens; accept electron density from
d) side chains; accept electron density from
3)As described in Chapter 10, the amino acid residues that help to selectively transport K+ are known as the “signature sequence” and are common to K+ channels; what are these amino acids and where are they in the primary structure of Chain A in 1BL8?
a) VGYGD; residues 76–80
b) TVGYG; residues 75–79
c) TTVG; residues 74–77
d) TATTVG; residues 72–77
4) As implied in Chapter 10, what role does Asp 80 from each subunit play in the transport of K+?
a) These residues assist in attracting positive ions.
b) These residues coordinate K+ and stabilize the protein-bound state of K+.
c) These residues assist in removing the hydration shell surrounding the K+ ion.
d) Each of these factors is involved in the transport of K+.
5) As explained in Chapter 10 and indicated by the stoichiometry shown in 1BL8, what promotes the flow of K+ into the channel during the initial stages of the transport and what prevents the K+ from getting stuck in the carbonyl-rich region?
a) K+ ions are forced into the channel due to the hydrophobic nature of the central region of the channel.
b) As indicated by the structural analysis, the protein undergoes a conformational change that promotes the flow of K+.
c) The interactions between K+ ions and the backbone carbonyls are not stabilizing.
d) K+ ions are essentially forced into the channel by electrostatic repulsions resulting from the binding of additional K+ ions.
6) During the transport of K+, what prevents the K+ from remaining bound within the protein channel?
a) K+ ions are essentially forced out due to the hydrophobic nature of the central region of the channel.
b) As indicated by the structural analysis, the protein undergoes a conformational change that ultimately releases the K+.
c) The interactions between K+ ions and the backbone carbonyls are not stabilizing.
d) K+ ions are essentially forced out due to the entry of additional ions into the channel.
7) What promotes the exit of K+ from the channel?
a) K+ ions are essentially forced out due to the hydrophobic residues located at the exit of the channel.
b) As indicated by the structural analysis, the protein undergoes a conformational change that ultimately releases the K+.
c) The interactions between K+ ions and the polar residues at the channel’s exit and the lower concentration of K+ on the exiting side of the channel assist in the exit of K+.
d) K+ ions are essentially forced out due to the entry of additional ions into the channel.
8)As indicated by the PDB pages for 4CHW and 4CHV, the K+ channel MloK1 is a _____________ and is controlled by __________.
a)ligand-gated channel; cyclic AMP
b) signal-gated channel; Ca2+
c) ligand-gated channel; cyclic ATP
d) signal-gated channel; Na+
9) As shown by the structural alignment of Chain A from 4CHW and 4CHV, the subunits possess _____________ conformation/s.
a) the exact same
b) drastically different
c) slightly different
10) This information suggests that binding of ______ induces a conformational change that results in __________ and increased transport of K+.
a) cAMP; closing of the channel
b) AMP; closing of the channel
c) cAMP; opening of the channel
d) AMP; opening of the channel
Explanation / Answer
KcsA is a prokaryotic potassium channel. This channel is considered modern channel because the KcsA structure provides a framework for understanding K+ channel conduction, which has three parts: Potassium selectivity, channel gating by pH sensitivity and voltage gated channel inactivation.
1. Option b. This protein is a homotetramer composed of hydrophobic alpha helices that form a central channel that is hydrophilic at the openings and hydrophobic in the middle.
2. Option a. The K+ are stabilized by backbone carbonyl oxygen atoms that point towards and donates electron density to the cations.
The K+ ions are stabilized by 8 carbonyl oxygen atoms for stabilization.
3. The wider end of the selectivity filter cone corresponds to the extracellular mouth of the channel made up of pore helices, that is formed by TVGYG (Threonine, Valine, Glycine, Tyrosine, Glycine). They are present in the residues 75- 79.
4. When there is low concentration of potassium, the Val-76 and Gly-77 amino acids of the selctivity filter shift due to hydrogen bonding of Glu-71 and Asp-80, causing a physical deformation of the selectivity filter and consequently closing the channel to prevent potassium from passing through. The KcsA-Fab complex transports the potassium ions by displacing water molecules and creating a antiprism of water molecules, which surrounds the potassium ions at the entrance of the channel so that it can pass through in that state.
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