Items 83-89
The following items deal with the molecular biology of different kinds of membrane proteins. Match the membrane protein in the answers below with the MOST appropriate description of its molecular biology in the items below. Answers may be used once, more than once, or not at all.
( A ) Acetylcholine receptor
( B ) GABA A receptor
( C ) Na + -glucose symport protein
( D ) Voltage – gated Na + -K + ATPase
( E ) ? subunit of Na + - K + ATPase
( F ) ? subunit of Na + - K + ATPase
( G ) Erythrocyte glucose transporter
83. Contains on single polypeptide with 11 membrane-spanning ?-helical domains.
84. Contains 5 subunits of 4 varieties.
85. Single polypeptide rich in basic amino acids.
86. Contains an S4 polypeptide rich in basic amino acids.
87. A single polypeptide with 8 membrane-spanning ?-helical domains.
88. An ion channel that produces inhibitory hyperpolarization of membrane.
89. Function unknown.
ANSWERS AND TUTORIAL ON ITEMS 83-89
The answers are: 83-C;84-A;85-G;86-D;87-E;88-B;89-F. The acetylcholine receptor ( A ) consists of five homologous polypeptide subunits with a total MW = 300,000. The 5 subunits occur in 4 varieties, 2 ? ( with the acetycloline binding site ), 1 ?, 1 ?, and 1 ? arranged around a central aqueous pore. When acetylcholine binds to the ? subunits, a conformational change occurs which opens the channel briefly, allowing ion flux. All three types of subunits have an ?-helical hydrophobic domain which spans the membrane 4 times. Portable Field Microscopes Professional High Power Microscopes Professional Low Power Microscopes
The GABA A receptor ( B ) also consist of 5 subunits of three types, the probable stoichiometry of which is 2 ?, 2 ?, and 1 ?. Each subunit has four membrane spanning hyperpolariation of the cell membrane.
The Na + - glucose symport protein ( C ) is a single polypeptide with 11 hydrophobic membrane-spanning ?-helical domains. Its N terminus is intracellular and its C terminus. This membrane protein is responsible for unidirectional couple transport of Na + and glucose.
The voltage-gate Na + channel ( D ) is a single large polypeptide with four large membrane-spanning “subunits “, each with 6 hydrophobic ?-helical domains. Both the N and C terminals are intracellular. Voltage-gated ion channels have one membrane spanning helix, called S4, which is rich in basic amino acids. Their positive charges interact strongly with the negative charges of membrane fatty acids. In the resting state, the S4 polypeptide spans the membrane. When the membrane becomes depolorized and becomes net positive on the cytoplasmic face, this pushes the S4 helix toward the extracellular side of the membrane, opening the ion channel.
The ? subunits of Na + - K + ATPase ( E ) consist of 8 membrane-spanning hydrophobic ?-helical domains. It has an intracellular Na + binding site. And extracellular K + binding site, and an intracellular ATP to drive coupled Na + and K + transport.
The ? subunit of Na + -K + ATPase ( F ) is a small glycoprotein with a single hydrophobic ?-helical domain. Its function is unknown.
The erythrocyte glucose transporter ( G ) is a large polypeptide with 12 membrane-spanning hydrophobic ?¬-helical domains. The N and C terminals are intracellular. Between the first and second hydrophobic domains, there is an extracellular glycosylation site. Glucose enters erythrocytes by facilitated diffusion, a process that is aided by this glucose permease.
