13.6
ATP Driven Pumps II: P-type Pumps
The P-type pumps are a large family of integral membrane transporter ATPases. They are divided into five major types based on substrate specificity, f…
The P-type pumps or P-type ATPases are a type of ATP-driven membrane transporters.
One of the most common examples of P-type pumps are the sarco/endoplasmic reticulum Ca-ATPase or SERCA present on the sarcoplasmic reticulum or SR membrane in the skeletal muscles.
The pump has a transmembrane domain and a cytoplasmic headpiece consisting of three domains: N, nucleotide-binding, P, phosphorylation, and A, actuator.
ATP is bound to the N-domain of the pump. Then, two calcium ions from the cytosolic side bind to the calcium-binding site present within the membrane-spanning domain.
The ATP is then hydrolyzed to ADP and inorganic phosphate. The inorganic phosphate attaches to the P-domain.
The ADP dissociates, followed by binding a new ATP molecule triggering a conformational change in the pump that opens the passageway to the SR lumen and releases the calcium ions. Two hydrogen ions from the SR lumen bind to the empty calcium-binding sites closing the passageway to the lumen.
After this, the inorganic phosphate from the P-domain dissociates. The hydrogen ions temporarily bound to the calcium-binding sites are released, and the pump returns to its initial conformation.
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Q1: What are the three main cytoplasmic domains found in P-type pumps?
P-type pumps contain three cytoplasmic domains: the nucleotide-binding (N) domain where ATP binds, the phosphorylation (P) domain where an aspartic acid residue is reversibly phosphorylated, and the actuator (A) domain that allows association of the N and P domains. These domains work together to enable proper pump functioning and solute translocation across the membrane.
Q2: How does the SERCA pump transport calcium ions across the sarcoplasmic reticulum membrane?
The SERCA pump binds two calcium ions from the cytosol at calcium-binding sites within its membrane-spanning domain. ATP hydrolysis provides energy, triggering a conformational change that opens a passageway to the sarcoplasmic reticulum lumen and releases the calcium ions. Hydrogen ions then bind to the empty sites, and the pump returns to its initial conformation, completing the cycle.
Q3: Why is maintaining low cytoplasmic calcium concentration important for muscle function?
Calcium pumps maintain cytoplasmic calcium concentration roughly 10,000 times lower than extracellular concentration, which is essential for cell signaling and muscle function. Failure to maintain this concentration gradient is one of the causes of muscle cramps. Calcium pumps account for about 80% of sarcoplasmic reticulum membrane protein in skeletal muscles, highlighting their critical role.
Q4: What is the role of phosphorylation in the P-type pump catalytic cycle?
During the catalytic cycle, inorganic phosphate attaches to the P-domain after ATP hydrolysis, forming a covalent phosphoenzyme intermediate. This phosphorylation at a highly conserved aspartic acid residue leads to conformational changes in the pump's transmembrane segments, enabling the pump to open its passageway and translocate solutes across the membrane.
Q5: Where are different types of calcium pumps located in cells?
Calcium pumps are found in multiple cellular locations. SERCA pumps are located on the sarcoplasmic and endoplasmic reticulum membranes in skeletal and heart muscles. PMCA pumps are expressed on the plasma membrane in various tissues including the brain. SPCA pumps are found on Golgi membranes. All these pumps are P-type ATPases that regulate intracellular calcium concentration.
Q6: How many ATP molecules are required for the SERCA pump to transport calcium ions?
The SERCA pump hydrolyzes one ATP molecule to transport two calcium ions into the sarcoplasmic reticulum against their concentration gradient. This stoichiometry of 2:1 (calcium to ATP) makes the pump highly efficient at maintaining the steep calcium concentration gradient necessary for primary active transport and cellular homeostasis.
Q7: What structural features connect the cytoplasmic domains to the membrane-spanning helices in P-type pumps?
The three cytoplasmic domains—nucleotide-binding, phosphorylation, and actuator domains—are connected to the membrane-spanning helices by short amino acid segments. This structural arrangement allows the domains to coordinate ATP hydrolysis and covalent phosphoenzyme intermediate formation, which are crucial parts of the catalytic cycle for solute translocation.