13.6
Bombas impulsadas por ATP II: bombas tipo P
Las bombas tipo P son una gran familia de ATPasas transportadoras de membrana integral. Se dividen en cinco tipos principales según la especificidad d…
Las bombas de tipo P o ATPasas de tipo P son un tipo de transportadores de membrana impulsados por ATP.
Uno de los ejemplos más comunes de bombas de tipo P son el retículo sarco/endoplásmico Ca-ATPasa o SERCA presente en el retículo sarcoplásmico o membrana SR en los músculos esqueléticos.
La bomba tiene un dominio transmembrana y un cabezal citoplasmático que consta de tres dominios: N, unión a nucleótidos, P, fosforilación y A, actuador.
El ATP está unido al dominio N de la bomba. Luego, dos iones de calcio del lado citosólico se unen al sitio de unión al calcio presente dentro del dominio que abarca la membrana.
A continuación, el ATP se hidroliza a ADP y fosfato inorgánico. El fosfato inorgánico se une al dominio P.
El ADP se disocia, seguido de la unión a una nueva molécula de ATP, lo que desencadena un cambio conformacional en la bomba que abre el paso hacia el lumen SR y libera los iones de calcio. Dos iones de hidrógeno del lumen SR se unen a los sitios vacíos de unión al calcio, cerrando el paso hacia el lumen.
Después de esto, el fosfato inorgánico del dominio P se disocia, los iones de hidrógeno unidos temporalmente a los sitios de unión al calcio se liberan y la bomba vuelve a su conformación inicial.
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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.