13.6
Bombas Acionadas por ATP II: Bombas tipo P
As bombas tipo P são uma grande família de ATPases transportadoras de membrana integral. Elas são divididos em cinco tipos principais com base na espe…
As bombas do tipo P ou ATPases do tipo P são um tipo de transportadores de membrana acionados por ATP.
Um dos exemplos mais comuns de bombas do tipo P são o retículo sarco/endoplasmático Ca-ATPase ou SERCA presente no retículo sarcoplasmático ou membrana SR nos músculos esqueléticos.
A bomba tem um domínio transmembrana e um capacete citoplasmático que consiste em três domínios: N, ligação de nucleotídeos, P, fosforilação e A, atuador.
O ATP está ligado ao domínio N da bomba. Em seguida, dois íons de cálcio do lado citosólico se ligam ao local de ligação ao cálcio presente no domínio que abrange a membrana.
O ATP é então hidrolisado em ADP e fosfato inorgânico. O fosfato inorgânico se liga ao domínio P.
O ADP se dissocia, seguido pela ligação de uma nova molécula de ATP, desencadeando uma mudança conformacional na bomba que abre a passagem para o lúmen SR e libera os íons de cálcio. Dois íons de hidrogênio do lúmen SR se ligam aos locais vazios de ligação ao cálcio, fechando a passagem para o lúmen.
Depois disso, o fosfato inorgânico do domínio P se dissocia. Os íons de hidrogênio temporariamente ligados aos locais de ligação ao cálcio são liberados e a bomba retorna à sua conformação 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.