21.3
細胞表面からのシグナル伝達
脂質不溶性（水溶性）のホルモンやリガンドと呼ばれる受容体に結合する分子は、細胞膜を越えて拡散することができません。細胞内に入らずに細胞に影響を与えるために、これらのホルモンは細胞膜上の受容体に結合します。ファーストメッセンジャーであるホルモンが受容体に結合すると、シグナルカスケードが起こり、セカンド…
オキシトシンのように 脂溶性ではないホルモンなど、一部のホルモンは細胞膜を通って拡散することができません。その代わりに、それらは細胞表面上の受容体に結合します。結合すると、第一メッセンジャーと見なされるホルモンは シグナル伝達カスケードを活性化します。例えば、Gタンパク質共役型受容体、すなわちGPCRが，細胞膜の外側の第一メッセンジャー によって活性化されると，内側では，ホスホリパーゼCのような酵素が リン脂質PIP2を二つの第二メッセンジャーである IP3とDAGに加水分解します。いったん形成されると、IP3は 細胞膜から小胞膜に移動し、IP3ゲートカルシウムチャンネルに結合してカルシウムイオン、3つ目の第二メッセンジャーを細胞質に放出し、筋肉収縮のような変化を誘導します。このようにして、ホルモンは、存在しているリガンドおよび受容体に応じて、活性化しない細胞に影響を及ぼし 多くの細胞プロセスを活性化することもできます。
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Q1: Why can't lipid-insoluble hormones cross the cell membrane directly?
Lipid-insoluble hormones are water-soluble molecules that cannot diffuse through the hydrophobic lipid bilayer of the cell membrane. Instead, these hormones bind to receptors on the cell surface, triggering a signaling cascade that produces cellular responses without the hormone entering the cell itself.
Q2: What happens when a first messenger binds to a G protein-coupled receptor?
When a hormone binds to a G protein-coupled receptor, the alpha subunit of the G protein dissociates from the receptor, activating an intracellular enzyme like phospholipase C. This enzyme hydrolyzes the membrane phospholipid PIP2 into IP3 and DAG, which are second messengers that trigger downstream cellular responses.
Q3: How do second messengers amplify hormonal signals inside the cell?
Second messengers like IP3 and calcium ions activate multiple proteins and pathways within the cell, amplifying the initial signal from a single hormone molecule. This multi-step process creates a cascade effect where each step depends on multiple reactions, verifying proper cellular activities occur and magnifying the response.
Q4: What are the three structural domains found in cell membrane receptors?
Cell membrane receptors contain an external ligand-binding domain that recognizes hormones, a transmembrane domain that spans the cell membrane, and an internal domain that initiates cellular responses. These three portions work together to translate extracellular signals into intracellular effects without requiring hormone entry.
Q5: How do ligand-gated ion channels differ from enzyme-linked receptors?
Ligand-gated ion channels undergo conformational changes when bound to a ligand, allowing ions to pass through a channel in the transmembrane region. Enzyme-linked receptors have an intracellular domain that is either associated with an enzyme or is an enzyme itself, directly activating other proteins when ligands bind.
Q6: What is signal transduction and why is it a multi-step process?
Signal transduction is the conversion of an extracellular stimulus into an intracellular response through multiple molecular interactions. Each step depends on other reactions, providing verification that proper cellular activities occur and allowing the response to be amplified before reaching the target outcome.
Q7: How can a hormone affect a cell without entering it?
Water-soluble hormones bind to cell surface receptors, triggering intracellular signaling cascades through second messengers like calcium ions. These messengers activate cellular processes such as muscle contraction depending on the ligands and receptors present, allowing hormones to produce effects without crossing the cell membrane. Understanding these mechanisms is central to the stress response and hypothalamic pituitary axis.