Calcium Signaling

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Calcium ions are vital for cellular signalling, as once they enter the cytoplasm they use allosteric effects caused by indirect transduction pathways such as G protein-coupled receptors. Calcium signalling is cause by gradients across the plasma membrane, as the resting concentration of Ca2+ in the cytoplasm is normally kept ˷100 nM, compared to the extracellular concentration which is ˷1.5mM. To continue low concentration within the cell, Ca2+ signals can be generated either from extracellular calcium or from within the intracellular stores. Some proteins within the cell act as sensors and buffers to ensure that the concentration remains within its range and does not rise above 10x-7M that can cause cell death.
When cells are stimulated, the
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The major sensors are the EF-hand proteins troponin C and calmodulin. Calmodulin is found intracellularly and has two globular domains each containing a pair of EF-hand motifs separated by region for a total of four Ca2+ binding sites. Upon binding to calcium, EF-hand allows calmodulin to sense intracellular calcium. When calcium binds to calmodulin it can indirectly affect smooth muscle contraction e.g. contraction and digestion of arteries. Similarly, Troponin C resembles calmodulin as it has two pairs of Ca2+-binding EF-hands. The Ca2+ released from the SR acts on Troponin C and allows skeletal muscle contraction. (2) when the muscle is relaxed, the tropomyosin in muscle is displaced out of the groove preventing the myosin interacting with the actin. During contraction, Ca2+ binds to the EF-hands on Troponin to induce a conformational change in the troponin complex that is transmitted to tropomyosin, causing it to move towards the groove, this then allows the myosin to interact with actin causing muscle contraction. (2.34/ …show more content…
Phosphorylase kinase is a Ca2+ sensitive enzyme that is regulated by calmodulin. The cyclic AMP signalling pathway can phosphorylate this enzyme and this can enhance its activity by increasing its sensitivity to Ca2 +. Phosphorylase kinase is mainly important in regulating the process of glycogenolysis both in skeletal muscle and in liver cells. Calcineurin which is known as protein phosphatase. As the cytosolic Ca2+ level rises, Ca2+ binds to Calcineurin to cause a conformational change, leading to exposure of the Calmodulin binding domain site. Ca2+ then activates Calmodulin, causing a further change so that it can start to hydrolyse its phosphorylated substrates. Calcineurin is important in synaptic plasticity where it acts to remove memories. It controls Ca2+ dependent gene transcription in glucagon-secreting α-cells Calcineurin dephosphorylates the transducer of regulated cAMP response element-binding, thus enabling it to enter the nucleus to switch on transcription.

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