10th November 2016 Endocrinology Calcium Signalling Recap Agonist GPCR By PIP2 DAG CLEAVAGE PLC3 + GTP IP 1) Understand how Ca2+ is able to be removed from the cytosol 2) Understand how Ca2+ is able to enter the cytosol 3) How Ca2+ is able to be detected and cause signal transduction within the cell Calcium ions One ion with two positive charges - intense electrostatic field as there is lots of positive charge in a small space - able to distort the protein by electrostatics - packs a lot of punch with such a small sized ion. Ca2+ binding can alter the local electrostatics within a protein because of its shape and charge. This can cause a conformational change in the protein, which can alter the structure/function of the protein. Therefore, Ca2+ can mediate signal transduction within a cell. Calcium homeostasis within the cell - a generic model Is calcium homeostasis important? The concentration of free Ca2+ within the cytosol of the cell is tightly regulated. This is because; extremely high levels are toxic and lead to cell death and to reduce the background noise, in order for Ca2+ to act as a second messenger. Between 10 and 100 nm concentration of calcium in a resting cell. Above 100 nm is when cell is activated. Above micromolar concentration is when the cell is in trouble: It triggers proteases, lipase, etc ... - toxic levels because it switches cell processes on that lead to apoptosis! Need calcium homeostasis.
10th November 2016 Endocrinology A resting cell The Ca2+ concentration is low in the cytosol. K EXTRACELLULAR Na NA K KT K Cal No Cal A B [Ca2+] 2 mM Ca2+ pumps and Ca2+ exchangers NCKX Ca2+-activated K and Cl channels PMCA ATPase NCX Ca2+ pumps include: - Plasma membrane Ca2+ATPase (PMCA) - Smooth endoplasmic Ca2+ATPase (SERCA) Ca2+ exchangers include: - Na+/Ca2+ exchanger (NCX) - 2) Na+/Ca2+/K+ exchanger (NCKX) (also sarcoplasmic reticulum) Massive concentration gradient in the extracellular matrix; is a problem getting calcium out against the concentration gradient. Na Na+ Na CYTOPLASM K+ PLC Ca2+ Ge Ca2+ release (IP3R or RyR) ENDOPLASMIC RETICULUM Ca Na 100 nM SERCA ATPase 100000 ? UM Move outside cell through plasma membrane or store it in the mitochondria - need to remove from the cytosol. Two machinery used to remove calcium - sodium calcium exchanger or potassium calcium exchanger - need to couple it with a molecule going down it concentration gradient; gives the energy to move the calcium outside the cell or into the mitochondria. Exchangers are good at moving calcium of low quantities - they don't have the affinity if there is too much calcium as they bind it at low affinity. Calcium pumps - used for when the calcium is high - pump the calcium out against its concentration gradient, removes the excess calcium. Most removed by exchangers and the rest by pumps. What changes the state of the system? How does calcium enter the cell? Open the flood gates ... Voltage-gated Ca2+-selective