Advanced Molecular Genetics-Biology 566 Calcium and Signal transduction
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Calcium ion concentrations

Ca2+ and Mg2+ are both divalent metal cations essential to the cell. Specific detection of Ca2+ even in the presence of high concentrations of Mg2+ is a challenge.

Mg2+ forms anionic interactions as a very regular octomer.

Ca2+ forms anionic interactions in range of 6-12 and often are arranged irregularly.

Chelators, such as EDTA and EGTA are fairly specific for Mg2+ and Ca2+ respectively.

Calcium pumped out of cytosol to exterior or into ER.

Plasma membrane Ca2+ pumps use the Na+ chemical gradient to drive export against a gradient four orders of magnitude higher.

Other Ca2+ pumps use ATPase.

Changes in cytosolic concentrations of Ca2+ were detected by Aequorin (Jellyfish) which releases flashes of light upon Ca2+ binding. (100 mM detection range).

The advent of Quin2, an EGTA like chelator, allowed the detection of Ca2+ in the range of 1 nM -> 1 µM through fluorescence spectropscopy. Quin2 was made membrane permeable by the addition of an ester group that is cleaved once inside the cytoplasm. Quin2 is specific for Ca2+ vs Mg2+.

Draw Model for calcium flow and pools.

Calreticulin in ER (low affinity, 20/1 w/w)

Proteins in cytosol (hi affinity, serve as buffers)

Two sources of Ca2+ from ER:

Ryanodine receptors

Type 1 - skeletal muscle, ER interact with voltage changes through Ca2+ channels in plasma membrane

Type 2 - cardiac muscle and some nerve cells, do not interact with Ca2+ channels in plasma membrane as directly

Type 3 - brain, muscle, some non-excitable cells.

Phopholipase activation.