Calcium-related proteins

1. Calsequestrin

Function: Calsequestrin is a calcium-binding protein that acts as a calcium buffer inside the sarcoplasmic reticulum (SR) of muscle cells.
Mechanism: It helps store large amounts of calcium in the SR, which is essential for muscle contraction. The release of this stored calcium triggers muscle contraction.
Location: Primarily found in the sarcoplasmic reticulum of skeletal and cardiac muscle.
Clinical Note: Mutations in the gene for cardiac calsequestrin (CASQ2) are linked to a type of arrhythmia called catecholaminergic polymorphic ventricular tachycardia (CPVT), which can cause sudden death.

2. Calmodulin

Function: Calmodulin is a calcium-activated protein that acts as a messenger. When calcium levels rise in the cell, calmodulin binds to it and then activates other proteins.
Mechanism: It's a key player in many cellular processes, including inflammation, metabolism, apoptosis, and muscle contraction. In the heart, it plays a critical role in regulating the channels that control the heart's electrical stability.
Location: Found in the cytoplasm of all eukaryotic cells.
Clinical Note: The three genes that code for calmodulin are highly conserved, meaning they have changed very little over the course of evolution. This highlights its importance for cellular function.

Function: Calcitonin is a hormone that helps regulate calcium levels in the blood.
Mechanism: When blood calcium levels get too high, the thyroid gland releases calcitonin. It works by slowing down the breakdown of bone (which releases calcium into the blood) and by increasing the amount of calcium the kidneys excrete.
Location: It's produced by the C-cells of the thyroid gland.
Clinical Note: Calcitonin has an opposing function to parathyroid hormone (PTH), which raises blood calcium levels. However, PTH is considered to have a more significant role in calcium regulation than calcitonin.

4. Calbindin

Function: Calbindin is another calcium-binding protein, but it has a different role. It acts as a calcium buffer, helping to transport calcium across cells and protecting cells from the damaging effects of high calcium levels.
Mechanism: It's particularly important for calcium absorption in the intestine. In the brain, it's found in certain neurons and is thought to protect them from cell death.
Location: Found in many tissues, including the intestine, kidneys, and brain.
Clinical Note: The production of calbindin in the intestine is stimulated by the active form of vitamin D, calcitriol.

5. Calreticulin

Function: Calreticulin has a dual role. It's a "chaperone" protein that helps other proteins fold correctly, and it also helps regulate calcium levels within the endoplasmic reticulum (ER).
Mechanism: It binds to misfolded proteins, preventing them from being sent to the wrong place in the cell. It also binds to calcium in the ER, helping to store it.
Location: Located in the endoplasmic reticulum.
Clinical Note: Calreticulin is involved in the immune response and can be found on the surface of some cancer cells, where it helps the immune system recognize and destroy them.

In a nutshell:

Calsequestrin "sequesters" (stores) calcium in muscle cells.
Calmodulin is a "modulator" that senses calcium and activates other proteins.
Calcitonin "tones down" calcium levels in the blood.
Calbindin "binds" to calcium to transport it and protect cells.
Calreticulin works in the endoplasmic "reticulum" to fold proteins and store calcium.