Calcium

Question 1

What is the concentration of calcium in the cytoplasm of most cells?

Answer 1

Less than 10-7 molar

Question 2

What is the calcium concentration of extracellular space?

Answer 2

roughly 10-3 molar

Question 3

What does the gradient of calcium between the inside of the cell and the outside mean for signaling via calcium/ calcium regulation?

Answer 3

The cell must work very hard to maintain this gradient. Lots of energy required to remove calcium signal as is working against the concentration gradient.
Also means very little calcium required to shift concentration inside cell - is a rapid signal. (mostly if cellular calcium concentration is increased it means a signal is switched on)

Question 4

What are the two main types of calcium pump found in a cell?

Answer 4

1 = P-Type calcium ATPases - 1/2 ATP require to pump one calcium ion. (found in all eukaryotic cells)
2 = Na Calcium pumps - utilise the Na gradient.
1 calcium pumped out for 3 Na pumped in OR 1 calcium and one K pumped in for 4 Na out. (found in calcium intensive cells such as muscle or nerve cells)

Question 5

What are the two types of calcium channels? (just names not details)

Answer 5

1 = IP3 gated calcium channel 
2 = Cell membrane depolarization channels (eg nerve impulses and muscle cells)

Question 6

Where are IP3 gated channels found?

Answer 6

Found in ER membrane

Question 7

How are IP3 gated calcium channels stimulated?

Answer 7

IP3 is cleaved from membrane bound PIP2 via Phospholipase C (signal from diff pathways including Gq subunit of GPCR or RAS-independent RTKS)
IP3 binds IP3 gated calcium channel and opens channel so calcium can flow with concentration gradient (ie out of ER into cytosol)

Question 8

What is an example of a calcium channel stimulated by membrane depolarization?

Answer 8

Ryanodine receptors found on sarcoplasmic reticulum in muscle cells.

Question 9

How does membrane depolarization open a calcium channel?

Answer 9

Nerve impulse depolarises the T-Tubule on the muscle cell plasma membrane. This opens voltage gated channels in the membrane and calcium flows into the cytosol.
This calcium binds ryanodine receptors on the SR which causes more calcium (stored in SR) to flow into the cytosol
(positive feedback loop)

Question 10

How can calcium act as a localised signal in a cell?

Answer 10

Calcium can act as a localised signal in a cell because the cell is full of buffers.

Question 11

What is the most common basic mechanism of calcium transducing a signal?

Answer 11

Most commonly, calcium binds a protein and causes a conformational change which is then recognised by a further system.

Question 12

How does calcium bind the protein calmodulin?

Answer 12

Binds cooperatively - requires 4 calcium ions to bind to activate the protein

Question 13

what is the general structure of calmodulin?

Answer 13

a long central helix with four EF hands

Question 14

How does binding of calcium affect calmodulin structure?

Answer 14

Binding to calcium exposes the hydrophobic surfaces of calmodulin (eg methionine residues) allowing it to bind to various diff targets. (The protein folds up around the target helices - ie the EF hands)

Question 15

How would you find CAM Kinase II in its resting state?

Answer 15

In the resting state, CAM Kinase II is autoinhibited. Its 2 kinase domains stick together preventing kinase activity

Question 16

What happens to CAM Kinase II when Ca-Calmodulin binds?

Answer 16

When Ca-loaded calmodulin binds, Kinase II phosphorylates part of itself. This alters the structure to make the kinase catalytically active.

Question 17

How does CAM Kinase II have calcium independent activity?

Answer 17

Even after the Ca-Calmodulin dissociates, the kinase requires a phosphatase to be deactivated. Thought of as having a memory of the signal

Question 18

What affect does calcium have when binding to C2 domains in proteins?

Answer 18

When calcium binds C2 domains in proteins it exposes hydrophobic residues making it more ‘sticky’, allowing it to relocate to the membrane (eg PLC or bacterial toxins)