Calcium

Question 1

Which properties of calcium allow it to interact with a wide range of varied molecules?

Answer 1

1. Divalent - strongly attracted to negatively charged/polarised points on biological molecules.
2. ‘Polarisable’ electron clouds - can bind in variety of irregular surfaces/enclosed spaces.
3. Squishiness - wide range of bonding angles and great variability in bond length.

Question 2

How does calcium binding to a protein change the activity of that protein?

Answer 2

1 - Ca2+ binds to exposed O2 atoms of protein’s aa.
2- Causes conformational change in protein structure - physical work or further signalling.
3- ATP-driven activity to remove Ca2+ - protein back to resting state.

Question 3

What is the proportionate conc. difference between extracellular and intracellular calcium?

Answer 3

10,000 x

Question 4

Name the intracellular calcium stores.

Answer 4

• Mainly smooth ER (or sarcoplasmic reticulum in muscle): rapid release store.
• But also mitochondria (buffer excessive [Ca2+]i levels): slow release

Question 5

What are the extracellular, intracellular and intracellular store concentrations of calcium?

Answer 5

• Extracellular: 1 x 10^-3 M (1 mM)
• Intracellular: 1 x 10^-7 M (100 nM)
• sER: 2-3 x 10^-4M (300 uM - 1mM)

Question 6

Which channels enable Ca2+ entry from the extracellular space into the cytosol? What do they open in response to?

Answer 6

1. Voltage-operated calcium channel (VOCC)
   
   • membrane depolarisation
2. Ligand-gated ion channel (LGIC)
   
   • binding of excitatory NTs
3. Store-operated channel (SOC)
   
   • low [Ca2+]SER and very active PMCA - opened by Ca2+ sensor in SER membrane

Question 7

Where are SOCs important? Why?

Answer 7

• In smooth muscle where prolonged states of stable contraction are required.
• Channel has very low conductance and operates over seconds - comparatively slow.

Question 8

Which molecules allow calcium efflux from the cytosol to the extracellular space?

Answer 8

1. Na+/Ca2+ exchanger (NCX)

2. Plasma membrane Ca2+ ATPase (PMCA)

Question 9

How do the NCX and PMCA transport Ca2+ out of the cell?

Answer 9

NCX:

• Utilises electrochemical energy gradient provided by large conc. of extracellular Na+.
• 3 Na+ in for 1 Ca2+ out.

PMCA:
- Uses 1 ATP molecule.

Question 10

When and where is each Ca2+ transporter used?

Answer 10

NCX

• Primary pump when high [Ca2+]i.
• Has lower Ca2+ affinity but high capacity as extensively expressed in many cell types.
• Esp active in excitable tissue, e.g. Nerve and muscle, where continuous large movement of Ca2+ underpins activity.

PMCA

• High Ca2+ affinity (esp when binds with calmodulin) but low expression - role in fine tuning [Ca2+]i.
• Expressed in all cell types.

Question 11

Which molecule allows calcium uptake from the cytosol to the intracellular store?

Answer 11

• Smooth/sarco endoplasmic reticulum calcium ATPase (SERCA)
• Enables rapid re-establishment of basal [Ca2+]i following external stimulus of cell.
• Requires ATP as transports Ca2+ against very steep concentration gradient.

Question 12

Which molecules enable calcium movement from the intracellular store to the cytosol?

Answer 12

• IP3R

- Ryanodine receptor (RyR)

Question 13

Describe how the opening of IP3R is regulated.

Answer 13

1. Ligand binds to GPCR - activates Gq.
2. Activates PLC - conversion of PIP2 to IP3.
3. IP3 binds to IP3R (LGIC) - calcium efflux from SER/ER.

Question 14

Describe how the opening of RyR is regulated?

Answer 14

Calcium induced calcium release:

1. T-tubule depolarisation - VOCC opens (and LGIC) - calcium influx.
2. Calcium acts as ligand for RyR (LGIC) - very large synchronous efflux of SER calcium into sarcoplasm.

Question 15

In which cell type is RyR mainly expressed?

Answer 15

Muscle cells

Question 16

How does the activation of RyR channels vary in the 3 major muscle types?

Answer 16

Smooth and cardiac:
- triggering calcium signal comes through activation of VOCC in t-tubule/caveolae.

Skeletal:

• T-tubule VOCCs are physically coupled to RyR
• Enables contraction by interaction of contractile proteins

Question 17

Why is the rate of calcium diffusion across the cytosol slower than expected?

Answer 17

• Buffer proteins - act to ‘smooth out’ and dampen down the very rapid entry of calcium.
• Can bind up to 50 calcium ions each.
• As [Ca2+]i drops, buffer bound Ca2+ would be gradually released - prolongs cytosolic availability.

Question 18

Name examples of calcium buffers.

Answer 18

In the cytosol:

• parvalbumin
• calbindin

In the SR:
- calsequestrin

In the SER:
- calreticulin

Question 19

Name an example of calcium sensor. What is the function of these proteins?

Answer 19

• Calmodulin (CaM)

- Mediate the interactions between Ca2+ and effector proteins

Question 20

What is the effect of calcium binding to CaM?

Answer 20

CaM can bind up to 4 Ca2+ ions - induces a conformational change in CaM - enables interaction with wide range of proteins.

Question 21

Give an example of CaM acting as a calcium sensor.

Answer 21

• When bound with Ca2+, CaM can bind to PMCA and increase its sensitivity to [Ca2+]i by about 10 fold.