Metal ions

Question 1

What happens when an element is limiting?

Answer 1

Low Nitrogen availability species use nucleotides which require fewer N atoms to encode same genes as in high N availability species

Nitrogen is limiting in natural ecosystems and N-content of transcriptome (complete set of mRNA expressed in an organism) is lower in undomesticated plants

Question 2

Strategies for elemental selection?

Answer 2

Physical retention by membranes:
membranes may provide a barrier to diffusion, as some elements can’t easily pass membrane (Na+, K+, Mg2+, Ca2+)

Binding to cellular components, such as proteins and polysaccharides:
Direct interaction with the macro-molecule (Fe3+, Mn3+, Ni2+, Cu2+, Zn2+, Mo, Si, B) or chelation (bonding between molecule and metal ion) by a ligand in the macro-molecule (Mg2+, Fe2+, Co2+, Ni2+)

Incorporation into cellular components through formation of kinetically stable covalent bonds:
Generally the main method for non-metals. Hydrogen, Carbon, Nitrogen, Oxygen, etc

Removal from the aqueous phase by precipitation:
Biomineralisation is important for several elements (Ca, Sr, Fe, Si, P, C, O)

Question 3

Metal ion chemistry?

Answer 3

Provide stable carriers of positive charge:

Essential for charge balance and cell energisation
Example: role of Na+, K+-ATPase in maintaining the electric potential difference across the plasma membrane of animal cells. Negative charge in cytosol

Metal ions interact selectively with organic ligands:

Essential for discriminating between ions, and for selecting particular ions for specific purposes
Example: specificity of the bacterial K+ channel allows transport of K+, whilst rejecting the smaller Na+

Metal ions form complexes with relatively flexible stereochemistry (3D arrangement of molecules):

Shapes can often be adjusted to optimise interactions between ions and its ligand. Allows Ca2+ to bind selectively despite higher concentrations of Mg2+

Ionic bonds can easily be altered by redistributing electric charge:

This feature of metal ion chemistry can be contrasted with the kinetic stability of many covalent bonds which have defined angles

Metal ions with high election affinities have considerable polarising power:

essential feature of many enzymes – polarising power of a metal ion can increase reactivity of the substrate, e.g. Zn2+ in hydrolytic enzymes, carboxypeptidase

Some metal ions have variable oxidation states:

Facilitates oxidation and reduction – reactions involving a transfer of electrons

Question 4

Electrochemical functions of metal ions?

Answer 4

Signalling and catalytic functions?

Question 5

Signalling functions of metal ions?

Answer 5

Elevated levels of cytosolic Ca2+ cause many cellular responses:
release of neurotransmitter at synapses, muscle contraction, stimulation of glycogen breakdown

Resting levels of cytosolic Ca2+ are kept low to prevent precipitation of phosphates

Elevation occurs when Ca2+ enters from outside the cell or from intracellular stores

Increase of Ca2+ leads to conformational changes in troponin C and calmodulin

Example: calmodulin – causes a conformational change exposing hydrophobic patches (not stable in aqueous soln) on the surface, which act as docking regions for the positively charged amphipathic helices of the target proteins.

Activates kinases (an enzyme that catalyses the transfer of a phosphate group from ATP to a specified molecule.)

Question 6

Catalytic functions of metal ions?

Answer 6

Approx. 30% of enzymes require metal ions.

Many of these metalloenzymes perform acid-base catalysis or redox catalysis.

Zn2+ is use in many hydrolytic enzymes to polarise the substrate and stabilise the intermediate

Cytochrome c oxidase which catalyses the last step in mitochondrial electron transport pathway has four prosthetic groups containing metal ions

Redox: Mo required for nitrogen fixation, Mn required for oxygen production, Fe/Cu often catalyse reactions involving oxygen