Metals in Biology

Question 1

What are the most common metals in most biological organisms and what are their functions?

Answer 1

Sodium (Na+) and Potassium (K+)

Function: Set up potentials across membranes

Animals: Sodium is higher concentration on the outside, potassium is higher concentration on the iniside

Question 2

How are magnesium ions and calcium ions able to interact?

What are their functions?

Answer 2

Formation of ions carrying two charges (Mg2+ and Ca2+) in aqueous solution= Able to form strong charge-charge interactions with proteins by interacting with negatively charged carboxylic acid groups in side chains

Functions: Important in stabilising membranes and cell walls by interacting with negative charges

Question 3

What are some specific functions of magnesium?

Answer 3

Magnesium: Associated with the negative charges carried on phosphate groups

• All enzymes that use or synthesise ATP or ADP and all enzymes that use other nucleotides need Mg2+ for activity
• Calcium cannot be used for this as calcium phosphates are insoluble
• Also interact with phosphate groups in nucleic acids and can be involved in maintaining tertiary structure in catalytically active RNAs
• Often play role in catalytic activity of enzymes
• Needed for photosynthesis in all green plants- found in chlorophyll= NOT held by charge-charge interactions but by interaction with nitrogen atoms (coordination compound) + Needed for loss of electrons to occur from chlorophyll when it is stimulated by light absorption

Question 4

What are some specific functions of calcium?

Answer 4

Calcium: Cell signalling processes as a range of proteins associated with cellular signalling can bind to calcium

Binding to calcium often causes conformational change in protein= changing functional activity

• Also stabilise 3D structures in proteins through ion-pair interactions with charged amino acid side chains= increases stability to heat and denaturants
• Can also play role in ligand binding= bound calcium ion is at the binding site for sugar residues and is needed for sugar to be bound

Question 5

What is calmodulin responsible for?

Answer 5

Many signalling processes caused by changes in intracellular [Ca2+] are mediated by calcium-binding protein calmodulin

Responds to a change in [Ca2+] and interacts with target proteins= transmitting a signal

Question 6

What are some of the d-bloc metals? What types of bonds do they form?

Answer 6

Mn, Fe, Co, Ni, Cu and Zn

Form coordination compounds which range of ligands= Metal atom and the ligand are essentially covalent

Metal atom in the coordination complex have different oxidation states

Question 7

What are some different geometries around the metal atom/ion?

Answer 7

Octahedral= 6 ligands (most common for Mn, Fe, Co, Ni)

Tetrahedral= 4 ligands (most common for Zn and Cu) = looks like +

Square planar= 4 ligands, look like X

Linear= 2 ligands

Question 8

What is the Irving-Williams Series?

Answer 8

Relative stabilities of complexes formed by transition metals:
Mg2+

Question 9

What are protoporphyrins?

Answer 9

Cofactors for many enzymes and other proteins + widely used for coordinating metal ions in biological systems

Structure contains ‘hole’ surrounded by 4 nitrogen ions, and the ‘hole’ is where a metal ion can be inserted

Found in haem groups such as haemoglobin and in chlorophyll

Octahedral coordination geometry

Question 10

Why does the formation of a coordination complex with a metal ion significantly alter its redox potential?

Answer 10

The coordinated ligands can potentially form BETTER interactions with one oxidation state than the other= Stabilises it= Changes its E° value