Group 12 elements: zinc and its role in biological systems

Group 12 elements: zinc and its role in biological systems

The three natural occurring elements of group 12 of the periodic table (12th vertical column) are zinc (Zn), cadmium (Cd) and mercury (Hg) (Figure 7.64).

There is no significant abundance of the metals zinc, cadmium and mercury in the earth’s crust, but they can be obtained from the respective ores. Zinc blende (ZnS) and sphalerite [(ZnFe)S] are the main sources of zinc, whereas CdS-containing ores are the only ores of importance for cadmium extraction. In order to obtain the pure metal, the relevant ores are roasted and the metal oxides are isolated. The corresponding metal is then extracted under high temperatures in the presence of carbon.

Mercury is liquid at room temperature, the only metal that shows this behaviour. Therefore, it was and still is often used in thermometers and barometers despite its toxicity. Mercury and cadmium are both highly toxic elements. Cadmium can often be found in batteries. Mercury is also well known for its formation of amalgams with other metals. Amalgams are formed by reacting mercury with other metals (except silver) and have been generally widely used as dental fillings, which mostly contain mercury and variable amounts of silver together with other metals such as tin and copper. Nowadays, there is an increased concern about the safety of those dental amalgams and alternative fillings are more and more used. Mercury(II) nitrate was used in the eighteenth and nineteenth century to cure felts for the production of hats. Hat makers who were exposed for long periods to the mercury compound showed symptoms of mercury poisoning including uncontrollable muscle tremors, confused speech patterns and, in extreme cases, hallucinations. This was the inspiration for the term mad as a hatter.

Zinc has many applications (e.g. for galvanisation or in batteries) and is often used in alloys. Zinc is also an essential element for living organisms, plays a vital role in their biochemistry and is often found as the active centre in many enzymes. Cadmium and mercury can compete with zinc at these enzyme-binding sites, which leads to their characteristic toxicity.

General chemistry

The elements of group 12 and their chemical behaviour differ from other d-block metals as they have a completely filled valence shell (d shell) and two electrons in the s shell. The latter two electrons are easily removed, leading to the divalent cation. Group 12 elements do not form ions with oxidation states higher than +2 as a result of the closed full d shell. The electronic configuration of group 12 elements is shown in Figure 7.65.

The chemistry of Zn²⁺ and Cd²⁺ is expected to be fairly similar to that of Mg²⁺ and Be²⁺. Indeed, there are some overlaps in regard to biological targets, but the chemical behaviour itself differs between members of group 2 and group 12 as a result of their different electronic configurations. It is important to note that mercury has some properties that are unique to this element and cannot be compared with the chemical behaviour of zinc or cadmium. Therefore, the chemistry of mercury will be discussed in less detail in this book.

Zinc and cadmium both dissolve in a variety of acids with the formation of hydrogen gas and the relevant metal cation M²⁺. In contrast, mercury is inert to reactions with acids. A similar trend is seen for the formation of oxides. Zinc and cadmium form the corresponding oxide when heated under oxygen. 

Mercury can also form the oxide, but the process is fairly slow. The resulting oxides, ZnO and CdO, are soluble in both acids and bases. ZnO will form salts when dissolved in acids, and the precipitate Zn(OH)₂ when dissolved in a base. Zn(OH)₂ can be dissolved in a strong base and the so-called zincates [Zn(OH)₃⁻, Zn(OH)₄²⁻] can be obtained. Cadmium oxides can also be dissolved in acids and bases, but the obtained Cd(OH)₂ is insoluble in even strong base solutions, but the hydroxide can be dissolved in ammonia. Halides of the metals (M) zinc and cadmium follow the general formula MX₂ and are either insoluble (X = F) in water or show a low aqueous solubility (Figure 7.66).

The role of zinc in biological systems

The average human body contains around 2 g of Zn²⁺. Therefore, zinc (after iron) is the second most abundant d-block metal in the human body. Zinc occurs in the human body as Zn²⁺ (closed d¹⁰ shell configura-tion), which forms diamagnetic and mainly colourless complexes. In biological systems, zinc ions are often found as the active centre of enzymes, which can catalyse metabolism or degradation processes, and are known to be essential for stabilising certain protein structures that are important for a variety of biological processes.

Already from ancient times, Zn²⁺ was known to have important biological properties. Zinc-based ointments were traditionally used for wound healing. Low Zn²⁺ concentrations can lead to a variety of health-related problems especially in connection with biological systems of high Zn²⁺ demand such as the reproductive system. The daily requirement for Zn²⁺ is between 3 and 25 mg, depending on the age and circumstances.

The enzymatic function of Zn²⁺ is based on its Lewis acid activity, which are electron-deficient species (see Chapter 4). In the following chapters, examples will be shown to further explain this. Carboanhydrase (CA), carboxypeptidase and superoxide dismutase are some examples for well-studied zinc-containing enzymes. 

The so-called zinc fingers have been discovered because of the crucial role of Zn²⁺ in the growth of organisms. Within the zinc finger, Zn²⁺ stabilises the protein structure and therefore enables its biological function.

1. Carboanhydrase (CA)

CAs are enzymes that catalyse the hydrolysis of carbon dioxide. These enzymes are involved in many bio-logical processes such as photosynthesis (CO₂ uptake), respiration (CO₂ release) and pH control.

H₂O + CO₂ ↔ HCO₃⁻ + H⁺

The human CA, form II(c), consists of 259 amino acids with a molecular weight of around 30 kDa. The catalytic site contains a Zn²⁺ ion which is coordinated by three neutral histidine (His) residues and a water molecule. The water molecule is believed to be important for structural reasons and enzymatic functionality (Figure 7.67).

A hypothetical mechanism for the mode of action for CA is shown in Figure 7.68. In a first step (i), a proton is transferred to His₆₄ from the coordinated water molecule. In a second step (ii), a buffer molecule (B) coor-dinates this proton and transports it away from the active site. The remaining hydroxide ligand reacts quickly and forms a transition state via hydrogen bonding with CO₂ (iii). Following some more transformations, HCO₃⁻ is released as it is replaced by another molecule of water (iv) and (v) [28].

2. Carboxypeptidase A (CPA)

Carboxypeptidase A (CPA) is an enzyme of the digestive system that is known to cleave amino acids favouring the C-terminal end as well as certain esters. This enzymatic activity depends on the metal at the catalytic site. Zn²⁺ and some Co²⁺-containing CPAs exhibit peptidase function, whilst esterase function has been seen by CPAs containing a variety of divalent d-block metals. CPA has a size similar to CA, consisting of about 300 amino acids and a molecular mass of 34 kDa. The metal centre is coordinated by two neutral histidine residues and one deprotonated glutamate residue as well as a water molecule.

At physiological conditions, the hydrolysis of proteins and peptides is a fairly sophisticated and slow chemical process. Within the catalysed reaction, the peptide or protein is attacked by an electrophile and a nucleophile, leading to a fast reaction. 

Several theories describing the mode of action have been published, and one of those is described below in order to give an idea about the catalytic processes. The electrophile (metal centre) is coordinated to the oxygen of the carbonyl group and therefore allows the glutamate-270 to attack the activated carbonyl group. A mixed anhydride is formed, which subsequently is hydrolysed to form the desired products [28] (Figure 7.69).

Another theory describes the CPA-catalysed hydrolysis of a peptide bond analogous to the CA mechanism. The Zn²⁺ ion is coordinated by one molecule of water, which acts as a nucleophile and can attack the carbonyl group of esters or peptide bonds. A complicated system of hydrogen bonding further facilitates the substrate binding and final steps of this hydrolysis process (Figure 7.70).

Angiotensin converting enzymes (ACEs) are zinc-containing CPAs that convert angiotensin I into angiotensin II. Angiotensin II regulates the reabsorption of water and sodium ions in the kidneys and contracts the blood vessels leading to an increase of the blood pressure. ACE inhibitors are a class of drugs that block the activity of these enzymes. These drugs are used to lower the blood pressure in patients with hypertension and there are a variety of drug examples, such as captopril and lisinopril. The mode of action of these ACE inhibitors is based on their ability to bind to the Zn²⁺ centre and the active site of these CPAs and thereby blocking their enzyme activity. Captopril contains a thiol (SH) group, which coordinates directly to Zn²⁺, whilst the carbonyl and carboxyl groups interact with the amino acid residues mainly via hydrogen bonding (Figure 7.71).

3. Zinc finger

It is well known that Zn²⁺ is essential for the growth of organisms and transcription of genetic material. It has been shown that there are special proteins that recognise certain DNA segments leading to the activa-tion or regulation of genetic transcription. These proteins contain residues that can coordinate Zn²⁺. This coordination leads to folding and a specific conformation, and they are called zinc fingers. Typically Zn²⁺ is coordinated by two neutral histidine (His) and two deprotonated cysteine (Cys) residues (Figures 7.72 and 7.73).

Zinc: clinical applications and toxicity

Clinical applications of Zn²⁺ range from its use in barrier creams and as a treatment option for Wilson disease to the use of zinc ions for the stabilisation of insulin. Long-acting human or porcine insulin is usually on the market as insulin zinc suspension. It is a sterile solution of usually human or porcine insulin, which is complexed by Zn²⁺.

Zinc sulfate in the form of either injection or tablets can be used to treat zinc deficiency and as supplemen-tation in conditions with an increased zinc loss. Zinc acetate is one treatment option for Wilson disease, as the zinc supplementation prevents the absorption of copper. It is important to note that zinc treatment has a slow onset time, which is crucial to take into account when switching from another therapy such as chela-tion therapy. Zinc acetate is usually offered to the market in an oral delivery form, mainly in capsules  (Figure 7.74).

Zinc ions are can also be found in barrier creams and lotions. Zinc oxide is present in barrier creams, for example, in creams used against nappy rash, often formulated with paraffin and cod-liver oil. Calamine lotion and creams are indicated for the treatment of pruritus, and both contain zinc oxide. It is interesting to note that the application of zinc oxide may affect the quality of X-ray images and it is therefore recommended not to apply these creams or lotions before X-ray tests .

Zinc is an essential element, but excess zinc can have a negative impact on human health. Zinc toxicity might be seen if the intake exceeds 225 mg. Typical symptoms include nausea, vomiting, diarrhoea and cramps. The so-called zinc-shakes are seen in workers, such as welders, who inhale freshly formed zinc oxide. Ingested metallic zinc dissolves in the stomach acid and zinc chloride (ZnCl₂) is formed. Zinc chloride is toxic to most organisms, depending on the concentration [29].