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The transition metals also have d-orbitals which are loosely bound. The first row transition metals generally form high spin complexes. The second and third row.
Table of contents
- Metalloproteins and Metalloenzymes
- Exploring the mechanism of tryptophan 2,3-dioxygenase
- Evolution of metal ions in biological systems
- introducing transition metals
In a cytochrome c, the heme iron is coordinated to the nitrogen atom of a histidine imidazole and the sulfur atom of a methionine thioether, in addition to the four nitrogen atoms provided by the porphyrin. In contrast to the blue copper proteins, two electron configurations are possible for both the oxidized and reduced forms of a cytochrome, and this has significant structural consequences.
Because cytochromes b and c are low spin in both their oxidized and reduced forms, the structures of the oxidized and reduced cytochromes are essentially identical. Hence minimal structural changes occur after oxidation or reduction, which makes electron transfer to or from the heme very rapid. Electron transfer reactions occur most rapidly when minimal structural changes occur during oxidation or reduction. Although all known bacteria, plants, and animals use iron—sulfur proteins to transfer electrons, the existence of these proteins was not recognized until the late s.
Iron—sulfur proteins transfer electrons over a wide range of reduction potentials, and their iron content can range from 1 to more than 12 Fe atoms per protein molecule. Consequently, only small structural changes occur after oxidation or reduction of the Fe—S center, which results in rapid electron transfer. Although they differ in the number of sulfur atoms provided by cysteine thiolates versus sulfide, in all cases the iron is coordinated to four sulfur ligands in a roughly tetrahedral environment.
Although small molecules, such as O 2 , N 2 , and H 2 , do not react with organic compounds under ambient conditions, they do react with many transition-metal complexes. Consequently, virtually all organisms use metalloproteins to bind, transport, and catalyze the reactions of these molecules.
Probably the best-known example is hemoglobin, which is used to transport O 2 in many multicellular organisms. Under ambient conditions, small molecules, such as O 2 , N 2 , and H 2 , react with transition-metal complexes but not with organic compounds. Many microorganisms and most animals obtain energy by respiration, the oxidation of organic or inorganic molecules by O 2.
Metalloproteins and Metalloenzymes
Because of their high surface area-to-volume ratio, aerobic microorganisms can obtain enough oxygen for respiration by passive diffusion of O 2 through the cell membrane. As the size of an organism increases, however, its volume increases much more rapidly than its surface area, and the need for oxygen depends on its volume. Consequently, as a multicellular organism grows larger, its need for O 2 rapidly outstrips the supply available through diffusion.
Unless a transport system is available to provide an adequate supply of oxygen for the interior cells, organisms that contain more than a few cells cannot exist. In addition, O 2 is such a powerful oxidant that the oxidation reactions used to obtain metabolic energy must be carefully controlled to avoid releasing so much heat that the water in the cell boils.
Consequently, in higher-level organisms, the respiratory apparatus is located in internal compartments called mitochondria, which are the power plants of a cell. Oxygen must therefore be transported not only to a cell but also to the proper compartment within a cell. Mammals, birds, reptiles, fish, and some insects use a heme protein called hemoglobin to transport oxygen from the lungs to the cells, and they use a related protein called myoglobin to temporarily store oxygen in the tissues.
Several classes of invertebrates, including marine worms, use an iron-containing protein called hemerythrin to transport oxygen, whereas other classes of invertebrates arthropods and mollusks use a copper-containing protein called hemocyanin. Despite the presence of the hem- prefix, hemerythrin and hemocyanin do not contain a metal—porphyrin complex. Myoglobin is a relatively small protein that contains amino acids.
The functional unit of myoglobin is an iron—porphyrin complex that is embedded in the protein Figure In myoglobin, the heme iron is five-coordinate, with only a single histidine imidazole ligand from the protein called the proximal histidine because it is near the iron in addition to the four nitrogen atoms of the porphyrin. A second histidine imidazole the distal histidine because it is more distant from the iron is located on the other side of the heme group, too far from the iron to be bonded to it.
Consequently, the iron atom has a vacant coordination site, which is where O 2 binds.
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The iron in deoxymyoglobin is five-coordinate, with one histidine imidazole ligand from the protein. Oxygen binds at the vacant site on iron. In the ferrous form deoxymyoglobin , the iron is five-coordinate and high spin. When O 2 binds to deoxymyoglobin to form oxymyoglobin, the iron is converted from five-coordinate high spin to six-coordinate low spin; Figure A vacant coordination site at a metal center in a protein usually indicates that a small molecule will bind to the metal ion, whereas a coordinatively saturated metal center is usually involved in electron transfer.
Hemoglobin consists of two subunits of amino acids and two subunits of amino acids, both similar to myoglobin; it is called a tetramer because of its four subunits. As shown in the curves, at low oxygen pressures, the affinity of deoxyhemoglobin for O 2 is substantially lower than that of myoglobin, whereas at high O 2 pressures the two proteins have comparable O 2 affinities. The physiological consequences of the unusual S-shaped O 2 -binding curve of hemoglobin are enormous.
In the lungs, where O 2 pressure is highest, the high oxygen affinity of deoxyhemoglobin allows it to be completely loaded with O 2 , giving four O 2 molecules per hemoglobin. In the tissues, however, where the oxygen pressure is much lower, the decreased oxygen affinity of hemoglobin allows it to release O 2 , resulting in a net transfer of oxygen to myoglobin. The curve for myoglobin can be described by a simple equilibrium between deoxy- and oxymyoglobin, but the S-shaped curve for hemoglobin can be described only in terms of a cooperative interaction between the four hemes.
The S-shaped O 2 -binding curve of hemoglobin is due to a phenomenon called cooperativity, in which the affinity of one heme for O 2 depends on whether the other hemes are already bound to O 2. Cooperativity in hemoglobin requires an interaction between the four heme groups in the hemoglobin tetramer, even though they are more than pm apart, and depends on the change in structure of the heme group that occurs with oxygen binding.
The structures of deoxyhemoglobin and oxyhemoglobin are slightly different, and as a result, deoxyhemoglobin has a much lower O 2 affinity than myoglobin, whereas the O 2 affinity of oxyhemoglobin is essentially identical to that of oxymyoglobin. Binding of the first two O 2 molecules to deoxyhemoglobin causes the overall structure of the protein to change to that of oxyhemoglobin; consequently, the last two heme groups have a much higher affinity for O 2 than the first two.
Oxygen is not unique in its ability to bind to a ferrous heme complex; small molecules such as CO and NO bind to deoxymyoglobin even more tightly than does O 2.
We can therefore represent the binding of O 2 to deoxyhemoglobin and its release as a reversible redox reaction:. Because the Fe—O—O unit is bent, while the Fe—C—O unit is linear, the imidazole group of the distal histidine in hemoglobin interferes with CO binding and decreases the affinity of hemoglobin for CO.
Exploring the mechanism of tryptophan 2,3-dioxygenase
Although CO has a much greater affinity for a ferrous heme than does O 2 by a factor of about 25, , the affinity of CO for deoxyhemoglobin is only about times greater than that of O 2 , which suggests that something in the protein is decreasing its affinity for CO by a factor of about Consequently, CO cannot bind to the heme in a linear fashion; instead, it is forced to bind in a bent mode that is similar to the preferred structure for the Fe—O 2 unit.
This results in a significant decrease in the affinity of the heme for CO, while leaving the O 2 affinity unchanged, which is important because carbon monoxide is produced continuously in the body by degradation of the porphyrin ligand even in nonsmokers. Severe carbon-monoxide poisoning, which is frequently fatal, has exactly the same effect. Thus the primary function of the distal histidine appears to be to decrease the CO affinity of hemoglobin and myoglobin to avoid self-poisoning by CO.
Hemerythrin is used to transport O 2 in a variety of marine invertebrates. It is an octamer eight subunits , with each subunit containing two iron atoms and binding one molecule of O 2. These invertebrates also contain a monomeric form of hemerythrin that is located in the tissues, analogous to myoglobin. Hemocyanin is used for oxygen transport in many arthropods spiders, crabs, lobsters, and centipedes and in mollusks shellfish, octopi, and squid ; it is responsible for the bluish-green color of their blood.
The protein is a polymer of subunits that each contain two copper atoms rather than iron , with an aggregate molecular mass of greater than 1,, amu. As with hemerythrin, the binding and release of O 2 correspond to a two-electron reaction:. Although hemocyanin and hemerythrin perform the same basic function as hemoglobin, these proteins are not interchangeable. In fact, hemocyanin is so foreign to humans that it is one of the major factors responsible for the common allergies to shellfish.
Myoglobin, hemoglobin, hemerythrin, and hemocyanin all use a transition-metal complex to transport oxygen. Many of the enzymes involved in the biological reactions of oxygen contain metal centers with structures that are similar to those used for O 2 transport. Many of these enzymes also contain metal centers that are used for electron transfer, which have structures similar to those of the electron-transfer proteins discussed previously. In this section, we briefly describe two of the most important examples: dioxygenases and methane monooxygenase.
Dioxygenases are enzymes that insert both atoms of O 2 into an organic molecule. In humans, dioxygenases are responsible for cross-linking collagen in connective tissue and for synthesizing complex organic molecules called prostaglandins, which trigger inflammation and immune reactions.
Iron is by far the most common metal in dioxygenases; and the target of the most commonly used drug in the world, aspirin, is an iron enzyme that synthesizes a specific prostaglandin. Aspirin inhibits this enzyme by binding to the iron atom at the active site, which prevents oxygen from binding. Methane monooxygenase catalyzes the conversion of methane to methanol.
The enzyme is a monooxygenase because only one atom of O 2 is inserted into an organic molecule, while the other is reduced to water:. Because methane is the major component of natural gas, there is enormous interest in using this reaction to convert methane to a liquid fuel methanol that is much more convenient to ship and store. Because the C—H bond in methane is one of the strongest C—H bonds known, however, an extraordinarily powerful oxidant is needed for this reaction.
The active site of methane monooxygenase contains two Fe atoms that bind O 2 , but the details of how the bound O 2 is converted to such a potent oxidant remain unclear. Because tight binding is usually the result of specific metal—ligand interactions, metalloenzymes tend to be rather specific for a particular metal ion. In contrast, the binding of metal ions to metal-activated enzymes is largely electrostatic in nature; consequently, several different metal ions with similar charges and sizes can often be used to give an active enzyme.
Metalloenzymes generally contain a specific metal ion, whereas metal-activated enzymes can use any of several metal ions of similar size and charge. A metal ion that acts as a Lewis acid can catalyze a group transfer reaction in many different ways, but we will focus on only one of these, using a zinc enzyme as an example. Carbonic anhydrase is found in red blood cells and catalyzes the reaction of CO 2 with water to give carbonic acid. Although this reaction occurs spontaneously in the absence of a catalyst, it is too slow to absorb all the CO 2 generated during respiration.
Without a catalyst, tissues would explode due to the buildup of excess CO 2 pressure. Thus the function of zinc in carbonic anhydrase is to generate the hydroxide ion at pH 7. An organic radical is an organic species that contains one or more unpaired electrons. Chemists often consider organic radicals to be highly reactive species that produce undesirable reactions. For example, they have been implicated in some of the irreversible chemical changes that accompany aging.
It is surprising, however, that organic radicals are also essential components of many important enzymes, almost all of which use a metal ion to generate the organic radical within the enzyme. These enzymes are involved in the synthesis of hemoglobin and DNA, among other important biological molecules, and they are the targets of pharmaceuticals for the treatment of diseases such as anemia, sickle-cell anemia, and cancer.
In this section, we discuss one class of radical enzymes that use vitamin B Vitamin B 12 was discovered in the s as the active agent in the cure of pernicious anemia, which does not respond to increased iron in the diet. Humans need only tiny amounts of vitamin B 12 , and the average blood concentration in a healthy adult is only about 3. In fact, vitamin B 12 has been called the most complex nonpolymeric biological molecule known and was the first naturally occurring organometallic compound to be isolated. Iron is a biologically important transition metal as it is also vital to life - it is one of the few trace elements needed for organisms to sustain life.
It has three main biological roles: 1. Transport oxygen from lungs to cells It is used to bind to enzymes throughout the body, such as in Hemoglobin to transport oxygen throughout the human body in blood. Energy Production Iron is used in the conversation of sugar, fats, and proteins into adenosine triphosphate, ATP. Catalase Production Iron is involved with the production of catalase and this is important because catalase protects the body from free radical damage. Rich sources of iron in food include: red meat, soybean, white flour products, seafood, and sunflower seeds Despite its uses in biological systems, an excess amount of iron can be detrimental to the human body.
First, iron can cause enzyme dysfunctions by replacing other vital minerals. All these essential minerals compete for binding sites in enzymes, and when iron replaces the competing mineral, it causes the enzyme to malfunction. Second, when iron replaces other elements in the body, it also causes inflammation. Iron attracts oxygen and when in excess, the free radical oxygen damages the surrounding body tissue.
In addition, as a carrier for oxygen, iron promotes bacterial growth by feeding it oxygen, leading to chronic infections. Iron can mostly be found in the pancreas, joints, liver, and intestines. Copper repairs the calcium in bones and connective tissue. Insufficiency or excess can lead to conditions like osteoporosis, bone spurs, and scoliosis. In the immune system, copper must be in balanced with zinc.
When these two elements are not balanced, the body is prone to infection, particularly yeast and fungal infections.
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Since copper is a critical element in aerobic metabolism, an improper level of copper allows the fungal organisms to flourish. Copper also plays a role in the reproductive system as it is required for pregnancy and fertility. An imbalance of copper can lead to premenstrual syndrome, ovarian cysts, miscarriages, and sexual dysfunctions. Studies have shown that woman with deficient estrogen and copper have a higher risk of miscarriage. Correcting the copper level by eating more meats, eggs, poultry, nuts, seeds, and grains can help with a normal pregnancy.
In the nervous system, copper plays a role in triggering the production of neurotransmitters epinephrine, norepinephrine and dopamine.
Evolution of metal ions in biological systems
As a result, copper imbalance can be associated with psychological, neurological, and emotional problems in humans. Copper is used to bind to enzymes throughout the body. It is used to defend the body against damage from free radicals. Foods that contain copper include shellfish i. Hemocyanin is an excellent example of the use in proteins. It is different from Hemoglobin in that in doesn't "tag along" with red blood cells, but is contained in hemolymph. Zinc is an inorganic compound that play an active role in biological settings. Its ability to adapt to various coordination geometries and its properties as a Lewis acid and redox inert makes it an important compound in structural and catalytic biochemistry.
Zinc undergoes rapid ligand exchange and is regulated by several proteins in cell signaling. For example, in the central nervous system, zinc is released from the synaptic vesicles at some glutamatergic nerve terminals to trigger signaling pathways which affect physiological functions such as synaptic plasticity, potentiation, and cell death. In addition, diabetes studies have shown that zinc is released along with insulin to control glucose levels. Besides from being regulated, zinc is also capable of regulating other proteins by shifting its concentration.
Zinc can influence the productivity of nitric oxide which changes the immune system. Lack of zinc in the body weakens the immune system and leaves the body prone for infections.
introducing transition metals
In the prostate glandular epithelium, a change in the normal concentration level can lead to complications in the prostate. In the nervous system, a concentration of zinc that is too high can mean mitochondrial dysfunctions. Pluth, Elisa Tomat, and Stephen J. Lippard Annual Review of Biochemistry, Vol. Cobalt is at the core of B 12 vitamins. The structure of this is based on the corrin ring.
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It is used to treat anemia because it stimulates the production of erythropoietin which makes red blood cells. Like any other element, a high concentration of cobalt is harmful to the human body. Excess intake of cobalt can result in vomiting, nausea, vision problems, heart problems, and thyroid damage. We mainly obtain it from the environment by breathing air, drinking water, and eating food that contain cobalt such as meats, dairy, and leafy green vegetables.
Radioactive cobalt can also cause health concerns. This type of radiation is sometimes used to treat cancer patients. Exposure affects include hair loss, diarrhea,and vomiting. There are several enzymes that contain cobalt and use it as a ligand to bind to methyls and adenosyl. It is thought that cobalt acts by inhibition of enzymes involved in oxidative metabolism and that the response is the result of tissue hypoxia.
Mercury was an important constituent of drugs for centuries-as an ingredient in many diuretics, antibacterials, antiseptics, skin ointments, and laxatives. The use of mercury in medicinal preparations has dramatically decreased due to the toxic effects that it has in the human body, such as nausea, vomiting, abdominal pain, bloody diarrhea, kidney damage, and death. Mercury readily forms covalent bonds with sulfur, and it is this property that accounts for most of the biological properties of the metal.