Biological roles

In cooperation with a Danish colleague Thai Janzen, George de Hevesy separated hafnium from 2arconium by fractional crystallization of fluorides. In 1925 they also isolated the element itself. The modem method for separation with liquid-liquid extraction is described in Chapter 19 Zirconium. 

A very special situation arose from the discovery that it was necessary to separate zirconium and hafnium if zircaloy was to be used in uranium reactors. Hafnium became available but was there any need for this new metal. Some applications have in fact been found. 

If the turbine vanes and combustion zones of jet aircraft engines can withstand higher working temperatures the engine efficiency is improved. Such an effect is obtained if 1-2% Hf is added to the nickel-based superalloys. 

An interesting property of hafnium is that the metal (like zirconium) can dissolve much hydrogen in its lattice. It is possible to reach a content corresponding to the formula HfH,. 

Hafnium is apparently not an essential element for humans. The element shows generally low toxicity. 

Retinol (I, in Formula 6.1) is of importance in protein metabolism of cells which develop from the ectoderm (such as skin or mucouscoated linings of the respiratory or digestive systems). Lack of retinol in some way negatively affects epithelial tissue (thickening of skin, hyperkeratosis) and also causes night blindness. 

Furthermore, retinol, in the form of 11-cis-retinal (II), is the chromophore component of the visual cycle chromoproteins in three types of cone cells, blue, green and red (Amax 435, 540 and 565 nm, respectively) and of rods of the retina. 

The chromoproteins (rhodopsins) are formed in the dark from the corresponding proteins (opsins) and 11-cis-retinal, while in the light the chromoproteins dissociate into the more stable all-trans-retinal and protein. This conformational change triggers a nerve impulse in the adjacent nerve cell. The all-trans-retinal is then converted to all-trans-retinol and through an intermediary, 11-cis-retinol, is transformed back into 11-cis-retinal (see Fig. 6.1 for the visual cycle reactions). 

Besides the peptides which we have described, there are numerous others which have not yet been studied to the point where they can be the subject of a detailed discussion. However, we should like to summarize what is known about them. 

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The reactivity of epoxides toward nucleophilic ring opening is responsible for one of the biological roles they play Squalene 2 3 epoxide for example is the biological... 

Fatty acids normally occur naturally as esters fats oils phospholipids and waxes all are unique types of fatty acid esters There is however an important class of fatty acid derivatives that exists and carries out its biological role m the form of the free acid This class of fatty acid derivatives is described m the following section... 

Section 27 20 The folding of a peptide chain is its tertiary structure The tertiary struc ture has a tremendous influence on the properties of the peptide and the biological role it plays The tertiary structure is normally determined by X ray crystallography... 

Short segments of poly(dG—dC) incorporated within plasmids, or citcular DNA, adopt the Z-conformation under negative superhehcal stress. This left-handed DNA may be important in genetic control. On the other hand, the stmctural alteration of the helix requited in a B-to-Z transition within a plasmid is radical, and would involve either a multistep mechanism or the complete melting and reformation of helix. The improbability of such transitions has led to questions concerning the feasibility of a biological role for Z-DNA. 

Two groups of substituted l,4-ben2oquiaones are associated with photosynthetic and respiratory pathways the plastoquinones, eg, plastoquinone [4299-57-4] (34), and the ubiquinones, eg, ubiquinone [1339-63-5] (35), are involved in these processes. Although they are found in all living tissue and are central to life itself, a vast amount remains to be learned about their biological roles. 

Biological Roles of Copper, no. 79, Ciba Foundation Symposium, Excerpta Medica and Elsevier-North Hodand, Amsterdam, The Netherlands, 1980,... 

Lipoxygenation is the major pathway of dioxygenation of arachidonic acid in blood platelets and leads to the 12-5-hydroperoxy acid 12-HPETE and the corresponding 12-hydroxy acid 12-HETE. Several pathways for the synthesis of 12-HETE have been developed. However, despite the availability of this substance, its biological role remains undetermined. 

Proteins are a diverse and abundant class of biomolecules, constituting more than 50% of the dry weight of cells. This diversity and abundance reflect the central role of proteins in virtually all aspects of cell structure and function. An extraordinary diversity of cellular activity is possible only because of the versatility inherent in proteins, each of which is specifically tailored to its biological role. The pattern by which each is tailored resides within the genetic information of cells, encoded in a specific sequence of nucleotide bases in DNA. 

Proteins are the agents of biolo ealfunction. Virtually every cellular activity is dependent on one or more particular proteins. Thus, a convenient way to classify the enormous number of proteins is by the biological roles they fill. Table 5.3 summarizes the classification of proteins by function and gives examples of representative members of each class. 

Kinetics is the branch of science concerned with the rates of chemical reactions. The study of enzyme kinetics addresses the biological roles of enzymatic catalysts and how they accomplish their remarkable feats. In enzyme kinetics, we seek to determine the maximum reaction velocity that the enzyme can attain and its binding affinities for substrates and inhibitors. Coupled with studies on the structure and chemistry of the enzyme, analysis of the enzymatic rate under different reaction conditions yields insights regarding the enzyme s mechanism of catalytic action. Such information is essential to an overall understanding of metabolism. 

K Systems and V Systems Fill Different Biological Roles... 

Schewe, T, and Kuhn, H., 1991. Do 15-lipoxygenases have a common biological role. Trends in Biochemical Sciences 16 369—373. 

Until the discovery in 1975 of nickel in jack bean urease (which, 50 years previously, had been the first enzyme to be isolated in crystalline form and was thought to be metal-free) no biological role for nickel was known. Ureases occur in a wide variety of bacteria and plants, catalyzing the hydrolysis of urea,... 

Metallic copper and silver both have antibacterial properties and Au thiol complexes have found increasing use in the treatment of rheumatoid arthritis, but only copper of this group has a biological role in sustaining life. It is widely distributed in the plant and animal worlds, and its redox chemistry is involved in a variety of... 

It is a remarkable contrast that, whereas Zn is biologically one of the most important metals and is apparently necessary to all forms of life,( Cd and Hg have no known beneficial biological role and are amongst the most toxic of elements. 

Terpenes (and terpenoids) are further classified according to the number of 5-carbon units they contain. Thus, monoterpenes are 10-carbon substances biosynthesized from two isoprene units, sesquiterpenes are 15-carbon molecules from three isoprene units, diterpenes are 20-carbon substances from four isoprene units, and so on. Monoterpenes and sesquiterpenes are found primarily in plants, but the higher terpenoids occur in both plants and animals, and many have important biological roles. The triterpenoid lanosterol, for example, is the precursor from which all steroid hormones are made. 

The important biological role of the isobacteriochlorins has decisively influenced the development of synthetic approaches leading to the isobacteriochlorin class of compounds. All of the naturally occurring isobacteriochlorins contain geminally dialkylated structural parts in the saturated pyrrole rings, which require special approaches for their synthesis. Until the discovery of siroheme and sirohydrochlorin, this structural element could only be found in vitamin B,2. Using the synthetic potential, which was invented during numerous syntheses of... 

Niehrs C (2006) Function and biological roles of the Dickkopf family of Wnt modulators. Oncogene 25 7469-7481... 

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