Biological Examples

Studies with yeast, heart, and brain have shown that concentrations of intermediates within the glycolytic pathway often follow an oscillating function. Continuous spectrophotometric recording techniques for determining the NAD /NADH ratio in cell-free extracts first revealed oscillations of the NADH level in these systems. These studies then led to the discovery of glycolytic oscillations in yeast cell and cell-free extracts, beef heart extracts, rat skeletal muscle extracts, and in ascites tumor cells, with concentrations of intermediates varying in the range between 10 and 10 M (Chance et al., 1973). 

Oscillatory behavior has also been demonstrated in mitochondrial preparations and appears to be mediated by a combination of the mitochondrial membrane and ion transport processes. Such oscillations involve volume changes, ion movements, and oxidation-reduction states of the respiratory chain proteins. Experiments have demonstrated that these oscillations require the simultaneous presence of a 

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Biological examples of pericyclic reactions are relatively rare, although one much-studied example occurs during biosynthesis in bacteria of the essential amino acid phenylalanine. Phenylalanine arises from the precursor chorismate,... 

Elimination reaction, 138. 383-393 biological examples of, 393 summary of, 393-394 Embden-Meyerhof pathway, 1143-1150 see also Glycolysis... 

Reductive animation, 930-932 amino acid synthesis and, 1026 biological example of, 932 mechanism of, 931 Refining (petroleum). 99-100 Regiospecific, 191... 

Synthetic examples include the poly(meth)acrylates used as flocculating agents for water purification. Biological examples are the proteins, nucleic acids, and pectins. Chemically modified biopolymers of this class are carboxymethyl cellulose and the lignin sulfonates. Polyelectrolytes with cationic and anionic substituents in the same macromolecule are called polyampholytes. 

Covalent attachment of enzymes to surfaces is often intuitively perceived as being more reliable than direct adsorption, but multisite physical interactions can in fact yield a comparably strong and stable union, as demonstrated by several biological examples. The biotin/streptavidin interaction requires a force of about 0.3 nN to be severed [Lee et al., 2007], and protein/protein interactions typically require 0.1 nN to break, but values over 1 nN have also been reported [Weisel et al., 2003]. These forces are comparable to those required to mpture weaker chemical bonds such as the gold-thiolate bond (1 nN for an alkanethiol, and even only 0.3 nN for a 1,3-aUcanedithiol [Langry et al., 2005]) and the poly(His)-Ni(NTA) bond (0.24 nN, [Levy and Maaloum, 2005]). 

Sayano et al. [97] generated the potential of mean force for interactions between amino acids and base pairs. Okamoto and coworkers [98, 99] have also developed replica-exchange multicanonical methods for proteins. Numerous additional biological examples can be found among the references within [94]. 

Lichtenberger O, Neumann D. Analytical electron microscopy as a powerful tool in plant cell biology examples using electron energy loss spectroscopy and x-ray microanalysis. Eur J Cell Biol 1997 73 378-386. 

The Colloidal Domain Where Physics, Chemistry, Biology, and Technology Meet (second edition) by D. Fennell Evans and Hakan Wennerstrom, Wiley, New York, 1999, is a superb book which satisfactorily demonstrates the interdisciplinary nature of the topic. Its biological examples are particularly good. They also present a nice discussion on pp. 602-603 of how colloid science contributed to the growth of several, disparate strands of science. 

A biological example of this relationship is provided by cysteine and cystine ... 

Reaction-diffusion systems have been studied for about 100 years, mostly in solutions of reactants, intermediates, and products of chemical reactions [1-3]. Such systems, if initially spatially homogeneous, may develop spatial structures, called Turing structures [4-7]. Chemical waves of various types, which are traveling concentrations profiles, may also exist in such systems [2, 3, 8]. There are biological examples of chemical waves, such as in parts of glycolysis, heart... 

I. Inorganic sulfur compounds containing another (usually more electropositive) element. When the other element is an alkali or alkaline earth, the sulfide is ionic in character. Metal sulfides often have unusual stoichiometries. Examples of sulfides include H2S, Na2S, FeS, and HgS. 2. Organic sulfides are also referred to as thioethers and have the general structure R—S—R. Biochemical examples of sulfides include methionine, cystathionine, and djenkolic acid. If the two R groups are identical, the substance can be referred to as a symmetrical sulfide (biological examples of which are lanthionine and homo-lanthionine). 

FIGURE 22-19 Biosynthesis of phenylalanine and tyrosine from chorismate in bacteria and plants. Conversion of chorismate to prephenate is a rare biological example of a Claisen rearrangement. 

A biological example of E° is the reduction of Fe(III) in the protein transferrin, which was introduced in Figure 7-4. This protein has two Fe(III)-binding sites, one in each half of the molecule designated C and N for the carboxyl and amino terminals of the peptide chain. Transferrin carries Fe(III) through the blood to cells that require iron. Membranes of these cells have a receptor that binds Fe(III)-transferrin and takes it into a compartment called an endosome into which H is pumped to lower the pH to —5.8. Iron is released from transferrin in the endosome and continues into the cell as Fe(II) attached to an intracellular metal-transport protein. The entire cycle of transferrin uptake, metal removal, and transferrin release back to the bloodstream takes 1-2 min. The time required for Fe(III) to dissociate from transferrin at pH 5.8 is —6 min, which is too long to account for release in the endosome. The reduction potential of Fe(IH)-transferrin at pH 5.8 is E° = —0.52 V, which is too low for physiologic reductants to reach. 

In the description and explanation of real experimentally testable situations, the lock-and-key model claims a success story however, in many cases - and among them are the majority of the more interesting biological examples -it fails to explain host-guest binding affinity and selectivity with reasonably ambitious satisfaction. For instance, anion binding in water very frequently shows endothermic rather than exothermic enthalpies of association, an observation that is incompatible with the naive complementarity model. 

The issues relevant in the construction of anion hosts cannot be grouped in a hierarchical order as there is an intimate interplay between all of them. Successful design mandates a balanced compromise in weighting the individual influences. Advice can and should be sought from pertinent calculations (molecular modelling/dynamics), from the successes and the very few examples of failure accumulated in the literature and collected in recent reviews [21-24], from inspiration provided by abundant biological examples and crystal structures, and not least from personal experience in the handling... 

I am not altogether sure how this approach is related to my own. First of all, the terms in which the two account are cast are difficult to compare. The teleological account is illustrated nearly exclusively by biological examples having to do with rather simple-minded animals. This is quite natural, for we do not understand much of the human mind in terms of biological functions. Conversely, my account is... 

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