Covalent conjugate addition

Small tfbiquitin-like modifier represents a family of evolutionary conserved proteins that are distantly related in amino-acid sequence to ubiquitin, but share the same structural folding with ubiquitin proteins. SUMO proteins are covalently conjugated to protein substrates by an isopeptide bond through their carboxyl termini. SUMO addition to lysine residues of target proteins, termed SUMOylation, mediates post-transla-tional modification and requires a set of enzymes that are distinct from those that act on ubiquitin. SUMOylation regulates the activity of a variety of tar get proteins including transcription factors. 

By using recombinant DNA techniques, modifications in the protein backbone, such as additions, deletions and alterations of amino acids, are easily achieved. These modifications can contribute to improved pharmacokinetic properties of the construct. Additions may consist of the introduction of residues that allow covalent conjugation of drug molecules. Deletions of amino acids can employed to remove membrane-bound regions of a protein, thereby increasing its solubility. Single amino acid modifications can be used to minimize antibody responses and alter the binding specificity and/or the three-dimensional structure of a certain protein. 

The first successful approaches were based on the, often Cu-mediated, conjugate addition of organolithium and organomagnesium (Grignard) reagents to o ,/ -unsaturated systems covalently modified with chiral auxiliaries (Scheme 1 ). ... 

The Lewis acidic organoboranes, on the other hand, are electrophilic in their chemical behavior.22 This is attributed to the high degree of covalent character in the boron-carbon bond, which results in the low reactivity of organoboranes versus organolithiums towards electrophiles. In spite of the low reactivity of the triorganoboranes with simple aldehydes and ketones, formal conjugate addition of alkyl (or aryl)... 

Aluminum nitrides, for semiconductor growth, 12, 2-3 Aluminum(III)-nitrogen bonds covalent and non-covalent, 9, 255 mixed covalent and non-covalent systems, 9, 258 Aluminum nucleophiles, in conjugate additions, 10, 389 Aluminum(III)-oxygen bonds covalent and non-covalent, 9, 252 mixed covalent and non-covalent systems, 9, 258 Aluminum(III)-phosphine bonds, covalent and non-covalent, 9, 259... 

Scanning force microscopy imaging provided further evidence for the successful conversion of the supramolecular polymers into covalent, conjugated polymers with retention of their hierarchical structure. First of all, SFM images obtained from any of the polymerizable macromonomers A-E looked virtually identical before and after polymerization. However, while the addition of a small amount of a deaggregating cosolvent such as hexafluoroisopropanol (HFIP) to the sample... 

In addition to the reaction of mercaptopropionic acid, mixed anhydrides were also formed and identified starting from leucine and phenylalanine in the presence of Ca2+ ions, showing that RNAs can replace protein aminoa-cyl fRNA synthetase catalysts for amino acid activation. The formation of a detectable amount of aminoacyl S -phosphalc polynucleotide seems to be in contradiction with the instability predicted for aminoacyl adenylates (Table 1), however it can be explained by the low pH value increasing their stability and the fact that the selected RNA structures are likely to stabilize the mixed anhydride moiety of the covalent conjugate by favorable intramolecular interactions induced by folding. 

Standard therapy of OP poisoning consists of the administration of a combination of atropine, oxime, and diazepam with other supportive measures when necessary. However, the possibility of addition of purified enzymes such as AChE, ChE, CarbE, and A-esterases to this therapeutic scheme has been considered and preliminary experiments in animals have shown much better protective effect after addition of exogenous enzymes. In this respect, protective effects of AChE, ChE, and CarbE are based on formation of covalent conjugates or phosphory-lated enzymes in the stoichiometric ratio 1 1. Capacity for binding of these enzymes is limited by the number of active sites on the enzyme to which OP molecules can be bound. This means that more enzymes have to be administered in order to achieve better detoxification of OPs which may not always be possible due to adverse effects. This can also be infiuenced by differences in the extent of spontaneous reactivation of these enzymes inhibited by OP. 

The transduction of chemical signals from the outside of a cell, across the cell membrane, further on into the interior of the cell, and ultimately to the cell nucleus is decisively influenced by proteins carrying covalently linked additional structural units, whose presence is vital to fulfill the designated biological functions of the biomacromolecules. To these so-called protein conjugates (Fig. 1) belong... 

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