Allosteric enzymes, protein-based

Altering the control of metabolic pathways can also be achieved by genetic manipulation. As proteins are generated using the template stored as a fragment of DNA, the structure of the allosteric enzyme may be altered so that there is little or no regulatory control. It is therefore possible to generate mutants that over-produce metabolites, and techniques based on this principle have been most widely exploited in amino-acid and nucleotide production 2 . 

A relative of the kinases is adenylate cyclase, whose role in forming the allosteric effector 3, 5 -cyclic AMP (cAMP) was considered in Chapter 11. This enzyme catalyzes a displacement on Pa of ATP by the 3 -hydroxyl group of its ribose ring (see Eq. 11-8, step a). The structure of the active site is known.905 Studies with ATPaS suggest an in-line mechanism resembling that of ribonuclease (step a, Eq. 12-25). However, it is Mg2+ dependent, does not utilize the two-histidine mechanism of ribonuclease A, and involves an aspartate carboxylate as catalytic base.906 All isoforms of adenylate cyclase are activated by the a subunits of some G proteins (Chapter 11). The structures907 of Gsa and of its complex with adenylate kinase905 have been determined. The Gsa activator appears to serve as an allosteric effector. 

Attack points are metal ion centers and specific cysteine residues of proteins. The mechanisms by which cysteine nitrosylation regulates protein functions can be broadly described in allosteric terms similar to protein phosphorylation. Often, 02-mediated redox reactions cooperate in the allosteric control by NO of protein functions. S-nitrosylation of target proteins is a redox-based signal with exquisite specificity based on the selective modification of single cysteine residues. The selectivity of S-nitrosylation has been shown to be provided by both the subcellular distribution of NOS enzymes and the sequence context of cysteine residues in target proteins. Two nitrosylation motifs have been identified. In one motif, the target cysteine is located between an acidic and a basic amino acid, as revealed in either the primary sequence or the tertiary structure. In the other motif, the cysteine is contained in a hydrophobic region. 

The mysterious behaviour of bio-macromolecules is one of the outstanding problems of molecular biology. The folding of proteins and the replication of DNA transcend all classical mechanisms. At this stage, non-local interaction within such holistic molecules appears as the only reasonable explanation of these phenomena. It is important to note that, whereas proteins are made up of many partially holistic amino-acid units, DNA consists of essentially two complementary strands. Nonlocal interaction in DNA is therefore seen as more prominent, than for proteins. Non-local effects in proteins are sufficient to ensure concerted response to the polarity and pH of suspension media, and hence to direct tertiary folding. The induced fit of substrates to catalytic enzymes could be promoted in the same way. Future analysis of enzyme catalysis, allosteric effects and protein folding should therefore be, more ambitiously, based on an understanding of molecular shape as a quantum potential response. The function of DNA depends even more critically on non-local effects. 

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