Adenylosuccinate synthetase mechanism

A three-substrate, three-product enzyme-catalyzed reaction scheme in which the three substrates (A, B, and C) and three products (P, Q, and R) can bind to and be released in any order. A number of enzymes have been reported to have this mechanism for example, adenylosuccinate synthetase , glutamate dehydrogenase, glutamine synthetase , formyltetrahydrofolate synthetase, and tubulin tyrosine ligase . See Multisubstrate Mechanisms... 

In the second control mechanism, exerted at a later stage, an excess of GMP in the cell inhibits formation of xanthylate from inosinate by IMP dehydrogenase, without affecting the formation of AMP (Fig. 22-35). Conversely, an accumulation of adenylate inhibits formation of adenylosuccinate by adenylosuccinate synthetase, without affecting the biosynthesis of GMP. In the third mechanism, GTP is required in the conversion of IMP to AMP (Fig. 22-34, step (T)), whereas ATP is required for conversion of IMP to GMP (step (4)), a reciprocal arrangement that tends to balance the synthesis of the two ribonucleotides. 

The purine nucleotide cycle of muscle consists of the conversion of AMP —> IMP AMP and requires AMP deaminase, adenylosuccinate synthetase, and adenylosuccinate lyase (Figure 27-24). Flux through this cycle increases during exercise. Several mechanisms have been proposed to explain how the increase in flux is responsible for the maintenance of appropriate energy levels during exercise (Chapter 21). 

The enzymes that convert IMP to XMP and adenylosuccinate are both regulated. GMP inhibits the activity of IMP dehydrogenase, and AMP inhibits adenylosuccinate synthetase. Note that the synthesis of AMP is dependent on GTP (of which GMP is a precursor), whereas the synthesis of GMP is dependent on ATP (which is made from AMP). This serves as a type of positive regulatory mechanism to balance the pools of these precursors when the levels of ATP are high, GMP will be... 

The Mechanism of Action of Hadacidin. J. Biol. Chem. 237, 1937 (1962). Shigeura, H.T. Structural Modifications of Hadacidin and Their Effects on the Activity of Adenylosuccinate Synthetase. J. Biol. Chem. 238, 3999 (1963). 

The adenylosuccinate synthetase reaction poses an interesting and general problem regarding the synthesis of carbon-nitrogen bonds coupled to nucleoside triphosphate hydrolysis. The three alternative mechanisms shown in Fig. 2 have been proposed for adenylosuccinate... 

In summary, a minimal regulatory mechanism for adenylosuccinate synthetase in E. coli contains the following elements ... 

The evidence suggested yeast adenylosuccinate synthetase catalyzed the first committed step in AMP biosynthesis and was also involved in the repression of synthesis of the early enzymes of the pathway. The mechanism by which this repression occurs remains unknown. Woods and co-workers (115,116) have isolated several prototrophic mutants of yeast defective in control of purine synthesis that are not allelic with adel2. Genetic analysis of these mutants suggested that the regulatory function of adenylosuccinate synthetase may be affected in conjunction with as many as three other gene products (116). 

Increased enzyme activity may be due to increased concentrations of nucleotide substrates, or by other mechanisms. For example, GTP is required as a substrate for adenylosuccinate synthetase, and ATP is required for guanylate synthetase in addition, ATP activates adenylate deaminase (33). 

The glutamine analogue, diazo-oxo-norleucine, and the aspartate analogue, hadacidin (iV-formyl hydroxyaminoacetic acid), inhibit guanylate synthetase and adenylosuccinate synthetase, respectively. Alanosine (2-amino-3-nitrohydroxylaminopropionic acid) is also an inhibitor of adenylate synthesis from inosinate, but its mechanism of inhibition is not yet clear (52). 

The results of this investigation are summarized in Figure 3. Rabbit liver adenine nucleotides formed as a consequence of in situ perfusion with adenine or hypoxanthine in these experiments, were converted to adenosine, probably at the cell membrane by a 5 -nucleotidase. The adenosine released from the cell was rapidly taken up by the circulating erythrocyte. Within the erythrocyte, phosphorylation to AMP occurred, thereby suggesting a possible mechanism for the maintenance and renewal of the adenine nucleotides in the absence of biosynthesis via the de novo pathway, and in the human cell, in the absence of adenylosuccinate synthetase. 

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