Cleavage multiple modes

As noted in the introduction, the effects of multiple modes of catalysis are often multiplicative rather than simply additive. Consequently, it is not surprising that a number of hydrolytic metalloenzymes have evolved that utilize a constellation of three metal ions in catalysis. Perhaps not coincidentally, all well-characterized examples of this class catalyze the hydrolytic cleavage of phosphate ester or phosphoric acid anhydride bonds, which represent a difficult and long-standing chemical problem. In every case but one, the metal ions in the trimetal centers are all zinc. As we shall see, alkaline phosphatase utilizes a Zn2Mg trinuclear center. It should be pointed out that in the older literature many of the enzymes discussed in this section have been described as containing dinuclear metal centers. Only in the last few years has it become clear that three metal ions are present and participate in catalysis by these systems. 

Vmax and Km values using pga as a substrate at the optimum pH 4.1 were calculated as 500 U/mg and 0.15 mg/ml and 2000 U/mg and 0.15 mg/ml for PGI and PGII, respectively. Mode of action analysis revealed a random cleavage pattern for PGII while for PGI multiple attack on a single chain was observed. For PGII a partial subsite map was obtained. 

The multiple cleavage modes in ketones sometimes make difficult the determination of the carbon chain configuration. Reduction of the carbonyl group to a methylene group yields the corresponding hydrocarbon whose fragmentation pattern leads to the carbon skeleton. 

On nickel, and even more so on cobalt, the large excess of methane relative to the simpler cleavage modes (C5 -I- Cj, C4 -I- C2) clearly shows that several consecutive C-C bond ruptures occur before desorption. Triple or multiple attachment to the metal may therefore be possible on these metals, resulting in a complete degradation to single-carbon species (CH ), which then desorb as methane (Scheme 58). 

The possibility of multiple cleavage modes of polypeptide P has not been thoroughly investigated with either mengovirus or poliovirus. Thus, this explanation cannot be excluded with any certainty at this point. 

In conclusion, at this time there is no definite explanation available for the increased production of capsid proteins in polio-and mengovirus infected cells. Certainly, the possibility of multiple cleavage modes has not been eliminated and premature termination has not been proven. Thus, the question of whether the inequalities in capsid protein production represents some kind of translational control is rather speculative at this time. 

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