= Ben F. Luisi
{c}
= B. F. Luisi
{c}
{synonym}
Ben F. Luisi coauthored a 2000 <X-ray crystallography> study of <polynucleotide phosphorylase>. It revealed a <protein trimer> with a central channel, providing a structural model for how a multicomponent <enzyme> might retain and process <RNA>.<A duplicated fold is the structural basis for polynucleotide phosphorylase catalytic activity, processivity, and regulation by M. F. Symmons and others>[Abstract, Results and Conclusions]{ref} The authors' 2002 account supplies the full byline Ben F. Luisi.<Running rings around RNA: a superfamily of phosphate-dependent RNases by Martyn F. Symmons and others>[Full byline]{ref}
The <polynucleotide phosphorylase central channel> was observed without bound <RNA>; a role in <processivity> was proposed rather than directly filmed.<A duplicated fold is the structural basis for polynucleotide phosphorylase catalytic activity, processivity, and regulation by M. F. Symmons and others>[Results: no bound RNA; Conclusions: possible channel function]{ref}
An original probability model illustrates the proposed retention advantage. Suppose an <enzyme> completes one cleavage, then remains associated for another with <probability> $0\leq p<1$, independently after each cleavage. The number $N$ of completed cleavages in one encounter has
$$
\Pr(N=n)=(1-p)p^{n-1},\qquad n\geq1,
\qquad \operatorname{E}N=\frac1{1-p}.
$$
Thus changing $p$ from $0.5$ to $0.9$ changes the mean run from $2$ to $10$ cleavages without specifying how quickly an individual cleavage happens. This invented <geometric distribution> separates <processivity> from <reaction rate>; it is not a fitted model or measured result from Luisi's paper.
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