= Polynucleotide phosphorylase central channel
The \b[polynucleotide phosphorylase central channel] is the passage through the ring-shaped <protein trimer> formed by <polynucleotide phosphorylase>. It helps organize <RNA> access to catalytic regions of the assembly. The channel is a property of the assembled enzyme, rather than a hollow tube inside one isolated polypeptide.<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, trimeric ring and channel]{ref}
Different experiments supply different levels of evidence. The 2000 structure contained a phosphate analogue but no bound RNA and proposed a channel role in <processivity>. A 2021 <cryo-electron microscopy> study captured an RNA-containing state of a mycobacterial enzyme, including a terminal oligonucleotide in an <active site> and an adenine base in the central tunnel. That directly places substrate features in the assembly without resolving every segment of a complete chain.<Structure and mechanism of Mycobacterium smegmatis polynucleotide phosphorylase by Mihaela-Carmen Unciuleac and others>[Abstract and Results, RNA-bound reconstructions; Figure4]{ref}
A conceptual geometry test illustrates the importance of routing. Imagine an RNA with a flexible single-stranded tail followed by a bulky paired region. A channel admitting the tail need not admit that paired region. Entry, retention and bond cleavage can therefore impose different constraints on the observed reaction, even when the chemical <active site> is unchanged.
The structural observation should not be interpreted as a molecular movie. Mobility, unresolved density and transitions between catalytic states require biochemical or kinetic evidence alongside the static structure. In particular, a ring-shaped assembly alone does not prove unlimited <processivity>.
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