What is the primary function of the peptide loops in the selectivity filter of K+ channels?

The primary function of the peptide loops in the selectivity filter of K+ channels is to narrow the channel and selectively bind K+ ions. The design of the selectivity filter is crucial for the channel's ability to differentiate between ions. The specific arrangement of amino acids in the peptide loops creates a unique environment that is ideal for the proper coordination of K+ ions.

This coordination occurs because the size and charge of the K+ ion allow it to interact with the carbonyl oxygen atoms in the selectivity filter, while the spatial arrangement of these peptide loops ensures that Na+ ions, which are smaller, do not fit as well into this selective binding site. This selective binding mechanism is essential for maintaining the ionic gradients across the membrane and ensuring that K+ can pass through the channel efficiently while excluding other ions. Thus, the peptide loops play a critical role in the selectivity and functionality of K+ channels.