[cmquer]

I am a new learner of Biochemistry, as I know, pyranose consist the lowest no. of carbon atoms (six). However, tetroses with only four carbon atoms can also form a ring readily. I don't under this phenomenon, could someone explain to me?

[Dan]

as I know, pyranose consist the lowest no. of carbon atoms (six).

This is not true. Pyranose form is a six-membered ring, but contains an endocyclic oxygen atom (ie. oxygen in the ring). Therefore, the minimum carbon chain length for a pyranose is 5 - the ring is composed of 5 carbon atoms and 1 oxygen to give a six membered ring. In other words: Ribose, arabinose, lyxose and xylose can all exist in pyranose forms.

Similarly, a tetrose, which has a 4 carbon chain, can form a five-membered ring- this is known as a furanose form. Erythrose and threose exist overwhelmingly (compared to open chain forms) as furanose forms in solution - but pyranose forms are not possible for these sugars.

Sugars of carbon chain length greater than 4 can exist in both pyranose and furanose forms.