DNA glycosylases recognize and remove formamidopyrimidine and 8-hydroxyguanine (both arising from purine oxidation), hypoxanthine (arising from adenine deamination), and alkylated bases such as 3-methyladenine and 7-methylguanine.

TLS Online TPP Program

#Id: 8474

#Unit 1. Molecules and their Interaction Relevant to Biology

β Sheets in proteins contain 2 to as many as 22 polypeptide strands, with an average of 6 strands. 

Each strand may contain up to 15 residues, the average being 6 residues.

TLS Online TPP Program

#Id: 8475

#Unit 1. Molecules and their Interaction Relevant to Biology

Repeat period is shorter for the parallel conformation (6.5 Å, vs. 7 Å for antiparallel)

TLS Online TPP Program

#Id: 8476

#Unit 1. Molecules and their Interaction Relevant to Biology

TLS Online TPP Program

#Id: 8477

#Unit 1. Molecules and their Interaction Relevant to Biology

Each single strand of the b-sheet structure can be pictured as a twofold helix, that is, a helix with two residues per turn.

The arrangement of successive amide planes has a pleated appearance due to the tetrahedral nature of the Ca atom.

TLS Online TPP Program

#Id: 8478

#Unit 1. Molecules and their Interaction Relevant to Biology

Parallel sheets are typically large structures; those composed of fewer than five strands are rare. 

Antiparallel sheets may consist of as few as two strands.

TLS Online TPP Program

#Id: 8479

#Unit 1. Molecules and their Interaction Relevant to Biology

Parallel sheets characteristically distribute hydrophobic side chains on both sides of the sheet