Figure 4

The serine, glutamine, and tyrosine ladders of Pb INP. A) Stereo view of the internal serine ladder. The colouring scheme is the same as in Figure 3. Hydrogen bonds are represented by black dashed lines. Hydrogen atoms are coloured white. Each serine -OH group forms two hydrogen bonds; one to the main-chain carbonyl oxygen from the first x residue of the x xxS motif of the same loop, and one to the main-chain amide nitrogen of the first x residue of the x xxS motif of the ensuing loop. The main-chain amide of each serine residue also hydrogen bonds to the main-chain carbonyl oxygen of the second x residue of the xx xS motif of the ensuing loop, further stabilizing the turn. B) Stereo view of the internal glutamine ladder. The side-chain amide of the glutamine forms hydrogen bonds with a side-chain carbonyl oxygen on the lower part of the loop, as well as with the main-chain carbonyl oxygen of the second glycine of the GYGS motif of the same loop and the serine of the GYGS motif of the ensuing loop. C) Stereo view of the external tyrosine ladder. The main chain of the protein is shown in cartoon format.