BioPHYSICAL chemistry

protein as well as some bound water mole-
cules (Figure 19.4). In some of the mutants,
the amino acid residues that form the chro-
mophore have been changed and the cofactor
is still expressed. These variants give rise to
different interactions between the pigment
and the surrounding protein and hence
different colors. For example, in the mutant
with Tyr-66 substituted by a His residue, the
cofactor is present in the same binding site
and the protein is still fluorescent. When
Ser-65 is changed to Thr, the chromophore
is located in the pocket but is displaced com-
pared to the wild type. The presence of the
altered chromophore results in small dis-
placements of the surrounding protein side
chains. In addition to these structural changes,
the hydrogen-bonding network surrounding
the chromophore is significantly altered. The
sum of these effects results in a chromophore
with altered fluorescent properties. The
sensitivity of the protein to point mutations
and the desirability of engineering fluorescent
proteins with altered optical properties has
led to the availability of GFPs with many
different optical properties. Indeed, one of the

CHAPTER 19 MOLECULAR IMAGING 407

clavGFP

dstrGFP

amajGFP

cgigGFP

rfloGFP

mcavGFP

dis3GFP

rmueGFP

dendGFP

ptilGFP

soubGFP1&2

zoanGFP

zoanYFP

rhoRFP

zoan2RFP

mcavRFP

DsRedRFP

dis2RFP

asulCP(3)

B-GFP hcriCP(4)

(1) C-GFP

(1)

GFP

Y-GFP(2)

Cyan-green cluster Orange-red cluster

ogigCP(4)

glenCP(4)

asulGFP

hcriGFP

400 450 500 550 600 750

nm

Figure 19.2A wide range of GFP and GFP-related fluorescent proteins are now available.
Modified from Matz et al. (2003).

Figure 19.3The structure of GFP showing the

wrapping of the β-strands into a barrel with the

chromophore located in the center of the protein.