The protein most famously known for being yellow is photoactive yellow protein (PYP).
Photoactive Yellow Protein (PYP)
PYP is a small, soluble protein found in certain bacteria. Its defining characteristic is its ability to absorb blue light and initiate a signaling pathway. This light absorption is due to a chromophore, p-coumaric acid, which is bound within the protein structure. It is the presence of this chromophore which gives the protein its yellow color.
Why is PYP yellow?
The color of a protein is determined by the wavelengths of light it absorbs. PYP absorbs strongly in the blue region of the visible spectrum. Because it absorbs blue light, it reflects or transmits the remaining wavelengths, which combine to produce a yellow color.
Function of PYP
PYP functions as a blue-light photoreceptor in bacteria. It is involved in a process called negative phototaxis, where the bacteria move away from blue light. This is thought to be a protective mechanism against potentially harmful radiation.
Other Yellow Proteins
While PYP is the protein most readily associated with the color yellow, it's important to note that other proteins can also exhibit a yellow hue, though perhaps less intensely. This coloration typically arises from:
- The presence of specific cofactors: Some proteins bind molecules (cofactors) that are colored, imparting their color to the protein complex. For example, flavoproteins contain flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN), which can contribute to a yellow appearance.
- Post-translational modifications: Certain chemical modifications to amino acids within the protein can alter its light absorption properties and result in a yellow tint.
- Metal ions: Proteins that bind certain metal ions, such as iron, can sometimes appear yellow or brownish-yellow.
Therefore, while PYP is the quintessential "yellow protein", other proteins can also display yellow coloration depending on their composition and interactions with other molecules.
