How do the 2 forms of the phytochrome pigment differ in the

How do the 2 forms of the phytochrome pigment differ in the type of light absorbed?

How is each form converted into the other?

When the ratio (red : far red) decreases, which type of light is becoming less abundant?

Solution

Answer

Phytochrome, a blue protein pigment with a molecular mass of about 125 kD exists in two photoreversible forms phytochrome red (Pr) and phytochrome far red (Pfr)

The red light-absorbing form of the pigment is designated Pr and the far-red-absorbing form Pfr.

Phytochrome is synthesized as the Pr, which accumulates in dark-grown tissue and is physiologically inactive.

When Pr absorbs red light, it is converted to the Pfr form, which is the physiologically active for most known responses.

When Pfr is exposed to far-red light it returns the pigment to the Pr form.

Phytochrome occurs as a dimer made up of two subunits.

Each subunit consists of two components: a light-absorbing pigment molecule called the chromophore, and a polypeptide chain called the apoprotein.which together make up the holoprotein.

The chromophore of phytochrome is a linear tetrapyrrole, termed as phytochromobilin.which is attached to the protein through a thioether linkage to a cysteine residue

Both chromophore and protein undergo uonformational changes upon absorption of light

Upon absorption of light, the Pr chromophore undergoes a cis–trans isomerization of the double bond between carbons 15 and 16 and rotation of the C14–C15 single bond.
The protein component of the phytochrome also undergoes a conformationalchange.

Pr is in Cis-configuration and Pfr is in Trans-configuration

The two forms are converted to each other just by absorption of light energy, and this energy is utilized in transformation of bond between C14-C15 into Cis-Trans and vice versa

If the red:far red ratio of phytochrome (Pr:Pfr) decreases, means Pfr is more that Pr, then red part of light is becoming more abundant and far-red part is becoming less abundant