FRET Peptides

Definition
Fluorescence resonance energy transfer (FRET) is a distance-dependent interaction between the electronic excited states of two dye molecules in which excitation is transferred from a donor molecule to an acceptor molecule without emission of a photon1. Following are some of the standard dye combinations used: a)Fluorescein and Dabcyl or Dabsyl b) Fluorescein and Tamra c) Methoxy-coumarin-acetic-acid(MCA) and 2,4-Dinitrophenyl(DNP).

Discovery
The first internally quenched fluorescent peptide reported was a substrate for angiotensin I-converting enzyme (ACE), namely Abz-Gly-Phe(NO2)-Pro where the fluorescence of the N-terminal ortho-amino benzoic acid (Abz) was quenched by the p-nitro-phenylalanine [Phe(NO2)] group (Carmel and Yaron 1978) 2. A new generation of fluorescence-quenched substrates was developed by Chagas et al., (1991), using the FRET peptide concept described substrates for tissue and plasma kallikrein containing Abz as the fluorescent group and EDDnp (2,4-dinitrophenyl ethylenediamine) as the quencher group. The FRET peptides introduced by Chagas et al., (1991) was a breakthrough in the study of proteases’ specificity, and the synthesis of different Abzpeptidyl-EDDnp sequences provided the opportunity for us to study the activity of various endopeptidases such as human rennin3.

Structural Characteristics
FRET peptides are labelled with two dye molecules. These can be identical, but in most applications, different dyes are commonly used. FRET describes the transfer of energy from an initially excited donor (dye 1) to an acceptor (dye 2). Typically, this donor emits light at a wavelength λd that overlaps with the absorption wavelength λa of the acceptor. If donor and acceptor dye are in close proximity (10 – 100 Å),

this energy transfer happens in one of two ways, depending on the chemical structure of the acceptor:

a) the transferred energy is converted to molecular vibrations (acceptor is dark quencher)

b) the transferred energy is emitted as light with a longer wavelength (acceptor is fluorescent).

Mode of Action
In a FRET peptide fluorescent donor group attached to one of the amino acid residues of the peptide transfers energy to a quenching acceptor attached to another residue in the sequence following the resonance mechanism. This process occurs whenever the emission spectrum of the fluorophore overlaps with the absorption spectrum of the acceptor (Sapsford et al., 2006)4. The FRET peptides exhibit internal fluorescence quenching when intact, but cleavage of any peptide bond between the donor/acceptor pair liberates fluorescence that can be detected continuously, allowing a quantitative measurement of the enzyme activity.

Functions
Fluorescence Energy Resonance Transfer (FRET) peptides are an excellent alternative for enzyme kinetic studies and for analysis of the enzymatic activity in biological fluids, crude tissue extracts or on the surface of cells in culture.

FRET peptides are used as suitable substrates in enzyme studies, such as:
1. Functional characterization of peptidases / proteases / kinases / phosphatases.
2. Kinetic characterization of peptidases / proteases / kinases / phosphatases.
3. Screening and detection of new proteolytic enzymes.
4. They can be used for conformational investigation of peptide folding.

References
1.Hossain MA, Mihara H, Ueno A (2003). Fluorescence resonance energy transfer in a novel cyclodextrin-peptide conjugate for detecting steroid molecules. Bioorg Med Chem Lett., 13(24):4305-4308.
2.Carmel A, Yaron A (1978). An intramolecularly quenched fluorescent tripeptide as a fluorogenic substrate of angiotensin-I-converting enzyme and of bacterial dipeptidyl carboxypeptidase. Eur J Biochem., 87(2):265-273.
3.Chagas JR, Juliano L, Prado ES (1991). Intramolecularly quenched fluorogenic tetrapeptide substrates for tissue and plasma kallikreins. Anal Biochem., 192(2):419-425.
4.Sapsford KE, Berti L, Medintz IL (2006). Materials for fluorescence resonance energy transfer analysis: beyond traditional donor-acceptor combinations. Angew Chem Int Ed Engl., 45(28):4562-4589.

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