Peroxidase
Definition
An enzyme that catalyses the transfer of oxygen from hydrogen peroxide to an electron donor, often producing a coloured product. Haemoglobin has pseudoperoxidase activity due to its haem iron centre, and this activity is exploited in presumptive blood tests including luminol, phenolphthalein, and tetramethylbenzidine (TMB).
- Function
- Catalyses oxygen transfer from hydrogen peroxide
- Output
- Coloured reaction product
- Relevant blood protein
- Haemoglobin, via pseudoperoxidase activity
- Test applications
- Luminol, phenolphthalein, TMB presumptive blood tests
Common questions
Why is haemoglobin's activity called pseudoperoxidase rather than true peroxidase?+
Haemoglobin is not a dedicated enzyme evolved for this reaction; its haem iron centre incidentally catalyses the same type of oxidation that true peroxidase enzymes perform, which is why the activity is described as peroxidase-like.
Why can presumptive blood tests based on peroxidase activity give false positives?+
Other substances containing metal catalysts or peroxidase-like enzymes, such as certain plant materials, bleach residues, or metal ions, can trigger the same colour or luminescent reaction, so a positive result requires confirmatory testing.
Does peroxidase activity degrade as a bloodstain ages?+
It can weaken over time and with environmental exposure, which is why very old or heavily degraded stains sometimes give faint or negative presumptive results even when confirmatory DNA testing later proves blood was present.
Related terms
- Amino Acid
- The monomer unit of proteins. All 20 standard amino acids share a central carbon bonded to an amino group, a carboxyl group,...
- Haemoglobin
- The iron-containing oxygen-transport protein in red blood cells, composed of four polypeptide subunits each with a haem prosthetic group. Its pseudoperoxidase activity...
- Isoenzyme (Isozyme)
- Multiple molecular forms of the same enzyme that differ in amino acid sequence but catalyse the same reaction. In forensic serology, isoenzyme...
- Protein Denaturation
- The loss of a protein's three-dimensional structure without breaking peptide bonds, caused by heat, pH change, UV radiation, or chemical agents such...
- Proteolysis
- The hydrolytic cleavage of peptide bonds by proteolytic enzymes (proteases). In biological evidence, proteolysis occurs via endogenous cellular proteases released during autolysis...