Haem Group
Definition
The iron-porphyrin prosthetic group at the centre of haemoglobin. The iron atom cycles between oxidation states during the peroxidase-like reaction, and it is this cycling that catalyses the indicator oxidation.
- Structure
- Iron-porphyrin prosthetic group
- Location
- Centre of haemoglobin molecule
- Mechanism
- Iron cycles oxidation states in peroxidase-like reaction
- Forensic role
- Catalyses indicator oxidation in presumptive blood tests
Common questions
Why does the haem group make blood detectable by a colour-change test?+
The iron at the centre of the haem group can catalyse the breakdown of hydrogen peroxide even outside a living cell, and this peroxidase-like activity oxidises a colourless indicator dye into a coloured product, which is what a presumptive test like Kastle-Meyer actually visualises.
Does the haem group's reactivity make presumptive blood tests fully specific to human blood?+
No. The same iron-catalysed reaction can be triggered by haem in animal blood, plant peroxidases, and some strong oxidising chemicals like bleach, which is why a positive presumptive result must be followed by a confirmatory test before blood is identified as human.
Related terms
- Confirmatory Test
- A test with sufficient specificity to establish the identity of a biological fluid for reporting purposes. Examples include Takayama and Teichmann crystal...
- False Positive
- A test result that indicates the presence of a target analyte when it is absent. In forensic serology this may mean incorrectly...
- Leuco-Phenolphthalein
- The reduced, colourless form of phenolphthalein used in the Kastle-Meyer reagent. It is oxidised back to the pink quinone form in the...
- Peroxidase-Like Activity
- The catalytic behaviour of haemoglobin (and other haem-containing proteins) that mimics the enzyme peroxidase: splitting hydrogen peroxide into water and oxygen. The...
- Presumptive Test
- A fast, low-cost, class-specific screen used to triage a caseload before instrumental work begins. Examples are Marquis reagent for opioids and amphetamines,...