Proteins and Enzymes in Biological Evidence

Proteins are polypeptides assembled from amino acids. The familiar 20 standard amino acids have an alpha-carbon framework with an amino group, carboxyl group, hydrogen and variable side chain; proline has a cyclic secondary amino group. Side-chain chemistry affects charge, solubility, interactions and reactivity.

Peptide bonds link amino-acid residues. In living cells, ribosomes form these bonds using activated aminoacyl-transfer RNAs and the energy invested in translation, rather than simply condensing free amino acids in water. Primary sequence constrains structure, but environment, chemical modifications and interaction partners also matter.

Hydrophobic effects, hydrogen bonds, ionic interactions and sometimes disulfide bonds help stabilise structure. Changing pH alters protonation and can change conformation or binding. Denaturation disrupts higher-order structure without necessarily cleaving the chain. Some changes are reversible; some proteins or fragments retain useful assay reactivity after substantial structural change.

Enzymes lower activation barriers and accelerate reactions without changing their equilibrium. Many enzymes are proteins, although catalytic RNAs also exist. Substrate recognition and catalytic behaviour depend on molecular interactions; induced fit describes changes that can accompany binding.

Haemoglobin is an oxygen-transport protein rather than a dedicated peroxidase enzyme. Its haem chemistry can nevertheless support peroxide-dependent oxidation reactions used in blood screening. Peroxide acts as an oxidant, with the haem system participating in the redox chemistry. This is more complex than a fixed ferric iron centre simply accepting electrons in every step.

Kastle–Meyer testing uses reduced phenolphthalein that can become pink on oxidation under the test conditions. Luminol produces chemiluminescence, while TMB-based systems produce a coloured oxidation product. The reagents, application order, observation window and controls follow the validated procedure.

Plant enzymes, oxidising cleaning products and some metals can produce interfering reactions. A substrate comparison may help assess local background but cannot exclude every interferent at the stain. Sensitivity depends on formulation, dilution, substrate and observation conditions; there is no fixed universal ranking that makes one reagent always more specific or more sensitive.

Heat or denaturation does not invariably abolish haem-driven screening activity. Alteration of the globin and release or modification of haem can have complex effects. A faint reaction cannot be assigned to denaturation, age or dilution without additional evidence. Sampling and reagent choice should also consider material reserved for later testing.

Presumptive blood tests indicate reactions compatible with blood, with known alternatives. Further testing may provide more specific information. The terms presumptive and confirmatory describe validated interpretive roles; they do not make a test infallible or erase specimen and cross-reactivity limits.

Takayama and Teichmann microcrystal tests form characteristic haem derivatives under their respective chemical conditions. Takayama uses a pyridine-based reagent to form haemochromogen crystals; Teichmann forms haemin crystals using acid, halide and heat. These approaches do not independently distinguish human from non-human blood.

Human-haemoglobin immunochromatographic tests use antibody recognition. ABAcard HemaTrace is an Abacus Diagnostics product; SERATEC’s corresponding product is HemDirect. They must not be presented as one manufacturer’s product. Targets, extraction buffers, reading windows and known cross-reactions depend on the actual device.

For the illustrated sandwich-format device, a valid control line is required. A test line within the prescribed time is interpreted using the product instructions and laboratory validation; absence of the control line makes the readout invalid. Faint lines are not a validated clock for stain age or direct measures of DNA quantity.

Possible false-negative causes include low target amount, degradation, poor extraction, interference and, for some assays, excessive antigen causing a high-dose hook effect. Dilution may be investigated where the procedure supports it. A negative human-haemoglobin result after a positive chemical screen does not establish animal blood or prove that the screen was false.

There is no universal US, UK and Indian rule requiring one particular confirmatory blood test before any related evidence can be admitted. Reporting must state the findings and limits; admissibility and expert duties vary by jurisdiction. Avoid claiming that a human DNA profile identifies the fluid it came from.

Proteins can undergo unfolding, oxidation, crosslinking and peptide-bond cleavage. Endogenous and microbial proteases can contribute to proteolysis. Temperature, moisture, pH and the local matrix influence rates; a single temperature threshold cannot describe when all human proteins begin to fail.

Assays depend on different properties. Loss of an enzyme’s native activity can affect an activity test, whereas an antibody may still recognise an accessible antigenic region or fail after its alteration. Haem chemistry may remain detectable when other parts of haemoglobin have changed. Protein damage therefore does not predict every serological result in the same direction.

The DNA in the same stain experiences different chemical and biological processes. DNA targets may survive when a protein assay is uninformative, and the reverse can occur. Ultraviolet photoproducts, for example, can interfere with DNA amplification without being equivalent to uniform peptide-bond destruction.

Do not infer fragmentation from faint protein-test lines alone or start an unvalidated high-cycle DNA procedure as an automatic response. Assess specimen history, suitable quantification, controls, available quantity and the intended method. Drying and appropriate storage can limit further change, but they do not repair prior damage.

Before routine DNA profiling, laboratories used inherited protein and enzyme markers to group or exclude possible donors. PGM, esterase D and other systems could produce informative electrophoretic patterns if sufficient activity or detectable marker remained. Interpretation depended on inheritance, population frequencies and assay quality.

Isoenzymes are molecular forms catalysing the same reaction; they may arise from different genes. Allozymes are allelic variants at a locus. These terms should not be reduced to a claim that every enzyme band corresponds to a uniquely identifying person-specific sequence.

ABO typing is related historical context but detects carbohydrate antigens, not a protein genotype directly. A and B transferases modify H structures on surface molecules, including glycoproteins and glycolipids. Common O alleles lack effective transferase activity; rare variants require more specific treatment.

Historical typing can be important when reviewing old case files. It is not generally a more sensitive fallback for material too small or degraded for modern DNA methods. Many activity-based markers are themselves labile. A new examination should assess the actual material and validated options, while avoiding reinterpretation of old group agreement as individualisation.

Blood contains haemoglobin in red cells and proteins such as albumin and immunoglobulins in plasma. Seminal material can contain PSA, also called p30, and semenogelins. PSA is associated with prostatic secretions but is not absolutely exclusive to semen; small quantities or cross-reactions in other contexts can matter for a sensitive assay.

Absence of sperm can occur after vasectomy, in azoospermia, with degradation or through sampling. A validated seminal-marker result may remain useful, but it does not establish ejaculation at a particular time, penetration, lack of consent or donor identity. Negative results also require qualified interpretation.

Amylase activity is useful in saliva screening but is not specific to human saliva. Phadebas uses dye-linked starch: amylase digestion releases blue dye. That mechanism is different from an iodine–starch test in which loss of starch changes an iodine reaction. A named commercial test must be described by its actual chemistry.

Hair is rich in keratin and associated proteins; bone and dentine contain substantial type I collagen and other matrix proteins. Protein mass spectrometry can support suitable species or sequence-variation studies. Genetically variant peptides in hair are an active forensic research and validation area, not permission to identify a person from any protein spectrum.

A useful workflow links the marker to the question: screening an area, assessing possible fluid, selecting material for genetic testing or performing a specialist protein comparison. It records the analytical result separately from the proposed donor or activity interpretation.