SN7 SHORT NOTE 5M

Classify proteins on the basis of composition, shape and function. Add a note on precipitation reactions.

Biochemistry · 1st Professional — Pre-Clinical

Classification BY Composition

Class	Definition	Examples
Simple proteins	Yield only amino acids on hydrolysis	Albumins (water-soluble), globulins (salt-soluble), glutelins and prolamines (plant), histones and protamines (basic, DNA-binding), scleroproteins (keratin, collagen, elastin)
Conjugated proteins	Yield amino acids plus a non-protein prosthetic group	Glycoproteins (carbohydrate — immunoglobulins, mucins); lipoproteins (lipid); phosphoproteins (casein); chromoproteins (haem — haemoglobin, cytochromes); metalloproteins (ceruloplasmin, ferritin); nucleoproteins
Derived proteins	Products of partial denaturation or hydrolysis	Proteans, metaproteins, proteoses, peptones, peptides

Classification BY Shape and BY Function

Basis	Type	Features and examples
Shape	Fibrous	Long parallel chains, insoluble in water, structural, resistant to digestion — keratin, collagen, elastin, fibrin, myosin
Shape	Globular	Compactly folded, soluble, functionally active — enzymes, haemoglobin, albumin, immunoglobulins, hormones
Function	Structural	Collagen, keratin, elastin
Function	Catalytic	All enzymes
Function	Transport	Haemoglobin, albumin, transferrin, lipoproteins
Function	Contractile	Actin, myosin, tubulin
Function	Defence	Immunoglobulins, complement, fibrinogen
Function	Regulatory and hormonal	Insulin, growth hormone, transcription factors
Function	Storage	Ferritin, casein, ovalbumin

CLINICAL PEARL

Fibrous and globular proteins differ in solubility because they differ in what they hide. A globular protein folds so that hydrophobic side chains are buried in the interior and polar groups face the water — hence solubility and a defined active site. A fibrous protein instead packs extended chains against one another, leaving hydrophobic surfaces exposed and cross-linked, which gives tensile strength and insolubility. Structure follows directly from function.

Precipitation Reactions

Proteins are held in solution by their hydration shell and by their surface charge; removing either causes precipitation
At the isoelectric pH the net charge is zero, so repulsion between molecules is minimal and the protein is least soluble and most easily precipitated — the basis of isoelectric precipitation, as in the clotting of milk casein by acid
Salting out — high concentrations of neutral salts (ammonium sulphate, sodium chloride) compete for water and remove the hydration shell; globulins precipitate at half saturation and albumins at full saturation, which is used to separate them
Salting IN — low salt concentrations increase solubility, which is why globulins dissolve in dilute salt but not in pure water
Organic solvents (alcohol, acetone) lower the dielectric constant and dehydrate the protein
Heavy metal ions (Hg2+, Pb2+, Ag+) bind negatively charged groups above the isoelectric point — the basis of using egg white or milk as a first-aid antidote in heavy metal poisoning
Alkaloidal reagents (tungstic, trichloroacetic, picric and sulphosalicylic acid) precipitate protein below the isoelectric point; used for deproteinising blood before analysis and for detecting urinary protein
Heat with dilute acid — the basis of the heat coagulation test for proteinuria

Applied Aspects

Serum protein electrophoresis separates albumin from the globulin fractions, and the pattern is diagnostic — a monoclonal band in myeloma, a diffuse polyclonal rise in chronic infection and liver disease, and a reduced gamma region in hypogammaglobulinaemia
The albumin to globulin ratio is normally about 1.2 to 2, and its reversal is a useful and cheap indicator of chronic liver disease or myeloma
Give milk or egg white in acute heavy metal poisoning as an immediate measure while arranging chelation; the protein binds the metal in the gut
Denaturation destroys function but not the peptide bonds, which is why a boiled enzyme is inactive yet still gives all the colour reactions of protein
Bence Jones protein precipitates on heating to 40–60°C and redissolves on boiling — an unusual behaviour that was the original basis of its detection in myeloma

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