Why an Egg White Turns Solid and White When You Boil It

Grind up a leaf or a piece of liver and analyse it, and you find the same kinds of chemicals again and again — water, sugars, amino acids, fats, nucleic acids and minerals. Life is built from a surprisingly small toolkit of biomolecules.

This part covers how tissues are analysed, primary and secondary metabolites, amino acids, and the structure and functions of proteins.

Living tissue is ground with trichloroacetic acid and strained: the filtrate, called the acid-soluble pool, holds small micromolecules, while the material left behind, the acid-insoluble fraction, holds large macromolecules along with lipids.

The method:

- Grind the tissue with trichloroacetic acid into a thick slurry
- Strain through cheesecloth or cotton
- Filtrate — the acid-soluble pool
- Retentate — the acid-insoluble fraction

Classification by molecular weight:

- Micromolecules — molecular weights from about $18$ to $800$ daltons: amino acids, sugars, nucleotides; found in the acid-soluble pool
- Macromolecules — molecular weights of $10000$ daltons and above: proteins, nucleic acids and polysaccharides; found in the acid-insoluble fraction

Inorganic content. Burning tissue oxidises the carbon compounds away as carbon dioxide and water, leaving ash that contains elements such as calcium and magnesium and compounds such as sulphates and phosphates.

An everyday example. A chapati left too long on the flame burns down to a little grey ash — the minerals that remain after the organic matter is gone.

The substance. Lipids end up in the acid-insoluble fraction even though their molecules are small, because they form membranes that break into insoluble vesicles.

Primary metabolites such as amino acids, sugars and nucleotides have clear roles in normal life processes, while secondary metabolites, found mainly in plants, fungi and microbes, are not needed for basic metabolism but include many substances valuable to humans.

Primary metabolites. Amino acids, sugars, nucleotides and fatty acids — the building blocks and fuels of every cell.

Secondary metabolites:

- Alkaloids — morphine, codeine
- Terpenoids — monoterpenes, diterpenes
- Essential oils — lemongrass oil
- Toxins — abrin, ricin
- Lectins — concanavalin A
- Drugs — vinblastin, curcumin
- Polymeric substances — rubber, gums, cellulose
- Pigments — carotenoids, anthocyanins

An everyday example. The bright yellow of haldi comes from curcumin, a secondary metabolite that is also studied for its medicinal properties.

The substance. For many secondary metabolites, their exact role in the organism that makes them is not yet fully understood — though their use to humans is clear.

An amino acid has an amino group, a carboxyl group, a hydrogen atom and a variable R group all attached to the same carbon, the alpha carbon, and amino acids are classed as acidic, basic or neutral according to their R group.

General structure:

$${\text{H}}_2\text{N}-\underset{\text{R}}{\underset{|}{C}}\text{H}-\text{COOH}$$

The R group decides the amino acid:

- R = H — glycine
- **R = CH${}_3$ — alanine
- R = CH${}_2$OH — serine

Proteins are built from twenty kinds of amino acids.

Classification by R group:

- Acidic — an extra carboxyl group: glutamic acid, aspartic acid
- Basic — an extra amino group: lysine, arginine
- Neutral — neither: valine, glycine
- Aromatic — a ring in the R group: tyrosine, phenylalanine, tryptophan

Worked example — identify the type. An amino acid with two carboxyl groups and one amino group, such as glutamic acid, is acidic.

Ionisable groups.** Because the amino and carboxyl groups can gain or lose H${}^{+}$, an amino acid's structure changes with pH; at a particular pH it exists as a zwitterion, carrying both a positive and a negative charge.

An everyday example. Dal and paneer are broken down in digestion into amino acids, which the body then reassembles into its own proteins.

The substance. All twenty amino acids share the same backbone — only the R group changes.

A protein's primary structure is its sequence of amino acids, its secondary structure is the helix or sheet the chain folds into, its tertiary structure is the full three-dimensional folding of that chain, and its quaternary structure is the arrangement of several chains together — and this final shape decides what the protein can do.

Levels of structure:

- Primary — the sequence of amino acids joined by peptide bonds, from the N-terminal amino acid to the C-terminal one
- Secondary — parts of the chain coil into a right-handed helix or form sheets
- Tertiary — the whole chain folds on itself like a crumpled woollen ball, giving a 3D shape essential for activity
- Quaternary — several polypeptide chains fit together; adult haemoglobin has four: two alpha and two beta chains

Functions:

- Transport — haemoglobin carries oxygen; GLUT-4 moves glucose into cells
- Defence — antibodies fight infections
- Catalysis — enzymes such as trypsin
- Structure — collagen, the most abundant protein in the animal world
- Hormones and receptors — insulin; receptors for smell, taste and hormones

RuBisCO, the enzyme that fixes carbon dioxide in photosynthesis, is the most abundant protein in the whole biosphere.

An everyday example. Boiling an egg turns the clear white solid and opaque, because heat unfolds its proteins and destroys their natural shape.

The substance. Changing even one amino acid can alter a protein's shape and function, as happens in sickle-cell haemoglobin.