Why an Egg White Turns Solid When You Cook It

Rice, dal, ghee and the enzymes in your saliva are all built from a handful of kinds of molecules: carbohydrates, proteins, lipids and nucleic acids. Their shapes decide what they do — which is why an egg white changes completely once heat alters the shape of its proteins.

This lesson covers carbohydrates, amino acids and protein structure, lipids, and enzymes with their cofactors and the factors that affect them.

Carbohydrates are classified by size into monosaccharides, the single sugar units; disaccharides, two units joined by a glycosidic bond; and polysaccharides, long chains of many units used for energy storage and structure.

Monosaccharides. Grouped by the number of carbon atoms: trioses such as glyceraldehyde, pentoses such as ribose and deoxyribose, and hexoses such as glucose, fructose and galactose.

Disaccharides:

- Sucrose — glucose plus fructose
- Lactose — glucose plus galactose
- Maltose — glucose plus glucose

Polysaccharides:

- Starch — the energy store of plants; its helical chains hold iodine and turn blue-black
- Glycogen — the energy store of animals; more branched than starch
- Cellulose — the structural polymer of plant cell walls, made of unbranched glucose chains
- Chitin — a structural polysaccharide containing amino sugars, in insect exoskeletons and fungal cell walls

An everyday example. Chewing a piece of plain roti for a while makes it taste slightly sweet, because amylase in saliva breaks starch down into maltose.

The substance. Starch and cellulose are both made of glucose, yet humans digest only starch — the glucose units in cellulose are linked differently, and human enzymes cannot break those bonds.

An amino acid has a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom and a variable R group; proteins are chains of amino acids folded through four levels of structure — primary, secondary, tertiary and quaternary.

Amino acids:

- A central carbon carrying an amino group $\mathrm{N}{\mathrm{H}}_2$, a carboxyl group $\mathrm{COOH}$, a hydrogen atom and an R group
- Twenty kinds occur in proteins, differing only in the R group
- Neighbouring amino acids are joined by peptide bonds formed by condensation

Four levels of protein structure:

- Primary — the sequence of amino acids, from the N-terminal end to the C-terminal end
- Secondary — local folding held by hydrogen bonds, as a right-handed alpha helix or a beta-pleated sheet
- Tertiary — the whole chain folded into a three-dimensional shape, like a ball of wool
- Quaternary — two or more folded chains joined together, as in haemoglobin, with two alpha and two beta chains

An everyday example. An egg white turning white and firm in a hot pan is protein denaturation — heat breaks the bonds holding the secondary and tertiary shapes, while the peptide bonds of the primary structure stay intact.

The substance. A single change in the primary sequence can alter the whole protein — in sickle cell anaemia, one amino acid swap in haemoglobin changes how the protein behaves.

Lipids are water-insoluble biomolecules classified as simple lipids such as fats, oils and waxes, compound lipids such as phospholipids, and derived lipids such as steroids; fats and oils are mainly triglycerides, made of glycerol joined to three fatty acids.

Classification of lipids:

- Simple lipids — esters of fatty acids with alcohols: fats and oils with glycerol, and waxes with long-chain alcohols
- Compound lipids — contain extra groups: phospholipids such as lecithin, found in cell membranes, and glycolipids
- Derived lipids — obtained from other lipids: steroids such as cholesterol

Structure of fats and oils:

- Glycerol joined to fatty acids by ester bonds forms monoglycerides, diglycerides or triglycerides
- Saturated fatty acids have no double bonds; unsaturated fatty acids have one or more

Fats versus oils:

- Fats — solid at room temperature and rich in saturated fatty acids; ghee, butter
- Oils — liquid at room temperature and rich in unsaturated fatty acids; groundnut oil, mustard oil

Functions. Long-term energy storage, insulation, cushioning of organs, cell membranes and steroid hormones.

An everyday example. A jar of coconut oil turning solid on a cold morning shows that coconut oil, unusually for a plant oil, is rich in saturated fatty acids.

The substance. Lipids are not true polymers — their molecules are small, but they gather into large structures such as membranes.

Enzymes are biological catalysts, almost all of them proteins, grouped into six classes by the reaction they catalyse; many need non-protein cofactors, and their activity depends on temperature, pH, substrate concentration and inhibitors.

Six classes of enzymes. Oxidoreductases carry out oxidation-reduction, transferases move a group between substrates, hydrolases break bonds by adding water, lyases remove groups to leave double bonds, isomerases convert one isomer into another, and ligases join two compounds.

Cofactors:

- The protein part is the apoenzyme; with its cofactor it forms the active holoenzyme
- Prosthetic groups are tightly bound organic cofactors, such as haem in peroxidase and catalase
- Coenzymes are loosely bound organic cofactors, such as NAD and NADP, which contain the vitamin niacin
- Metal ions, such as zinc in carboxypeptidase, help bind the substrate at the active site

Factors affecting enzyme activity:

- Temperature and pH — each enzyme works best at an optimum; high temperature denatures it, while low temperature only slows or stops it
- Substrate concentration — the rate rises, then levels off once every enzyme molecule is busy
- Inhibitors — a competitive inhibitor resembles the substrate and blocks the active site, as malonate blocks succinic dehydrogenase

An everyday example. Keeping fruit in a refrigerator slows ripening and spoilage, because low temperature makes the enzymes involved work more slowly.

The substance. Cold and heat affect enzymes differently — cooling is reversible, but heating well beyond the optimum permanently destroys the enzyme's shape.