Why Boiling an Egg Changes Its Proteins Forever

Hair, muscle and the enzymes that digest food are proteins; tiny amounts of vitamins keep the body running; and DNA carries the instructions for making every protein.

This part covers amino acids and peptides, protein structure and denaturation, enzymes and vitamins, and nucleic acids with hormones.

Amino acids carry both an amino group and a carboxyl group; they are essential or non-essential depending on whether the body can make them, acidic, basic or neutral by their extra groups, exist as dipolar zwitterions in solution, and join through –CO–NH– peptide linkages.

Classification:

- Essential — must come from food, such as valine, leucine and lysine
- Non-essential — made in the body, such as glycine and alanine
- Neutral, acidic or basic — balanced groups as in glycine, an extra carboxyl as in aspartic acid, or an extra amino group as in lysine

Zwitterion. In water the –COOH passes a proton to –NH${}_2$, giving an ion with both a positive and a negative charge. This explains why amino acids are crystalline solids with high melting points, why they dissolve in water, and why they are amphoteric.

Peptide linkage. The –COOH of one amino acid condenses with the –NH${}_2$ of the next, losing water.

Worked example. Glycine ($M=75$ g mol${}^{-1}$) and alanine ($M=89$ g mol${}^{-1}$) form the dipeptide glycylalanine with loss of one water molecule:

$$75+89-18=146{\text{ g mol}}^{-1}$$

An everyday example. Dal, milk, eggs and paneer matter in Indian diets because they supply essential amino acids the body cannot make.

The substance. An amino acid is never simply neutral in water — even glycine exists mostly as a zwitterion carrying both charges.

**A protein's primary structure is its sequence, its secondary structure an $\alpha$-helix or $\beta$-pleated sheet, its tertiary structure the overall fold, and its quaternary structure the arrangement of several chains; denaturation destroys all but the sequence.

Levels of structure:

- Primary — the sequence of amino acids; changing even one can alter the protein's function
- Secondary — the $\alpha$-helix, a right-handed coil held by hydrogen bonds within one chain, or the $\beta$-pleated sheet, chains side by side held by hydrogen bonds between them
- Tertiary — further folding into a fibrous or globular shape, stabilised by hydrogen bonds, disulphide links, van der Waals and electrostatic forces
- Quaternary — two or more subunits together, as in haemoglobin, which has four

Denaturation. Heat, a change in pH or certain chemicals break the hydrogen bonds holding the fold, so the protein unfolds and loses its biological activity, although its peptide bonds survive.

Worked example.** A protein chain of $150$ amino acids has $149$ peptide bonds. With an average residue mass of about $110$ g mol${}^{-1}$, its molar mass is roughly $150\times 110=1.65\times 10^4$ g mol${}^{-1}$.

An everyday example. Boiling an egg turns the clear, runny albumin white and solid — the protein is denatured and cannot return.

The substance. Denaturation leaves the primary structure untouched — only the folding is lost.

Enzymes are protein catalysts that bind only substrates fitting their active sites; vitamins are needed in small amounts and are either fat-soluble (A, D, E, K) and stored in the body, or water-soluble (B group, C) and needed regularly.

Enzymes:

- Specific — each binds a particular substrate at its active site, like a key in a lock
- Efficient — they lower activation energy greatly and work at body temperature and near-neutral pH

Vitamins and deficiency diseases:

- A — fish liver oil, carrots, milk; xerophthalmia and night blindness
- D — sunlight on skin, fish, egg yolk; rickets and osteomalacia
- E — vegetable oils, wheat germ; fragile red blood cells and muscular weakness
- K — green leafy vegetables; delayed blood clotting
- **B${}_1$ — yeast, milk, cereals; beriberi
- C — citrus fruits, amla, green vegetables; scurvy

Worked example.** Vitamin C, C${}_6$H${}_8$O${}_6$, has molar mass $6(12)+8(1)+6(16)=176$ g mol${}^{-1}$, so a $250$ mg tablet contains $\frac{0.250}{176}\approx 1.42\times 10^{-3}$ mol.

An everyday example. Amla and citrus fruits are rich in vitamin C, which long boiling destroys because it is water-soluble and heat-sensitive.

The substance. Fat-soluble vitamins can build up to harmful levels, while excess water-soluble vitamins are mostly lost in urine.

Nucleic acids are chains of nucleotides — sugar, base and phosphate — joined by phosphodiester links; DNA has deoxyribose and thymine in a double helix, RNA has ribose and uracil in a single strand, and hormones act as chemical messengers.

Building blocks:

- Nucleoside — base plus sugar
- Nucleotide — base plus sugar plus phosphate
- Phosphodiester linkage — a phosphate joins the 5' carbon of one sugar to the 3' carbon of the next

DNA versus RNA:

- Sugar — DNA: 2-deoxy-D-ribose; RNA: D-ribose
- Bases — DNA: A, G, C and T; RNA: A, G, C and U
- Strands — DNA: a double helix; RNA: usually single-stranded

Double helix. Two antiparallel strands coil round each other, held by hydrogen bonds between complementary bases: A pairs with T, G with C.

Functions:

- DNA — stores hereditary information and passes it on when cells divide
- RNA — carries out protein synthesis as messenger, ribosomal and transfer RNA
- Hormones — messengers such as insulin and glucagon, which control blood glucose, and thyroxine, which regulates metabolism

Worked example. If $30$ percent of the bases in a DNA sample are adenine, thymine is also $30$ percent, leaving $40$ percent for guanine and cytosine together — so each is **$20$ percent.

An everyday example. DNA tests used to confirm family relationships rely on base sequences being inherited from parents.

The substance. Base pairing is why DNA can copy itself** — each strand serves as a template that fixes the sequence of the new one.