(a) Essential amino acids
Amino acids that cannot be synthesised in the human body and must therefore be supplied through the diet are called essential amino acids.
Ten of the twenty amino acids found in proteins are essential — for example valine, leucine, isoleucine, lysine and phenylalanine.
The remaining ten, which the body can make for itself, are the non-essential amino acids, such as glycine and alanine.
(b) Peptide bond
The amide linkage $-CO-NH-$ formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of a water molecule, is called a peptide bond.
$$H_2N-CHR-COOH + H_2N-CHR'-COOH \rightarrow H_2N-CHR-\underset{\text{peptide bond}}{CO-NH}-CHR'-COOH + H_2O$$
The bond has partial double bond character, which makes it planar and rigid — an important factor in how proteins fold.
(c) Denaturation
Denaturation is the process in which a protein loses its native three-dimensional shape, and with it its biological activity, when subjected to a change in physical or chemical conditions such as heating or a change in pH.
What is destroyed : the secondary and tertiary structures, as the hydrogen bonds and other weak forces holding them are broken. The globular protein uncoils into a random, fibrous form.
What survives : the primary structure remains intact, since the covalent peptide bonds are not broken.
Everyday examples : the coagulation of egg white on boiling; the curdling of milk.
