(a) Native protein
Definition : A protein that possesses its natural, folded three-dimensional structure — the form in which it is found in a biological system — and is therefore biologically active, is called a native protein.
Key features :
- Its secondary and tertiary structures are intact, held by hydrogen bonds and other weak forces
- Globular native proteins are generally soluble in water
- It performs its proper biological function — an enzyme still catalyses, a hormone still signals
On denaturation — by heating or a change in pH — the secondary and tertiary structures are destroyed and the protein becomes biologically inactive. The primary structure survives, since the covalent peptide bonds are not broken.
Everyday example : the coagulation of egg white on boiling is the denaturation of a native protein.
(b) Nucleotide
Definition : A compound made of three components — a nitrogenous base, a pentose sugar and a phosphate group — is called a nucleotide.
$$\text{Base} + \text{Sugar} \rightarrow \text{Nucleoside} \xrightarrow{H_3PO_4} \text{Nucleotide}$$
The base joins C-1 of the sugar by a glycosidic linkage; the phosphate esterifies the $-OH$ at C-5.
Its importance : the nucleotide is the repeating unit of nucleic acids. Nucleotides are joined to one another through phosphodiester linkages — the phosphate bridging C-5 of one sugar and C-3 of the next — building up the DNA or RNA chain.
Contrast with a nucleoside, which has only two components and lacks the phosphate, so it cannot form the backbone.
(c) Essential amino acid
Definition : An amino acid that cannot be synthesised by the human body and must therefore be obtained from the diet is called an essential amino acid.
Key points :
- Ten of the twenty amino acids found in proteins are essential
- Examples : valine, leucine, isoleucine, lysine, phenylalanine, threonine, tryptophan, methionine, histidine, arginine
- A deficiency in the diet leads to poor growth and protein deficiency disorders such as kwashiorkor
The remaining ten, which the body can make for itself, are the non-essential amino acids — glycine, alanine, aspartic acid, glutamic acid, serine and others.
