1. Cell structure and Function
1.6 b , Chemical constituents of cell -Proteins
Chemical constituents of cell
Dr V Malathi and Mrs Sushumna Rao
The next abundant constituent of cell is Proteins.
These are made from amino acids
They function as structural components of cells and subcellular entities.
They act as sources of nutrients, as atom- and energy-storage reservoirs, and
They are functional species such as hormones, enzymes, receptors, and transport molecules.
Aminoacids
These are organic molecules .
They contain a hydrogen atom, a carboxyl group (–COOH), and an amino group (–NH2) and all of these are bonded to the same carbon atom, called α carbon.
The fourth group bonded to the α carbon varies among the different amino acids and is called a residue or a side chain, represented in structural formulas by the letter R.
When two or more amino acids combine, water molecule is removed and the two amino acids are connected by a covalent bond called the which is formed by the reaction of the carboxylic acid group of one amino acid molecule with the amine group of another amino acid molecule. The resulting molecule is called a Peptide.
Prefixes are often used to specify these numbers of amino acids that join for example dipeptides (two amino acids), tripeptides (three amino acids) etc.,
In general oligopeptides are formed by joining up to approximately 20 amino acids, whereas polypeptides are synthesized from up to approximately 50 amino acids.
“Proteins” by OpenStax is licensed under CC BY 4.0
When the number of amino acids linked together becomes very large, or when multiple polypeptides are used as building subunits, the macromolecules that result are called proteins.
Protein Structure
The size (length) and specific amino acid sequence of a protein are major determinants of its shape, and the shape of a protein is critical to its function.
Protein structure is categorized in terms of four levels: primary, secondary, tertiary, and quaternary.
The primary structure : is simply the sequence of amino acids that make up the polypeptide chain.
The interactions of the functional groups and R groups of the amino acids form hydrogen, ionic, and disulfide bonds, along with polar/nonpolar interactions. These interactions lead to the formation of secondary, tertiary, and quaternary protein structures.
These groups are composed primarily of carbon, hydrogen, oxygen, nitrogen, and sulfur, in the form of hydrocarbons, acids, amides, alcohols, and amines.
The secondary structure : When the chain is sufficiently long, hydrogen bonding may occur between amine and carbonyl functional groups within the peptide backbone (excluding the R side group),
This results in localized folding of the polypeptide chain into helices and sheets.
These shapes constitute a protein’s secondary structure. The most common secondary structures are the α-helix and β-pleated sheet.
In the α-helix structure, the helix is held by hydrogen bonds between the oxygen atom in a carbonyl group o