Proteins: Definition structure and functions

Definition: Twenty percent of the human body is made up of proteins.
Proteins are the large, complex molecules that are critical for normal functioning of cells.
• They are essential for the structure, function, and regulation of the body’s tissues and organs.
• Proteins are made up of smaller units called amino acids, which are building blocks of proteins. They are attached to one another by peptide bonds forming a long chain of proteins.

Amino acid structure and its classification

• An amino acid contains both a carboxylic group and an amino group. Amino acids that have an amino group bonded directly to the alpha-carbon are referred to as alpha amino acids.
• Every alpha amino acid has a carbon atom, called an alpha carbon, Cα ; bonded to a carboxylic acid, –COOH group; an amino, –NH2 group; a hydrogen atom; and an R group that is unique for every amino acid.


Peptide bonds

• Amino acids are linked together by ‘amide groups’ called peptide bonds.
• During protein synthesis, the carboxyl group of amino acid at the end of the growing polypeptide chain reacts with the amino group of an incoming amino acid, releasing a molecule of water. The resulting bond between the amino acids is a peptide bond.

peptide bond

Structure of proteins

• The sequence of a protein is determined by the DNA of the gene that encodes the protein (or that encodes a portion of the protein, for multisubunit proteins).
• A change in the gene’s DNA sequence may lead to a change in the amino acid sequence of the protein. Even changing just one amino acid in a protein’s sequence can affect the protein’s overall structure and function.
• To understand how a protein gets its final shape or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and quaternary.


Primary Structure

• The simplest level of protein structure, primary structure is simply the sequence of amino acids in a
polypeptide chain.
• The hormone insulin has two polypeptide chains A, and B. The sequence of the A chain, and the sequence
of the B chain can be considered as an example for primary structure.

Secondary structure

• secondary structure, refers to local folded structures that form within a polypeptide due to interactions between atoms.
• The most common types of secondary structures are the α helix and the β pleated sheet. Both structures are held in shape by hydrogen bonds, which form between the carbonyl O of one amino
acid and the amino H of another.

Tertiary structure

• The overall three-dimensional structure of a polypeptide is called its tertiary structure. The tertiary structure is primarily due to interactions between the R groups of the amino acids that make up
the protein.
• Important to tertiary structure are hydrophobic interactions, in which amino acids with nonpolar, hydrophobic R groups cluster together on the inside of the protein, leaving hydrophilic amino acids on the outside to interact with surrounding water molecules.
• Also, Disulfide bonds, covalent linkages between the sulfurcontaining side chains of cysteines, are much stronger than the other types of bonds.

Quaternary structure

• When multiple polypeptide chain subunits come together, then the protein attains its quaternary
• An example for quaternary structure is hemoglobin. The hemoglobin carries oxygen in the blood and is made up of four subunits, two each of the α and β types.

Functions of Proteins

Positive negative attractions between different atoms in the long amino acid strand cause it to coil on itself again and again to form its highly complex shape. Folded proteins may combine with other folded proteins to form even larger more complicated shapes.

The folded shape of a protein molecule determines its role in body chemistry. Structural proteins are shaped in ways that allow them to form essential structures of the body. Collagen, a protein with a fibre shape, holds most of the body tissues together. Keratin, another structural protein forms a network of waterproof fibres in the outer layer of the skin.

Functional proteins have shapes that enable them to participate in chemical processes of the body. Functional proteins include some of hormones, growth factors, cell membrane receptors, and enzymes.

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