Protein

Proteins are biochemical compounds consisting of one or more polypeptides folded into a fibrous or globular form. Proteins are coded by the genetic information on DNA.

There are 7 classes of protein:

Ø  Structural proteins (function in the cell membrane, muscle tissue, and the silk of spiders, fibers, and mammal hair.)

Ø  Contractile proteins (work with structural elements, provide muscular movement.)

Ø  Storage proteins (such as albumin, the main substance of egg white.)

Ø  Defensive proteins (antibodies)

Ø  Transport proteins (hemoglobin, which transport oxygen from lungs to other parts of the body.)

Ø  Enzymes (the most important class, promote and regulate the chemical reactions in the cell.)

Ø  Hormones (can act as chemical messengers such as insulin.)

Proteins are amino acid polymers folded into specific three-dimensional shapes. The functions are determined by the specific structural characteristics. An amino acid is an organic molecule, including a central carbon atom which is attached by an amino group, a carboxyl group, an H atom and a variable group of atoms called a side chain (R). There are 20 different amino acids which are different in their side chains.

*amino and carboxyl groups are ionized.

*since amino acids contain both acidic (carboxyl) and basic (amino) groups, amino acids are considered amphiprotic.

*amino acids may be polar, nonpolar, or acidic, basic, depending on the nature of their side chains.

*9 amino acids are considered essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

      

An amino acid polymer is called a polypeptide. The amide linkage that holds amino acids together in polypeptides are called peptide bonds. Peptide bonds are formed by a condensation reaction between the amino group of one amino acid and the carboxyl group of an adjacent amino acid.

1 A.A →a monomer;

2 A.As →dipeptide (has one bond);

Many A.As →polypeptides→protein molecule.

Primary protein:

       Primary protein is the unique sequence of amino acids in a polypeptide chain. The polypeptide chain will always have an amino at one end, called the amino terminus, and a carboxyl group at the other, called the carboxyl terminus. Each of the amino acids in a polypeptide is referred to as a residue. Primary structure is determined by the nucleotide sequence in a gene in DNA. Changing the sequence by one amino acid could alter the three-dimensional shape to the point that the protein loses function or is rendered useless.

Secondary protein:

Secondary protein contains coils and folds in a polypeptide caused by hydrogen bonds between hydrogen and oxygen atoms near the peptide bonds.

l  α helix: a type of polypeptide secondary structure characterized by a tight coil that is stabilized by hydrogen bonds.

i.e. keratin.

l  β-pleated sheet: polypeptide secondary structured that form between parallel stretched of a polypeptide and are stabilized by hydrogen bonds.

i.e. silk protein.

*α helix is more flexible than β-pleated sheet, and β-pleated sheet structure is stronger.

Tertiary protein:

Supercoiling of a polypeptide that is stabilized by side-chain interactions.

l  Hydrophobic and van der Waals interactions

l  Proline kink

l  Hydrogen bond

l  Disulfide bridge

l  Ionic bond

 

Quandary protein:

Two or more polypeptide subunits forming a functional protein.

i.e. Hemoglobin: composed of four polypeptides in tertiary structure: two identical α-chains and two identical β-chains. Each subunit has a nonproteinaceous heme group containing an iron atom that binds oxygen.

Protein denaturation:

Protein denaturation is a change in the three-dimensional shape of a protein caused by changes in temperature, PH, ionic concentration, or other environmental factors. A denatured protein cannot carry out its biological functions; all their five types of tertiary interactions are disrupted.

Use of protein denaturation:

Dangerous: prolonged fever above 39℃ could denature critical enzymes in the brain, leading to seizures and possibly death.

Useful: heat can be used to denature the proteins in hair, allowing people to temporarily straighten curly hair or curl straight hair.