Understanding Proteins : Definition, Structure and Functions

What is protein

Proteins are Macromolecules essential for the structure, function, and regulation of body tissues and organs. They are complex structures made up of small basic units called amino acids, which form long chains to form protein molecules. The information for protein synthesis is encoded in DNA and translated into cells to make proteins. These proteins are essential for an organism’s growth, tissue repair, and maintenance of all cells. This article explores the nuances of protein structure, function, and the significant roles they play in biological processes.

What is a simple definition of protein?

proteins are made by amino acids which are essential for regulating the body’s function and structure.

Structure of Proteins

Amino acids

The fundamental units of proteins are amino acids, which are organic compounds containing amino (-NH2) and carboxyl (-COOH) groups. Proteins are constructed from 20 different amino acids, each distinguished by its unique R group that determines its properties. The sequence of amino acids in a protein, dictated by the genetic code, ultimately defines the protein’s structure and function.

Nonpolar Amino Acids

Polar (Uncharged) Amino Acids

Charged Amino Acids

Positively Charged (Basic)

Negatively Charged (Acidic)

What is the difference between polar and nonpolar R groups?

Polar amino acids are hydrophilic, they like to interact with water and aqueous solution and non-polar amino acids contain R groups, which do not interact or avoid with water and other liquids.

Levels of Protein Structure

We know that proteins are complex molecules determined by their amino acid sequence. Based on their structure, proteins are divided into four different states: primary, secondary, tertiary and quaternary structures.

Primary Structure

When proteins are synthesized, the amino acids are joined together in a specific order to form a linear chain called the primary structure of the protein. One example of a primary protein structure is the hormone insulin.

Secondary Structure

The secondary structure of proteins is determined by hydrogen bonds between amino acids, which shape the local spatial organization of the polypeptide backbone, while not involving the side chains. The most common second structure is alpha helices and beta sheets,

Alpha Helix

• Alpha helices are right-handed helical structures where the polypeptide chain forms a helical shape.
• Structure: These helices are formed by hydrogen bonds between the carbonyl group of an amino acid residue and the amide hydrogen of an amino acid residue located four residues down the chain.
• Characteristics: Alpha helices are compact and stable structures commonly found in protein regions that require structural support or rapid folding and unfolding.

Beta Sheets

• Beta sheets align polypeptide chains side by side to form a flat sheet-like structure.
• Strands run in opposite directions (anti-parallel) or in the same direction (parallel), stabilized by hydrogen bonds between the backbone atoms of adjacent strands.
• Beta sheets are often found in the core of globular proteins.
• They play an important role in providing strength and stability to protein structures

Tertiary Structure

The tertiary structure refers to the three-dimensional shape of a single polypeptide chain, which is determined by interactions between the R groups (side chains) of amino acids. These interactions include:

• Hydrogen bonds: Weak bonds between hydrogen and electronegative atoms like nitrogen or oxygen in other molecules.
• Disulfide bridges: Covalent bonds between two cysteine amino acids, formed due to sulfur-containing cysteine residues, which stabilize the protein structure.
• Ionic bonds: Electrostatic attractions between positively and negatively charged R groups.
• Hydrophobic interactions: Nonpolar R groups cluster together in the protein core to minimize contact with water molecules.

The specific arrangement and combination of these interactions dictate the folded shape of the protein. This structure is crucial as it directly influences the protein’s stability, functionality, and ability to interact with other molecules.

Quaternary Structure

The quaternary structure involves the assembly of multiple polypeptide chains (subunits) to form a functional protein complex. Similar interactions observed in tertiary structure—such as hydrogen bonds, disulfide bridges, ionic bonds, and hydrophobic interactions between the R groups of different subunits—stabilize the quaternary structure.

The stability and function of many proteins depend on their quaternary structure. It allows for the synthesis of larger, more complex molecules capable of performing intricate biological processes such as molecular transport, cellular signaling, and enzyme activity.

In summary, while tertiary structure pertains to the three-dimensional shape of a single polypeptide chain, quaternary structure involves the arrangement of multiple polypeptide chains to form a functional protein complex. Both structures are essential for understanding protein function and their roles in biological systems.

the structures of proteins (primary, secondary, tertiary, and quaternary) along with examples and their functions

Functions of Proteins

 Proteins play a vital role in the body’s functional, structural, and regulatory activity here are some main functions of proteins;

Enzymatic Activity:

Enzymes are 3D-shaped protein molecules that act as biocatalysts. They increase the rate of biochemical reactions by reducing the activation energy. Examples of enzymes include amylase, cellulase, and protease.

Structural Support

Proteins form the structure of cells and tissues. Actin, collagen, myosin, and keratin are examples of structural proteins that build cell structures and contribute to the formation of cartilage, bones, hair, and muscles.

Transport and Storage

The protein found in red blood cells called hemoglobin carries oxygen from the lungs to the body’s tissues. In the liver, ferritin stores iron.

Cell Signaling

Proteins play a role in cell signaling pathways, including the one that controls blood glucose levels through insulin.

Immune Response

Proteins called antibodies are able to identify and eliminate foreign substances such as bacteria and viruses.

Movement

The proteins myosin and actin allow muscles to contract and move.