Structure of Proteins

Secondary structure of proteins

What are proteins?

Proteins are large, complex molecules and are heteropolymers containing amino acids as the monomeric units which are attached in long chains. Proteins play many critical roles in the organism’s body such as they are required for the structure, function, and regulation of the body’s tissues and organs.

What is meant by the structure of proteins?

Primary Structure of proteins

Image Source: Campbell Biology 10th edition

There are 20 different types of amino acids. Proteins are not simply in a straight line. The protein strands tend to twist, bend and fold into particular shapes. The location of each amino acid in the long string of proteins determines its three-dimensional shape and its specific function. 

There are four structural levels of proteins:

Primary structure

The amino acid sequence in a polypeptide chain forms the primary structure of a protein. The first amino acid is also termed as N-terminal amino acid and the last amino acid is termed as C terminal amino acid.

Secondary structure

The polypeptide chain segments of proteins are coiled in very specific patterns that give rise to the protein’s shape. The secondary structure comprises the coils and folds. The stability in the secondary structure is achieved by hydrogen bonds between the constituents of the polypeptide backbone occurring in a repetitive manner.

The two main types of secondary structures are α helix and β pleated sheet.

α helix

α helix is a right-handed coil that is held together by hydrogen bonding. This H-bonding is between every fourth amino acid.

alpha helix and beta sheets

β pleated sheet

The segments (2 or more) of the polypeptide chain lie parallel to each other and are termed β strands and are connected by means of hydrogen bonds. The core of globular proteins is specifically formed by β pleated sheets.

Secondary structure

Image Source: Campbell Biology 10th edition

Tertiary structure

Tertiary structure is a three-dimensional view of a protein. The tertiary structure of polypeptide results particularly from interactions between R groups (the side chains) of the amino acids. It reflects the spatial coordination of secondary structural elements. The tertiary structure is the most crucial because the proteinaceous enzymes which have the role to perform many functions in our body have a tertiary structure.

The interactions in the tertiary structure are

1. Hydrophobic interactions.

2. Weak van der Waals forces that stabilize the protein.

3. Hydrogen bonds.

4. Ionic interactions.

In an aqueous cellular environment, these interactions are weak, but their additive effect helps give the protein attain a unique shape. Covalent bonds called disulfide bridges may also be found.

Quaternary structure

Quaternary structure is the overall architecture of a protein that results from the aggregation of these polypeptide subunits. These are arranged in a special manner with respect to each other, for instance, spheres are arranged consecutively in the form of a plate or a cube or even spheres in a linear string. Some examples of proteins with the quaternary structure are haemoglobin, collagen, etc.

There are four subunits in adult haemoglobin (Hb)-

  1. 2 alpha chains
  2. 2 beta chainscollagen haemoglobin

 Each level of the protein structure gives rise to the next level.

 

quaternary structure

Image Source: Campbell Biology 10th edition

What is the importance of the structure of proteins?

Now let’s know why these proteins form these structures. These conformations are responsible for the catalytic and structural function in the cell. They form enzyme complexes, protein filaments, ribosomes and membranes.

Hope the article could explain to you the importance of four structural levels of proteins which are the machinery of living tissues.

Thank you for reading at MBD!

Team MBD 

 

For more on BiochemistryBiochemistry Archives – My Biology Dictionary

 

 

 

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