Proteins: Structure And Functions

What are Proteins?

Proteins are long-chain molecules made of amino acids that are vital in the body and are involved in several activities within the body. The major roles of proteins include enzymes that catalyse biochemical reactions, the transportation of molecules using haemoglobin, and the structural framework provided by collagen.

Proteins are made of 20 different types of amino acids that are characterised by an amino group (NH2), carboxyl group (COOH), hydrogen atom, and a variable R group that varies depending on the type of amino acid. An important fact that defines proteins, is the uniqueness of amino acids’ sequence and structure, which determines their function in biological processes.

Protein Structure

The protein structure is described below:

Primary Structure

The main secondary building block of a protein is its amino acid sequence, held together by peptide bonds to form a polypeptide chain.

Role of peptide bonds: Peptide bonds that occur between the carboxyl group of one amino acid and the amino group of the next define the linear sequence of the protein through the backbone.

Secondary Structure

The secondary structure is the regular, repeated patterns of the polypeptide chain, primarily stabilized by hydrogen bonds

There are two main categories and these include the alpha helix and the beta-sheet.

• A right-handed alpha helix is where each amino acid donates a hydrogen bond to the carbonyl oxygen of the amino acid four units before it.

• Beta sheets are constructed by two or more strands in parallel or antiparallel in the polypeptide chain through the hydrogen bonds.

Role of hydrogen bonds: These interactions are between the backbone atoms of the amino acids and they assist in the stabilisation of the secondary structure.

Tertiary Structure

The tertiary structure can be defined as the overall form of a polypeptide where the covalent bonds are between the side chains (R groups). Examples of tertiary structure include lysozymes, immunoglobulins.

Types of interactions:

• Hydrophobic interactions: Nonpolar side chains cluster to or out of water.

• Ionic bonds: The side chains with opposite charges are attracted towards one another.

• Hydrogen bonds: Interpolar side chains.

• Disulfide bridges: Disulfide linkages of two cysteine residues in the side chains.

Quaternary Structure

Quaternary structure is the level of protein structure where several polypeptide chains combine to form a functioning protein complex. Examples include haemoglobin, DNA polymerase

Importance in multi-subunit proteins: The quaternary structure supports proteins that require cooperative billing interactions and the stability and regulatory mechanisms of the protein.

Types of Proteins

The types of proteins are discussed below:

Fibrous Proteins

Characteristics:

• Fibrous proteins are also long and insoluble

• These are the structural proteins of cells and tissues because they offer support and strength.

• These proteins contain simple sequences of amino acids and create tension structures like a rope.

Examples:

• Collagen contributes to the structural framework of the body such as the skin, bones, tendons, and even joint capsules.

• Keratin is responsible for the cuticle, cortex, and medulla of hair, nails, and the outermost layer of skin, thus offering the mechanical barrier.

Globular Proteins

Characteristics:

• Globular proteins are dense, water-soluble, and possess a high structural complexity with some kind of tertiary or quaternary structures.

• There are several roles of enzymes, some of them include acting as a catalyst, transportation of substances, and control of various activities.

Examples:

• Antibodies, to detect in the human body unwanted and dangerous intruders as bacteria and viruses

Membrane Proteins

Characteristics:

• Membrane proteins are located within the cell membrane, either partially or wholly. They may be partially in the internal layer of the membrane or may be located on the surface layer and partially in the interior of the cell.

• They are involved in carrying out numerous functions inclusive of communication and transport across the cell membrane.

Examples:

• Receptors include seven transmembrane domain receptors, and G-protein coupled receptors (GPCRs).

• Transport proteins also facilitate the movement of ions across the cell membrane.

Functions of Proteins

The functions of proteins are discussed below:

Enzymatic

Proteins are enzymes that promote reactions of biochemical processes by decreasing the activation energy hence enhancing metabolic reactions.

Example: Amylase breaks down starch into their simplest compounds, sugars, while digestion. Lipase helps to break down the fats into glycerol and fatty acids, known to be the essential substances in the human body.

Structural

Connective tissues involve structural proteins which are the framework of the cells and tissues to offer stability and form.

Example: Collagen fibres are found in connective tissues to provide tensile strength and elasticity.

Transport

Transport proteins have the function of moving molecules and ions across membranes into cells and through the blood.

Example: Hemoglobin transports oxygen in the tissues and takes carbon dioxide in the tissues back to the lungs for exhalation.

Regulatory

The biochemical function of the proteins is to transport glucose across the plasma membrane. These proteins act like conductors of cells to regulate certain genes which dictate the functioning and reaction of the cells to different conditions.

Example: Attach to specific sites on DNA to control the process of conversion of genes into mRNA.

Signalling

They are involved in the transfer of signals from one cell to another to regulate several physiological activities.

Example: Insulin Influences the amount of sugar that is allowed into the cells to ensure that the body’s blood sugar levels are not raised. Growth hormones promote cell growth and division and influence the developmental changes of cells.

Defensive

Role in immune response: Protective proteins also exist to defend the body from disease and other materials that are foreign to the body.

Example: Antibodies recognize and eliminate hazardous foreign bodies such as bacteria and viruses.

Protein Synthesis

The process of protein synthesis is given below-

Transcription

Transcription is the process where the DNA sequence of a gene is transcribed into the messenger RNA that contains the required information for synthesizing proteins.

Translation

It is one of the core processes of genetic decoding that takes place on the Ribosome where an mRNA designates a certain polypeptide.

Role of tRNA, ribosomes, rRNA

tRNA transfers specific amino acids to position the anticodon element to the reach of the ribosome to fix the corresponding mRNA codons.

Ribosomes bring aminoacyl-tRNA molecules carrying specific amino acids. With the help of rRNA and ribosomal proteins, these amino acids are incorporated into a growing polypeptide chain.

rRNA facilitates the formation of peptide bonds and properly aligns the tRNA and mRNA.

Posttranslational Modifications

PTMs are modifications to the protein after it has been synthesized and are changes to the primary structure that alter the protein's function and activity.

Protein Folding and Misfolding

The folding and misfolding are discussed below:

Importance of Correct Folding

Protein folding is essential for the functionality of proteins because the misfolding of proteins could lead to the generation of nonfunctional products. Folded proteins have a well-defined tertiary structure that is ideal for normal physiologic functions, including catalysis, signaling, or structural support

If the proteins are folded incorrectly, they are useless or destroy other proteins and can cause diseases since cellular processes are heavily reliant on protein folding.

Role of Chaperones

Some proteins that have at least one domain with a solvent-exposed hydrophobic surface are designated as chaperones, if they fold with specified components of the target protein that interact with it. They assist proteins in folding, which is an intensive process during protein synthesis and when the proteins are under stress.

Diseases from Misfolding

Any alteration in the structure of proteins can change its function and may cause diseases in humans:

Parkinson's Disease: When the proteins, alpha-synuclein gets misfolded and starts to aggregate, the Lewy bodies have toxic effects on neurons and can cause neurodegeneration.

Alzheimer’s Disease: It primarily involves beta-amyloid protein clumps and tau protein tangles that damage brain cells and disrupt function.

Prion Diseases: These are fatal brain diseases that are caused by misfolded proteins called prions, that accumulate in the brain and damage nerve cells.

Techniques to Study Proteins

The techniques are discussed below:

• Xray Crystallography

• NMR Spectroscopy

• Mass Spectrometry

• Western Blotting

NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Different types of Protein Structures

• Types of Proteins

Practice Questions for NEET

Q1. Alpha helix and beta pleated sheets are absent in__.

1. Primary structure

2. Secondary structure

3. Tertiary structure

4. Quaternary structure

Correct answer: 1) Primary structure

Explanation:

The alpha helix and beta pleated sheets are absent in the primary (1°) structure of proteins, which is simply the linear sequence of amino acids. These structures form later in the secondary structure as the polypeptide chain folds, with hydrogen bonds stabilizing the alpha helix and beta sheets.

Hence, the correct answer is option 1) Alpha helix and beta pleated sheets are absent in the 1° structure.

Q2. The primary structure of a protein molecule has

1. Two ends

2. One end

3. Three ends

4. No ends

Correct answer: 1) Two ends

Explanation:

The primary structure of a protein molecule consists of a specific sequence of amino acids linked by peptide bonds. This sequence is determined by the gene encoding the protein and dictates its overall structure and function. Any change or mutation in the sequence can alter the protein's properties, potentially affecting its biological activity. The primary structure serves as the foundation for the protein's higher-order structures, including secondary, tertiary, and quaternary forms.

Hence, the correct answer is option 1) Two ends.

Q3. The primary structure of protein represents

1. The linear sequence of amino acids joined by peptide bond

2. 3D structure of protein

3. Helical structure of protein

4. Subunit structure of protein

Correct answer: 1) The linear sequence of amino acids joined by peptide bond

Explanation:

The primary structure of protein represents a linear sequence of amino acids joined by a peptide bond. The primary structure of a protein represents a linear sequence of amino acids joined by peptide bonds. It is unique to each protein and serves as the foundation for its higher-order structures, such as secondary, tertiary, and quaternary forms. This sequence is determined by the genetic code within an organism's DNA. The primary structure plays a crucial role in defining the protein's overall shape, stability, and specific biological function. Even a single change in the sequence, caused by a mutation, can alter the protein's activity or lead to diseases. Understanding the primary structure is essential for studying protein folding and function.

Hence, the correct answer is option 1) Linear sequence of amino acids joined by peptide bonds.

Also Read:

• MCQs on Types of Structure of Proteins

• Metabolites

• Biomolecules

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