Proteins: Structure And Functions

Proteins are referred to as the building blocks of life because they are the most abundant molecules in the body, accounting for approximately 60% of the dry weight of cells. Protein is a topic of the chapter Biomolecules in Biology.

What are Proteins?

Proteins are long-chain molecules made of amino acids that are very vital in the body and are involved in many 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.

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This Story also Contains

1. What are Proteins?
2. Protein Structure
3. Types of Proteins
4. Functions of Proteins
5. Protein Synthesis
6. Protein Folding and Misfolding
7. Techniques to Study Proteins
8. Recommended Video On Proteins

Proteins are made out 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 it 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

Definition:

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

Definition:

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

Types:

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

A righthanded 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

Definition:

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

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.

Examples of tertiary structure: Lysozomes, Immunoglobulins

Quaternary Structure

Definition:

Quaternary structure is the level of protein structure where several polypeptide chains combine to form a functioning protein complex.

Examples:

Haemoglobin, DNA polymerase

Importance in multisubunit proteins:

The quaternary structure supports proteins that require cooperative billing interactions and the stability and regulatory mechanisms of the protein.

Also Read-

• MCQ Practice on Proteins
• Biomolecules
• Metabolites
• Water

Types of Proteins

The types of proteins are discussed below:

Fibrous Proteins

Characteristics:

Fibrous proteins are also usually long and insoluble as well as 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 often possess a high level of 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 those proteins that 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 Function

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

Example:

Amylase: This divides starches into their simplest compounds, sugars, while digestion.

Lipase: Helps to divide the fats into glycerol and fatty acids known to be the essential substances in the human body.

Structural Function

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

Example:

That is where collagen fibres come into play, they are found in connective tissues to provide tensile strength and elasticity.

Transport Function

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 Function

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 in turn 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.

Signaling Function

Role in cell communication:

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.

Defence Function

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, and rRNA:

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

Ribosomes: Aminoacyl units come to be when to help in the incorporation into polypeptide chains with the help of rRNA and proteins.

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.

Types:

Phosphorylation, where phosphate groups are attached, for example, to serine, threonine, or tyrosine amino acids, and have an impact on the protein function and the signalling processes.

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, signalling, 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.

Chaperone proteins and their role:

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.

Consequences of misfolding:

Parkinson's Disease: Alpha-synuclein protein when it gets misfolded and starts to aggregate forms the Lewy bodies has toxic effects on neurons and can cause neurodegeneration.

Techniques to Study Proteins

The techniques are discussed below:

• Xray Crystallography

• NMR Spectroscopy

• Mass Spectrometry

• Western Blotting

Also Read-

Recommended Video On Proteins