Proteins: Structure And Functions

Edited By Irshad Anwar | Updated on Jul 02, 2025 05:55 PM IST

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.

This Story also Contains

What are Proteins?
Protein Structure
Types of Proteins
Functions of Proteins
Protein Synthesis
Protein Folding and Misfolding
Techniques to Study Proteins
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Proteins: Structure And Functions

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.

Primary structure of Protein

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.

Secondary structure of Protein

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

Tertiary structure of lysozyme

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.

Quaternary structure of Haemoglobin

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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

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Frequently Asked Questions (FAQs)

1. Describe the structure of protein and their levels?

Proteins have four levels of structure: Proteins have four levels of structure:

Primary Structure: Polymer of amino acids connected in a linear arrangement through peptide bonds.

Secondary Structure: Recurring forms of protein secondary structures, which consist of hydrogen bondings such as alphahelix and betasheet.

Tertiary Structure: In gross form, overall 3D shape with multiple interactions between the amino acid side chains.

Quaternary Structure: Dimerisation and other oligomeric assembly processes which bring together separate polypeptide subunits to form a welldefined complex if required.

2. How do proteins function as enzymes?

Protein works as an enzyme hence its role in metabolism is to catalyze biochemical reactions. They reduce the enthalpy of activation that is needed for the reactions to happen and as a result, enhance the rates of reactions and are usually not consumed in the process. In enzymes, substrates combine with the enzyme at the active region, and interactions within this region enable the formation of products from the substrates.

3. What is the importance of protein folding?

Protein folding is essential because a protein's jobs depend on its shape and size and is regulated by molecular chaperones. Structural correctness is required for proteins to perform their biological activities in areas like enzymatic catalysis, structural support, transportation, and signaling. OF, failure of proteins to correctly fold may cause loss of function, aggregate, and form toxic structures that cause diseases such as Alzheimer’s and Parkinson’s.

4. How are proteins synthesised in the cell?

Protein synthesis occurs in two main stages: The process of protein synthesis takes place in two principal phases:

Transcription: In the nucleus translations occur and DNA is written over to mRNA. mRNA transports the information of the DNA to the ribosomes where translation occurs.

Translation: In the ribosome, the message of the mRNA is read along with the creation of the sequence of particular amino acids. tRNA transports amino acid to the ribosome based on the code provided on mRNA to form a polypeptide chain.

5. What are some common diseases caused by protein misfolding?

Protein misfolding can lead to several diseases, including Some of the diseases that can arise as a result of protein misfolding include:

Parkinson's Disease: Alphasynuclein protein breakdowns and aggregates as Lewy bodies inhibit neural performance and cause cell death.

Prion Diseases: It changes ordinary proteins to its own using the recipe, and causes diseases such as CreutzfeldtJakob in human beings and the infamous mad cow disease in cattle.

6. What are proteins and why are they important?

Proteins are large, complex molecules made up of amino acids. They are crucial for life, serving as building blocks for cells and tissues, catalyzing biochemical reactions as enzymes, transporting molecules, and playing roles in immune function, cell signaling, and more. Their diverse functions make proteins essential for nearly all biological processes.

7. How do allosteric regulators affect protein function?

Allosteric regulators are molecules that bind to a protein at a site other than the active site, causing a conformational change that affects the protein's activity. This can either activate or inhibit the protein's function. Allosteric regulation allows for fine-tuning of protein activity in response to cellular needs and is a common mechanism in metabolic pathways.

8. What is the significance of the "lock and key" model in enzyme function?

The "lock and key" model describes how enzymes work by having a specific shape that complements their substrate, like a key fitting into a lock. This model explains the high specificity of enzymes - they can only catalyze reactions for substrates that fit precisely into their active site. While this model is simplified (enzymes are actually somewhat flexible), it helps explain the basics of enzyme-substrate interactions.

9. How do intrinsically disordered proteins challenge the structure-function paradigm?

Intrinsically disordered proteins (IDPs) lack a fixed 3D structure under physiological conditions. This challenges the traditional view that a specific structure is necessary for function. IDPs can perform important functions precisely because of their flexibility, often involved in signaling and regulation. They can adopt different conformations when interacting with different partners, allowing for versatility in function.

10. How do post-translational modifications affect protein function?

Post-translational modifications (PTMs) are chemical changes made to proteins after they are synthesized. These can include adding chemical groups (like phosphates or sugars), cutting the protein, or changing amino acids. PTMs can dramatically alter a protein's function, activity, stability, or localization within the cell. For example, phosphorylation often activates or deactivates enzymes in signaling pathways.

11. How does the primary structure of a protein determine its function?

The primary structure, or the specific sequence of amino acids, determines how the protein will fold into its secondary, tertiary, and quaternary structures. This folding ultimately determines the protein's shape and chemical properties, which in turn dictate its function in the body.

12. How do disulfide bridges contribute to protein structure?

Disulfide bridges are covalent bonds formed between the sulfur atoms of two cysteine amino acids. They help stabilize the tertiary and quaternary structure of proteins by creating strong links between different parts of the polypeptide chain or between different chains, enhancing the protein's stability and shape.

13. What role do hydrogen bonds play in protein structure?

Hydrogen bonds are weak interactions that form between hydrogen atoms and electronegative atoms like oxygen or nitrogen. In proteins, they play a crucial role in maintaining secondary structures (like alpha helices and beta sheets) and in stabilizing the overall 3D shape of the protein. They contribute significantly to the protein's final conformation and stability.

14. How does protein denaturation occur, and what are its consequences?

Protein denaturation is the process by which a protein loses its higher-level structures (secondary, tertiary, and quaternary) while maintaining its primary structure. It can be caused by heat, extreme pH, or certain chemicals. Denaturation disrupts the protein's 3D shape, usually resulting in a loss of its biological function. This is why cooking an egg changes its texture - the proteins in the egg white denature and form new interactions.

15. What is the relationship between a protein's structure and its function?

A protein's structure directly determines its function. The specific 3D arrangement of atoms in a protein creates unique surfaces and chemical environments that allow the protein to interact with other molecules in specific ways. For example, an enzyme's active site is shaped to fit its substrate perfectly, allowing it to catalyze a specific reaction.

16. What are the four levels of protein structure?

The four levels of protein structure are:

17. What is the difference between alpha helices and beta sheets?

Alpha helices and beta sheets are two common types of secondary protein structure. Alpha helices are spiral-shaped structures held together by hydrogen bonds within the same polypeptide chain. Beta sheets are flat, pleated structures formed by hydrogen bonds between different regions of the polypeptide chain or between different chains.

18. What is the relationship between protein structure and evolutionary conservation?

Protein structure is often more conserved across species than protein sequence. This is because structure is directly linked to function, and many proteins perform similar functions across different organisms. Regions of proteins that are crucial for function (like active sites in enzymes) tend to be highly conserved in both sequence and structure. Understanding structural conservation can provide insights into protein function and evolution.

19. What is protein folding, and why is it important?

Protein folding is the process by which a protein assumes its functional 3D structure from its initial linear chain of amino acids. It's crucial because the function of a protein depends on its specific 3D shape. Misfolded proteins can be non-functional or even harmful, potentially leading to diseases like Alzheimer's or cystic fibrosis.

20. What is the role of hydrophobic interactions in protein folding?

Hydrophobic interactions play a crucial role in protein folding. In an aqueous environment, hydrophobic amino acids tend to cluster together in the protein's interior, away from water. This "hydrophobic collapse" is often one of the first steps in protein folding and contributes significantly to the stability of the protein's tertiary structure.

21. How do amino acids form proteins?

Amino acids join together through peptide bonds to form long chains called polypeptides. These bonds form between the carboxyl group of one amino acid and the amino group of another, releasing a water molecule. Multiple polypeptides can then fold and interact to create the final protein structure.

22. What is the role of protein-nucleic acid interactions in cellular processes?

Protein-nucleic acid interactions are fundamental to many cellular processes:

23. How do protein-protein interaction networks contribute to cellular function?

Protein-protein interaction networks are fundamental to cellular function:

24. What is the significance of protein post-translational modifications in cell signaling?

Post-translational modifications (PTMs) are crucial in cell signaling:

25. What is the relationship between protein dynamics and function?

Protein dynamics, or the constant motion and flexibility of proteins, is crucial to their function:

26. What is the significance of protein-protein interactions?

Protein-protein interactions are essential for many cellular processes. They allow for:

27. How do prions challenge our understanding of protein function?

Prions are misfolded proteins that can induce other proteins of the same type to also misfold. This challenges our understanding because:

28. How do molecular chaperones prevent protein aggregation?

Molecular chaperones prevent protein aggregation by:

29. How do intrinsically disordered regions in proteins contribute to function?

Intrinsically disordered regions (IDRs) in proteins lack a fixed 3D structure but are functional. They contribute by:

30. How do protein conformational changes contribute to cellular signaling?

Protein conformational changes are key to cellular signaling:

31. What are the main differences between globular and fibrous proteins?

Globular proteins are roughly spherical in shape and are typically soluble in water. They often have dynamic functions like enzymes or signaling molecules. Fibrous proteins, on the other hand, are long, string-like structures that are usually insoluble in water. They often have structural roles, providing strength and support to cells and tissues. Examples include collagen in connective tissue and keratin in hair and nails.

32. How do proteins contribute to cell membrane structure and function?

Proteins are integral components of cell membranes, serving various roles:

33. How do intrinsically disordered proteins contribute to phase separation in cells?

Intrinsically disordered proteins (IDPs) play a key role in cellular phase separation:

34. How do chaperone proteins assist in protein folding?

Chaperone proteins help other proteins fold correctly by preventing inappropriate interactions between partially folded proteins. They can bind to exposed hydrophobic regions of unfolded proteins, shielding them from the cellular environment and giving them time to fold properly. Some chaperones can also actively assist in unfolding and refolding misfolded proteins.

35. What is the significance of protein domains in protein function?

Protein domains are distinct functional and/or structural units within a protein. They are significant because:

36. What is the role of water molecules in protein structure and function?

Water molecules play crucial roles in protein structure and function:

37. What is the significance of protein quaternary structure in cellular function?

Quaternary structure, the arrangement of multiple protein subunits, is significant because:

38. What is the significance of protein-ligand binding in drug design?

Protein-ligand binding is crucial in drug design because:

39. How do molecular recognition and specificity arise from protein structure?

Molecular recognition and specificity in proteins arise from:

40. How do protein-lipid interactions contribute to membrane function?

Protein-lipid interactions are essential for membrane function: