Protein: Definition, Structure, and Examples

Protein Definition

In biology and biochemistry, Proteins are defined as a chain of amino acids joined together by peptide bonds. They are nutrient-rich macromolecules that provide 4cal/gram food energy.

Etymology: The word protein has derived from the French word “protein” and Greek word “protos”, meaning “first”.

What is Protein?

Proteins are a type of biomolecules made up of amino acid chains. Amino acids are considered a monomeric unit of proteins that join together by peptide bonds to form a polypeptide chain or protein. All living organisms produce different biomolecules such as carbohydrates, lipids, and nucleic acids.

Proteins are considered as one of the major biomolecules that are made up of carbon, hydrogen, oxygen, sulfur, nitrogen, and sometimes phosphorus.

They have a different composition, sequence, function, and spatial configuration than other biomolecules. The protein folding and its particular 3D configuration is determined by the amino acid sequence. The specific protein configuration further determines the activity and function of the protein.

Proteins have various important functions in a biological system, for example- some proteins are structural material whereas others help in transport, immunity, and act catalysts (enzymes).

Protein vs Peptide

The amino acid consists of a compound called a peptide, which is connected by a peptide bond. The bond is formed between the carboxyl group of one amino acid and the amino group of another amino acid. When a peptide is composed of two amino acids, it is considered a dipeptide.

Similarly, peptides are made up of several amino acid residues from an unbranched linear chain that is called a polypeptide. The polypeptide may have as many as 4,000 amino acid residues or sometimes contain only 20 to 30 amino acid residues. A complex of polypeptides or 3D structure of a polypeptide is called a protein.

Protein Structure

The amino acid residues are the monomeric unit of protein. These amino acids are joined together by peptide bonds. All polypeptide chains are composed of different amino acids where the DNA specifies the sequence of amino acids in the protein.

There are usually 20 standard amino acids that define the genetic code however the genetic code of some Archaea and other organisms specifies more amino acids. The structure and function of a particular protein are determined by the sequence of amino acids.

The proteins can form a complex either with another protein or with any other biomolecule. The co-factors or prosthetic groups are also a type of non-peptide group in a protein.

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Based on the structural proteins are divided into four types-

(1) Primary Structure

(2) Secondary Structure

(3) Tertiary Structure

(4) Quaternary Structure

The unbranched sequence of amino acids in a polypeptide chain is concerned as the primary structure. The secondary structure is stabilized by hydrogen bonds that include α-helix, β-sheet, and turns and loops.

The tertiary structures are also called folds that are stabilized by various nonlocal interactions such as disulfide bonds, salt bridges, etc. The protein complexes are often found in the quaternary structure.

The tertiary and quaternary structure is also known as conformations and the transition between these two structures are called conformational change. The binding of substrate to the protein induces conformational changes in the structure of the substrate.

Types of Protein

Based on the forms and functions of proteins, they are categorized into different types, such as globular proteins, fibrous proteins, or membrane proteins. Most of the enzymes are globular proteins whereas fibrous proteins have structural roles such as collagen, keratin, etc.

The membrane receptors or channel proteins are a type of membrane proteins that helps in the transportation of polar molecules.

Biosynthesis of Proteins

Proteins are synthesized inside the cell in most living organisms. In eukaryotes, the process starts from the nucleus, where the transcription completes and results in the formation of an mRNA script.

Later, the mRNA script moves into the cytoplasm, and the ribosomes along with tRNA use this script for translation into an amino acid sequence. In the case of prokaryotes, the nucleus is absent therefore the transcription also completes in the cytoplasm of a prokaryotic cell.

The process of creating protein is called the biosynthesis of protein synthesis. The process involves transcription, amino acid synthesis, and translation. The set of biochemical reactions that produce amino acids from carbon sources is called amino acid synthesis.

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The process includes two major processes named transcription and translation. The conversion of DNA into mRNA is called transcription, this mRNA molecule act as a template for translation.

The process of translating a nucleotide sequence into an amino acid sequence is called translation. The process completes in the cytoplasm of the cell where the ribosomes are located.

The translation completes in the following steps-

(1) The binding of mRNA to a ribosome

(2) The ribosome begins matching tRNA anticodon sequences to the mRNA codon

(3) The amino acid carried by the tRNA gets added to the elongating chain

(4) A stop codon releases the polypeptide chain and the mRNA. The process also includes proteolysis, post-translational modification, and protein folding.

Protein Degradation

Different kinds of proteins have a different lifespans. Some proteins remain for years while some proteins degrade within few minutes of production. The misfolded proteins can cause instability and dysfunctions in the body thus they are degraded to prevent the causes.

Proteins may be degraded in proteasomes or in eukaryotes the proteins are degraded in lysosomes of the cell. The lysosome is a membrane-bound organelle that contains digestive enzymes such as proteases that helps in the digestion of endocytosed proteins.

Biological Functions of Protein

Proteins are considered one of the major biomolecules. They have various significant roles in the biological system. Structural proteins such as keratins are the structural components of hairs, and actin and myosin are the components of muscles.

Most of the enzymes are proteins that catalyze various biochemical reactions. Some other important functions of protein include transportation, antibodies, and as regulators of gene expression. Some of the proteins are acquired through diet whereas others are biosynthesized in the cell. Thus the dietary proteins serve as a food source.

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