TLDR;
This video provides a detailed overview of proteins, starting with their etymology and importance, and moving through their structure, classification, and functions. It covers the building blocks of proteins (amino acids), the different levels of protein structure (primary, secondary, tertiary, and quaternary), and the various roles proteins play in biological systems, including enzymatic activity, hormonal regulation, structural support, immunity, and transport.
- Proteins are the most important and abundant biomolecules in protoplasm, essential for all cellular functions.
- Amino acids, the building blocks of proteins, are classified into standard (20) and non-standard types, with the 20 standard amino acids being crucial for protein synthesis.
- Proteins exhibit four levels of structural organization: primary (linear sequence of amino acids), secondary (alpha helices and beta sheets), tertiary (3D folding), and quaternary (arrangement of multiple polypeptide chains).
- Proteins are categorized based on their shape (fibrous or globular) and function (structural or functional), highlighting their diverse roles in biological processes.
Introduction to Proteins [0:00]
The video begins by explaining that the word "protein" comes from the Greek word "Protos," meaning "first," highlighting its importance among biomolecules. Proteins are the most abundant biomolecules in protoplasm and are essential for all cellular functions. Examples of protein functions include oxygen transport by hemoglobin in red blood cells and muscle contraction/relaxation facilitated by actin and myosin. Proteins determine the functional specificity of cells, as every gene product is a protein. They also play key roles in chemical coordination through hormones, metabolic processes via enzymes, and immunity through antibodies. Many bodily structures, such as skin, bones, hair, and nails, are also made of protein. Proteins constitute more than 50% of the body's dry mass.
Protein Structure and Amino Acids [1:29]
Proteins are polymer molecules called heteropolymers, composed of 20 different types of amino acids. Unlike starch, which consists of repeating glucose units, proteins are made from diverse amino acids like methionine, glutamine, leucine, and aspartic acid. These amino acids polymerize to form proteins. There are two types of amino acids: standard (primary) and non-standard (secondary), with 20 standard amino acids used in protein synthesis. The number and order of these amino acids vary in each protein, similar to how the arrangement of alphabets varies in different words.
Standard and Non-Standard Amino Acids [2:48]
The video explains that the 20 standard amino acids are encoded by genetic code and used to make proteins. Non-standard amino acids, such as citrulline and ornithine, are not used in protein synthesis but have other biological roles, like in the urea cycle. GABA (Gamma-Aminobutyric Acid) is another example of a non-standard amino acid that functions as a neurotransmitter in the brain but is not used to build proteins.
Typical Amino Acid Structure [4:07]
A typical amino acid structure consists of five parts: an alpha carbon, a carboxyl group (COOH), an amino group (NH2), a hydrogen atom, and a variable R group. The alpha carbon is directly attached to the carboxylic acid group, which is given the highest priority in chemical nomenclature. The amino group is basic and acts as a proton acceptor, while the carboxyl group is acidic and acts as a proton donor. Amino acids are named for containing both an amino and an acidic group. The properties of amino acids are determined by their R groups, which can be acidic, basic, polar, non-polar, or aromatic.
R Groups and Elemental Composition [6:06]
The properties of amino acids depend on their variable R groups, which determine whether an amino acid is acidic, basic, polar, non-polar, or aromatic. There are five varieties of amino acids based on their R groups. The elemental composition of amino acids includes carbon, hydrogen, oxygen, and nitrogen. Sulfur is present in only two amino acids: methionine and cysteine.
Peptide Bonds and Polypeptides [8:42]
Amino acids join together to form proteins through peptide bonds, which are covalent bonds between the carboxyl group of one amino acid and the amino group of another. This reaction releases a water molecule. A dipeptide is formed from two amino acids, a tripeptide from three, an oligopeptide from two to ten, and a polypeptide from more than ten. A polypeptide with a molecular weight exceeding 10,000 is considered a protein. Examples include antidiuretic hormone (9 amino acids, an oligopeptide), oxytocin (9 amino acids, a short peptide), insulin (51 amino acids, a polypeptide hormone), and growth hormone (199 amino acids, a protein hormone).
Protein Terminals and Amino Acid Isomers [11:04]
A protein chain always has a COOH group at one end, called the C-terminal, and an NH2 group at the other end, called the N-terminal. Amino acids can exist in two isomeric forms: L-amino acids and D-amino acids. The body uses L-amino acids to make proteins, while D-amino acids are found in some bacterial cell walls.
Levels of Protein Structure [12:16]
Proteins have four levels of structural organization: primary, secondary, tertiary, and quaternary. The primary structure is the linear sequence of amino acids with no folding. The secondary structure is formed when the chain folds into alpha helices or beta sheets, stabilized by hydrogen bonds. The tertiary structure is the further folding of the polypeptide into a ball-like or globular structure, stabilized by ionic and disulfide bonds. The quaternary structure involves the combination of multiple tertiary structures.
Stabilizing Forces and Examples of Protein Structures [13:00]
The stabilizing force in the primary structure is the covalent peptide bond. In the secondary structure, it is the hydrogen bond. Tertiary structures are stabilized by hydrogen bonds, ionic bonds, and disulfide bonds. Quaternary structures are held together by hydrophobic interactions. Examples of proteins include insulin, fibrous proteins like collagen and elastin (found in bones and skin), keratin (found in hair and nails), and myoglobin (a protein inside muscle that stores oxygen). Hemoglobin has four polypeptide chains (two alpha and two beta) and is an example of a quaternary structure.
Stability and Shape-Based Classification of Proteins [16:05]
The primary structure is the most stable, followed by secondary, tertiary, and quaternary structures, which is the least stable. Proteins are classified based on their shape as either fibrous (fiber-like) or globular (ball-like). Fibrous proteins often have secondary structures, while globular proteins can have secondary, tertiary, or quaternary structures. Fibrous proteins are often structural, while globular proteins are often functional.
Structural vs. Functional Proteins and Their Roles [17:33]
Structural proteins, like those in hair and bones, create structures. Functional proteins, like enzymes and hormones, perform functions. Enzymes, which drive the body's metabolism, are globular proteins. Hormones, responsible for chemical coordination, are mostly proteins. Structural components like skin, nails, hair, and bones contain fibrous proteins. Antibodies, which provide immunity, are globular proteins. Proteins also transport respiratory gases, such as oxygen stored by myoglobin in muscles. About 75% of dietary amino acids are used to make new proteins, while 25% can be used for energy production.