Proteins and Amino Acids

Proteins and Amino Acids

• Proteins are the unbranched polymers of L- α-amino acids.
• They are made up of 20 standard amino acids in different sequences and numbers.
• Proteins contain only L-α-amino acids.
• In most of the amino acids, an amino group is attached to α-carbon atom next to the carboxyl group.

Biomedical Importance of Proteins

• Enzymes: proteins that catalyze biochemical reactions.
• Immunoglobulins: proteins involved in immune response.
• Hormones: proteins that act as messengers in the body.
• Mechanical support: proteins provide structural support to cells and tissues.
• Receptors: proteins that receive and respond to signals.
• Transport: proteins involved in transporting molecules and ions.
• Storage proteins: proteins that store and release nutrients.
• Oxygen binding proteins: proteins that bind to oxygen.
• Energy source: proteins can be broken down to provide energy.

Composition of Proteins

• Proteins are composed of carbon, hydrogen, oxygen, and nitrogen.
• They also contain small amounts of sulfur and phosphorus.
• Few proteins contain other elements such as iodine, copper, manganese, zinc, and iron.
• The monomers of proteins are called amino acids.

Amino Acids

• There are 20 standard amino acids that serve as building blocks of body proteins.
• Some proteins contain additional amino acids that arise by modification of an amino acid already present in a peptide.
• Amino acids are classified based on their properties, such as size and shape, charge, polarity, hydrophobicity, and aromaticity.

Classification and Structure of Amino Acids

• Amino acids are classified into neutral, acidic, and basic amino acids.
• They are also classified into hydrophobic and hydrophilic amino acids.
• Polar and non-polar amino acids are another classification.
• Aromatic amino acids contain a ring structure.
• Sulphur-containing amino acids have a sulphur atom in their side chain.

Special Amino Acids

• Glycine is the smallest amino acid.
• Proline is an imino acid.
• Both glycine and proline are aliphatic, non-polar, and hydrophobic.

Aliphatic Amino Acids

• Aliphatic amino acids contain carbon and hydrogen atoms in their side chains.
• Examples include glycine, alanine, valine, leucine, and isoleucine.

Hydrophobic Amino Acids

• Hydrophobic amino acids do not contain oxygen or nitrogen atoms in their side chains.
• Examples include methionine, phenylalanine, tyrosine, and tryptophan.

Acidic Amino Acids

• Acidic amino acids contain a carboxyl group in their side chains.
• Examples include aspartic acid and glutamic acid.

Basic Amino Acids

• Basic amino acids contain an amino group in their side chains.
• Examples include lysine, arginine, and histidine.

Aromatic Amino Acids

• Aromatic amino acids contain a ring structure in their side chains.
• Examples include phenylalanine, tyrosine, and tryptophan.

Nutritional Importance

• Essential amino acids cannot be synthesized by the body and must be obtained from the diet.
• Examples include methionine, arginine, tryptophan, and threonine.
• Non-essential amino acids can be synthesized by the body.
• Examples include alanine, glycine, serine, and tyrosine.

Biosynthesis of Non-essential Amino Acids

• Non-essential amino acids are synthesized by transamination or de novo synthesis.
• Examples include alanine, asparagine, aspartate, and glutamate.

Essential Amino Acids are Difficult to Synthesize

• The number of enzymes required to synthesize essential amino acids varies from 5 to 13.
• Examples include arginine, histidine, isoleucine, and tryptophan.

Rare and Unusual Amino Acids

• Rare and unusual amino acids are not found in proteins but play important roles in metabolism.
• Examples include ornithine, citrulline, and arginino succinic acid.

New Amino Acids

• Two new amino acids have been recently described for protein synthesis.
• Selenocysteine is the 21st amino acid and occurs at the active site of several enzymes.
• Pyrrolysine is the 22nd amino acid and is found in archaea and bacteria.

Peptide Linkage

• The amino acid components of peptides and proteins are linked together by amide bonds.
• The sequence of a peptide is always written from the N-terminus to the C-terminus.

Biomedically Important Peptides

• Peptides have various biological functions, such as sweetening, antioxidant, and vasodilatory activities.
• Examples include aspartame, glutathione, and bradykinin.

Structural Organization of Proteins

• Protein structure is normally described at four levels of organization: primary, secondary, tertiary, and quaternary.
• Primary structure refers to the sequence of amino acids in a polypeptide chain.
• Secondary structure refers to the folding of short segments of polypeptide into geometrically ordered units.
• Tertiary structure refers to the complete folding pattern of a polypeptide chain.
• Quaternary structure refers to the interaction of subunits.

Protein Folding

• Protein folding is the process by which a polypeptide chain assumes its native conformation.
• Protein folding enzymes, such as disulfide isomerase and cis-trans prolyl isomerase, aid the folding process.
• Chaperons, such as chaperonins, also aid protein folding by preventing the formation of aggregates.

Denaturation of Proteins

• Denaturation is the loss of native conformation of a protein.
• Denaturation can be caused by high temperature, extreme pH, urea, guanidine, UV radiation, sonication, and vigorous shaking.
• Denatured proteins are inactive and have altered physical, chemical, and biological properties.