Proteins
Chemistry of Proteins
Learning Objectives
Categorize amino acids based on functional group and polarity
Amino acids have functional groups like amino and carboxyl groups.
Explain the amphoteric property of amino acids
Due to the presence of amino and carboxyl groups.
Illustrate the formation of peptides from different amino acids
Determine amino acid sequences and peptide names
Importance of small peptides
Describe levels of protein structure and their interrelationship
Relate protein structure to physiological function
Differentiate hydrolysis and denaturation of proteins
Overview
Protein: Crucial biomolecules in the body, derived from the Greek word proteios.
Proteins: account for 15% of cell mass and half of its dry weight.
Proteins: are unbranched polymers of amino acids linked by peptide bonds.
Structure: Collagen and keratin provide structural support as fibrous proteins.
Catalysis: Enzymes catalyze reactions in organisms for various functions.
Movement: Proteins like myosin and actin enable muscle mobility.
Transport: Hemoglobin transports molecules in blood and across membranes.
Amino Acids: Building blocks of proteins containing carboxyl and amino groups.
Essential Amino Acids: Body cannot synthesize them adequately; must be obtained from the diet.
Standard Amino Acids: Humans have 20 standard amino acids with distinct side chains called.
Stereoisomers: Standard amino acids have stereogenic centers, existing as D or L enantiomers.
Fischer Projection: Representation method for D and L stereoisomers of amino acids.
Side Chains: Distinguish amino acids and group them by the polarity of their side chains.
essential amino acids: 10 amino acids crucial for a child's normal growth and development.
Arginine is essential for infants for normal growth but becomes nonessential as they grow.
Premature infants may lack sufficient quantities of some nonessential amino acids, making them conditionally essential until maturity.
Conditionally essential amino acids must be obtained through the diet, found in human milk and infant formula.
Complete dietary proteins contain all essential amino acids in adequate amounts.
Proteins from animal sources are usually complete, while incomplete proteins lack one or more essential amino acids.
Gelatin is an incomplete protein with tryptophan as its limiting amino acid.
Proteins from plant sources are generally incomplete, with lysine, methionine, and tryptophan as common limiting amino acids.
Soy protein is a complete plant protein, while a mix of plant proteins like rice and beans can provide complete dietary protein.
Eating rice and beans together forms complementary dietary protein.
Amphoteric Properties of SAAs
Amino acids have both acidic (-COOH) and basic (-NH2) groups in one molecule.
At neutral pH, -COOH loses a proton becoming carboxylate, while -NH2 accepts a proton becoming a quarternary ammonium ion.
In aqueous solutions, -COOH is deprotonated, and -NH2 is protonated in SAAs.
pH Effects on Amino Acid Structure
Amino acid structure varies at different pH levels.
In acidic solutions, carboxyl group ionizes first, while in basic solutions, the ammonium group ionizes first.
Different structures are observed at varying pH levels for acidic and basic amino acids.
Specific Ionization Patterns
Histidine, lysine, and arginine have distinct ionization patterns at different pH levels.
Histidine's imidazolium group ionizes first, while lysine and arginine have the quat. Ammonium group of the a-carbon ionizing first.
Acidic to basic solution pH solutions
Acidic amino acids are amphoteric, can be both acidic and basic
Carboxyl group can be acidic or deprotonated near neutral pH
Protonated NH2 becomes positively charged NH3
Conjugate acid pattern and conjugate base formation explained
Peptides
Peptide formation by condensing carboxyl group and amino group to form a peptide bond
Nomenclature of peptides
C-terminal amino acid residue keeps its full name
Other residues named with -yl suffix
Naming sequence starts at the N-terminal acid residue
Isomeric peptides contain similar amino acid residues but in different orders
Oxytocin and Vasopressin are small peptides with physiological importance
Functions and roles of oxytocin and vasopressin explained
Glutathione as an antioxidant peptide
Primary structure of proteins defined by the number, kind, and sequence of amino acids
Importance of primary structure in determining the protein's three-dimensional structure and function
Importance of knowing the primary structure of a protein
Primary structure provides the sequence of amino acids in a protein.
Helps in understanding the function and properties of the protein.
Example of amino acid sequences in different animals provided.
Secondary Structure of Proteins
Refers to the regular localized arrangement of the polypeptide backbone.
Stabilized by hydrogen bonds between C=O and N-H groups.
Examples include alpha-helix and beta-pleated sheet structures.
Factors disrupting the alpha-helix structure
Presence of proline, causing a break in the helix.
Adjacent similarly charged groups leading to electrostatic repulsion.
Adjacent bulky groups causing steric repulsion.
Tertiary Structure
Overall three-dimensional structure of proteins.
Results from interactions of side chains of amino acid residues and prosthetic groups.
Quaternary Structure
Non-covalent association of protein subunits into a supramolecule.
Not all proteins have quaternary structures.
Different quaternary structures based on the number of subunits
Examples: Dimer (Insulin), Trimer (Thrombin), Tetramer (Hemoglobin).
Protein Hydrolysis
Causes disruption of peptide bonds, liberating free amino acids.
Can be carried out using strong acids, bases, or enzymes (proteases).
Protein Denaturation
Leads to the unfolding of a protein's three-dimensional structure.
Results in the loss of biological activity.
Denaturation can be reversible or irreversible.
Factors causing denaturation
Denaturing agents like heat, urea, detergents, acids, bases, salts, reducing agents, heavy metals, and alcohol.