Chapter 5 Part B: (Protein Structure and Function: Study Notes for General Biology)

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Proteins: Diversity of Structure and Function

Introduction to Proteins

Proteins are essential biological macromolecules that perform a vast array of functions in living organisms. Their structural complexity enables them to participate in nearly every cellular process.

Proteins are the most structurally complex biological molecules.
Humans produce tens of thousands of different proteins, each with a specific structure and function.
Nearly every function of a living being depends on proteins.

Protein Functions

Major Functions of Proteins

Proteins carry out a wide range of functions, unmatched by any other class of macromolecule.

Enzymes (Catalysts): Accelerate chemical reactions (e.g., digestive enzymes).
Defensive Proteins: Protect against disease (e.g., antibodies).
Storage Proteins: Store amino acids (e.g., casein in milk, ovalbumin in eggs).
Transport Proteins: Transport substances (e.g., hemoglobin transports oxygen, membrane channels move molecules across membranes).
Hormonal Proteins: Coordinate organismal activities (e.g., insulin regulates blood sugar).
Receptor Proteins: Respond to chemical stimuli (e.g., hormone and neurotransmitter receptors).
Motor and Motile Proteins: Enable movement (e.g., actin and myosin in muscle contraction).
Structural Proteins: Provide support (e.g., collagen in connective tissue, keratin in hair and nails).

Table: Major Classes of Protein Functions

Protein Class	Function	Example
Enzymatic	Selective acceleration of chemical reactions	Digestive enzymes
Defensive	Protection against disease	Antibodies
Storage	Storage of amino acids	Casein, ovalbumin
Transport	Transport of substances	Hemoglobin, membrane transport proteins
Hormonal	Coordination of activities	Insulin
Receptor	Response to chemical stimuli	Neurotransmitter receptors
Motor/Motile	Movement	Actin, myosin
Structural	Support	Collagen, keratin

Polypeptides, Proteins, and Amino Acids

Basic Structure

Proteins are polymers made from amino acid monomers, linked by peptide bonds. The terms polypeptide and protein are often used interchangeably, though a protein may consist of one or more polypeptides folded into a functional shape.

Amino acids (AAs): Organic molecules with both a carboxyl group (-COOH) and an amino group (-NH2).
Each amino acid has a unique side chain (R group) that determines its chemical properties.
Amino acids are linked by peptide bonds to form polypeptides.

General structure of an amino acid:

Central carbon (α-carbon)
Amino group ()
Carboxyl group ()
Hydrogen atom
R group (side chain)

Formula:

Example: Glycine has a hydrogen as its R group; serine has a hydroxymethyl group.

Properties of Amino Acids

There are 20 common amino acids used in proteins.
R groups can be non-polar (hydrophobic), polar (hydrophilic), or charged (acidic/basic).
The sequence and properties of amino acids determine protein structure and function.

Peptide Bonds and Polypeptide Formation

Formation of Peptide Bonds

Amino acids are joined by peptide bonds through a dehydration reaction, forming a covalent linkage between the carboxyl group of one amino acid and the amino group of the next.

Polypeptides have a N-terminus (amino end) and a C-terminus (carboxyl end).
Polypeptides can range from a few to thousands of amino acids in length.

Peptide bond formation equation:

Levels of Protein Structure

Primary Structure

The primary structure of a protein is its unique linear sequence of amino acids, determined by genetic information.

Analogous to the order of letters in a word.
Changes in primary structure can affect protein function (e.g., sickle-cell disease).

Secondary Structure

The secondary structure refers to regular, repeating structures formed by hydrogen bonds between backbone atoms in the polypeptide chain.

Major types: α-helix (coiled structure) and β-pleated sheet (folded structure).
Stabilized by hydrogen bonding.

Tertiary Structure

The tertiary structure is the overall three-dimensional shape of a polypeptide, resulting from interactions among R groups.

Interactions include hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces, and covalent disulfide bridges.
Determines the protein's functional properties.

Quaternary Structure

The quaternary structure arises when two or more polypeptide chains (subunits) associate to form a functional protein complex.

Subunits are held together by non-covalent interactions.
Not all proteins have quaternary structure (e.g., hemoglobin has four subunits).

Table: Four Levels of Protein Structure

Level	Description	Stabilizing Forces
Primary	Linear sequence of amino acids	Peptide bonds
Secondary	Regular repeating structures (α-helix, β-sheet)	Hydrogen bonds
Tertiary	3D folding of polypeptide	R group interactions (hydrogen, ionic, hydrophobic, van der Waals, disulfide bridges)
Quaternary	Association of multiple polypeptides	Non-covalent interactions

Protein Structure and Function Relationship

Specificity and Binding

The function of a protein depends on its ability to recognize and bind to other molecules, which is determined by its three-dimensional structure.

Example: Endorphin receptors bind endorphins and morphine due to structural compatibility.
Enzymes bind specific substrates to catalyze reactions.

Factors Affecting Protein Structure

Denaturation and Environmental Effects

Protein structure can be affected by physical and chemical conditions such as temperature, pH, and salt concentration. Loss of native structure is called denaturation, which inactivates the protein.

High temperature, extreme pH, or chemicals can denature proteins.
Denatured proteins lose their functional shape.

Protein Folding and Misfolding

Proteins fold into their functional shapes through complex processes, often involving intermediate structures. Misfolding can lead to diseases such as Alzheimer's and Parkinson's.

Protein folding is guided by the primary sequence but is difficult to predict.
Chaperone proteins may assist in proper folding.

Case Study: Sickle-Cell Disease

Impact of Primary Structure Change

A single amino acid change in the primary structure of hemoglobin leads to sickle-cell disease, affecting protein function and causing health problems.

Normal hemoglobin has glutamic acid; sickle-cell hemoglobin has valine at a specific position.
This change causes hemoglobin molecules to aggregate, reducing oxygen-carrying capacity.

Key Concepts Summary

Proteins have diverse structures and functions.
Protein function depends on structure, which is determined by amino acid sequence.
Four levels of protein structure: primary, secondary, tertiary, quaternary.
Environmental factors can affect protein structure and function.
Protein folding is crucial for function; misfolding can cause disease.