Protein Structure and Function: Study Notes for General Biology

Protein Structure and Function

Introduction

Proteins are the most abundant and versatile macromolecules in living organisms. They are essential for nearly every biological process, and their structure determines their diverse functions in cells. This chapter explores the composition, structure, and roles of proteins, focusing on the relationship between amino acid sequence and protein function.

Amino Acids: The Building Blocks of Proteins

Structure of Amino Acids

Amino acids are organic molecules that serve as the monomers of proteins. Each amino acid shares a common structure but differs in its side chain, which imparts unique chemical properties.

• Central Carbon (α-carbon): The core of every amino acid, bonded to four groups.

• Amino Group (–NH2): Acts as a base, accepting protons.

• Carboxyl Group (–COOH): Acts as an acid, donating protons.

• Hydrogen Atom: Attached to the central carbon.

• R Group (Side Chain): Unique to each amino acid; determines its properties.

In aqueous environments, amino acids exist in ionized form, with the amino group gaining a proton and the carboxyl group losing one.

Classification of Amino Acids

The 20 standard amino acids are classified based on the chemical nature of their side chains:

• Charged (Acidic and Basic): Side chains carry a positive or negative charge; hydrophilic.

• Polar (Uncharged): Side chains contain polar covalent bonds; hydrophilic.

• Nonpolar: Side chains lack polar bonds; hydrophobic.

This classification affects how amino acids interact within proteins and with their environment.

Polymerization of Amino Acids: Formation of Polypeptides

Peptide Bond Formation

Amino acids are linked together by peptide bonds through a condensation (dehydration) reaction:

• The carboxyl group of one amino acid reacts with the amino group of another, releasing water and forming a covalent bond.

The resulting chain of amino acids is called a polypeptide, which has structural polarity (N-terminus to C-terminus).

Equation for Peptide Bond Formation:

$ext{Amino Acid}_1 + ext{Amino Acid}_2 ightarrow ext{Dipeptide} + ext{H}_2 ext{O}$

Levels of Protein Structure

Primary Structure

The primary structure of a protein is its unique sequence of amino acids. This sequence is determined by genetic information and dictates all higher levels of structure.

• Example: Somatostatin sequence: Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser

• Even a single amino acid change can alter protein function.

Secondary Structure

The secondary structure arises from hydrogen bonding between backbone groups (carbonyl and amino groups) within the polypeptide chain.

• α-Helix: A coiled structure stabilized by hydrogen bonds.

• β-Pleated Sheet: Sheet-like arrangement formed by hydrogen bonds between parallel or antiparallel strands.

Tertiary Structure

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

• Includes hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions, and van der Waals forces.

• Determines the protein's functional conformation.

Quaternary Structure

Some proteins consist of two or more polypeptide chains (subunits) that assemble into a functional complex. This is known as quaternary structure.

• Example: Hemoglobin is composed of four polypeptide subunits.

Protein Folding and Stability

Denaturation and Chaperones

Protein function depends on proper folding. Disruption of hydrogen bonds or disulfide bridges can lead to denaturation, rendering the protein nonfunctional.

• Molecular chaperones assist in proper folding by preventing inappropriate interactions.

Prions

Some proteins, called prions, can misfold and induce other proteins to misfold, leading to infectious diseases.

• Example: Prion diseases such as Creutzfeldt-Jakob disease.

Protein Function in Cells

Diverse Roles of Proteins

Proteins perform a wide variety of functions essential for cellular life:

• Catalysis: Enzymes accelerate chemical reactions.

• Defense: Antibodies protect against pathogens.

• Movement: Motor proteins enable cellular and organismal movement.

• Signaling: Hormones and receptors transmit signals.

• Structure: Structural proteins provide support and shape.

• Transport: Transport proteins move substances across membranes.

Enzymes: Biological Catalysts

Enzymes are proteins that catalyze biochemical reactions by lowering activation energy and increasing reaction rates.

• Enzymes are highly specific for their substrates.

• Substrates bind to the enzyme's active site, where the reaction occurs.

Equation for Enzyme-Catalyzed Reaction:

$ext{E} + ext{S} ightarrow ext{ES} ightarrow ext{E} + ext{P}$

Where E = enzyme, S = substrate, ES = enzyme-substrate complex, P = product.

Case Study: SARS-CoV-2 Spike Protein

Role in Viral Entry

The spike protein (S) of SARS-CoV-2 is a key example of protein function. It mediates viral entry into host cells by binding to the human ACE2 receptor.

• Other viral proteins include membrane protein (M), envelope protein (E), nucleoprotein (N), and ribonucleic acid (RNA).

• Understanding protein structure is crucial for developing antiviral strategies.

Table: Classification of Amino Acids by Side Chain Properties

Type	Properties	Examples
Charged (Acidic)	Negative charge, hydrophilic	Aspartic acid, Glutamic acid
Charged (Basic)	Positive charge, hydrophilic	Lysine, Arginine, Histidine
Polar (Uncharged)	Polar covalent bonds, hydrophilic	Serine, Threonine, Asparagine, Glutamine
Nonpolar	No polar bonds, hydrophobic	Valine, Leucine, Isoleucine, Methionine, Phenylalanine

Summary

Proteins are fundamental to life, with their structure intricately linked to their function. Understanding the levels of protein structure, the properties of amino acids, and the mechanisms of protein folding and catalysis is essential for studying biology and biochemistry.