Proteins, Amino Acids, and Metabolism: Foundations of Cellular and Molecular Biology

Proteins: Structure and Function

Introduction to Proteins

Proteins are the most important chemicals in life, serving as the primary macromolecules responsible for the vast majority of cellular functions. Every biological activity in a cell is carried out by one or more proteins, which act as molecular machines specialized for a variety of tasks.

• Definition: Proteins are polymers made of amino acids linked by peptide bonds.

• Functions: Proteins perform structural roles, catalyze chemical reactions (as enzymes), transport substances, regulate cellular processes, and more.

• Examples: Hemoglobin (oxygen transport), enzymes (catalysis), antibodies (immune defense).

Amino Acids: Building Blocks of Proteins

Proteins are chains of small molecules called amino acids. There are 20 major amino acids, each with a unique side chain (R group) that determines its chemical properties.

• Structure: Each amino acid contains a central carbon atom bonded to an amino group (NH2), a carboxyl group (COOH), a hydrogen atom, and a variable R group.

• Classification: Amino acids are grouped by chemical properties: hydrophobic (nonpolar), hydrophilic (polar), acidic, and basic.

Group	Amino Acids	Properties
Hydrophobic (Nonpolar)	Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Proline	Nonpolar side chains, often found in the interior of proteins
Hydrophilic (Polar, Uncharged)	Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine	Polar side chains, often found on protein surfaces
Acidic	Aspartic acid, Glutamic acid	Negatively charged at physiological pH
Basic	Lysine, Arginine, Histidine	Positively charged at physiological pH

Peptide Bonds and Protein Structure

A peptide bond is a covalent bond that links two amino acids together, forming a polypeptide chain. Proteins may consist of one or multiple polypeptide subunits.

• Primary Structure: Sequence of amino acids in a polypeptide.

• Secondary Structure: Local folding patterns (e.g., alpha helices, beta sheets).

• Tertiary Structure: Overall 3D shape of a single polypeptide.

• Quaternary Structure: Arrangement of multiple polypeptide subunits.

Enzymes: Catalysts of Biological Reactions

Enzyme Structure and Function

Enzymes are proteins that catalyze chemical reactions, increasing reaction rates by lowering activation energy. They are highly specific for their substrates and often require cofactors for activity.

• Active Site: The region of the enzyme where substrate binding and catalysis occur.

• Cofactors: Non-protein chemical helpers (e.g., metal ions like Mg2+, Zn2+, or organic molecules) required for enzyme activity.

• Regulation: Enzyme activity can be regulated by other proteins, chemical modifications (e.g., phosphorylation), or environmental conditions (pH, temperature).

Example: TEV Protease

The TEV protease enzyme contains a catalytic triad of three key amino acids (aspartate, histidine, cysteine) in its active site, which work together to cleave peptide bonds in substrate proteins.

Metabolism: Chemical Reactions in Cells

Overview of Metabolism

Metabolism is the sum of all chemical reactions occurring in an organism. These reactions are organized into metabolic pathways, which transform molecules through a series of enzyme-catalyzed steps.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones, requiring energy input.

Metabolic Pathways

Metabolic pathways consist of multiple steps, each catalyzed by a specific enzyme. The product of one reaction becomes the substrate for the next.

Step	Enzyme	Substrate	Product
1	Enzyme 1	Molecule A	Molecule B
2	Enzyme 2	Molecule B	Molecule C
3	Enzyme 3	Molecule C	Molecule D (end product)

Example: Glycolysis

Glycolysis is a metabolic pathway that breaks down glucose to produce energy. It involves a series of enzyme-catalyzed steps:

• Substrate: Glucose

• Products: Pyruvate, ATP, NADH

Key reactions in glycolysis:

• $\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

Metabolic Intermediates and Biosynthesis

Metabolic intermediates produced during pathways like glycolysis can be used to synthesize other important biomolecules, such as amino acids, nucleotides, and lipids.

• Example: 3-phosphoglycerate (from glycolysis) can be converted into serine, an amino acid.

Summary Table: Key Terms and Concepts

Term	Definition	Example/Application
Protein	Polymer of amino acids with diverse cellular functions	Enzymes, structural proteins, antibodies
Amino Acid	Building block of proteins; 20 major types	Glycine, serine, lysine
Enzyme	Protein that catalyzes chemical reactions	DNA polymerase, TEV protease
Metabolism	Sum of all chemical reactions in an organism	Glycolysis, citric acid cycle
Metabolic Pathway	Series of enzyme-catalyzed reactions transforming molecules	Glycolysis, fatty acid synthesis

Additional info: These notes expand on the original content by providing definitions, examples, and structured tables for clarity. The chemical structures of amino acids and the glycolysis pathway are described in text, as images cannot be rendered in HTML tables.