Proteins: The Most Abundant and Versatile Macromolecules

Overview of Proteins

Proteins are essential macromolecules that perform a vast array of functions in living organisms. They are composed of 20 different amino acids, each with unique side chains, and account for more than 50% of the dry mass of most cells. The structure and function of proteins are determined by the sequence and chemical properties of these amino acids.

• Proteins are polymers made from amino acid monomers.

• They possess three key attributes of life: information, replication, and evolution.

• Proteins speed up chemical reactions, provide structural support, and play roles in defense, storage, transport, communication, and movement.

Main Functions of Proteins in Cells

Proteins are classified based on their diverse roles in living cells. The following table summarizes the major types and their functions:

Amino Acids: Building Blocks of Proteins

Structure of Amino Acids

All amino acids share a common structure centered around a central carbon atom (α-carbon) bonded to four different groups:

• A hydrogen atom (H)

• An amino group (–NH2)

• A carboxyl group (–COOH)

• A variable side chain (R group) that determines the properties of each amino acid

The general structure can be represented as:

(with the R group attached to the central carbon)

Classification of Amino Acids

Amino acids are classified based on the chemical nature of their side chains (R groups):

• Nonpolar (hydrophobic)

• Polar (hydrophilic)

• Acidic (negatively charged)

• Basic (positively charged)

Example: Glycine is the simplest amino acid with a hydrogen as its side chain; glutamic acid has a carboxyl group in its side chain, making it acidic.

Protein Structure: Four Levels of Organization

Primary Structure

The primary structure of a protein is its unique sequence of amino acids, determined by genetic information. This sequence dictates all higher levels of structure and ultimately the protein's function.

Secondary Structure

The secondary structure refers to local folding of the polypeptide chain into structures such as:

• α-helix: A coiled structure stabilized by hydrogen bonds

• β-pleated sheet: Sheet-like structure formed by hydrogen bonds between parallel segments

These structures are stabilized by hydrogen bonds between the backbone atoms (not the side chains).

Tertiary Structure

The tertiary structure is the overall three-dimensional shape of a polypeptide, resulting from interactions among side chains (R groups), including:

• Hydrogen bonding

• Hydrophobic interactions

• Van der Waals interactions

• Disulfide bridges (covalent bonds between cysteine residues)

• Ionic bonds

Quaternary Structure

The quaternary structure arises when two or more polypeptide subunits assemble into a functional protein complex. Examples include:

• Hemoglobin: Composed of four subunits, each capable of binding oxygen

• Collagen: A fibrous protein with three polypeptide chains wound together

Protein Structure and Function Relationship

The specific shape of a protein determines its function. Even a single amino acid change can have dramatic effects, as seen in sickle-cell hemoglobin, where a mutation causes hemoglobin molecules to aggregate, reducing oxygen-carrying capacity.

Protein Denaturation

Proteins can lose their native structure and function through denaturation, which can be caused by changes in pH, salt concentration, temperature, or other environmental factors. Denatured proteins are usually biologically inactive, though sometimes renaturation is possible.

Enzymes: Biological Catalysts

Enzymes are proteins that act as catalysts, speeding up chemical reactions without being consumed. They are highly specific for their substrates and can be used repeatedly to carry out the processes of life.

Nucleic Acids: Information Storage and Transfer

What is a Nucleic Acid?

Nucleic acids are polymers made of nucleotide monomers. They store and transmit genetic information in all living organisms.

• DNA (deoxyribonucleic acid): Stores genetic information; contains deoxyribose sugar

• RNA (ribonucleic acid): Involved in protein synthesis and gene regulation; contains ribose sugar

Structure of Nucleotides

Each nucleotide consists of three components:

• A phosphate group

• A five-carbon sugar (ribose or deoxyribose)

• A nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil replaces thymine in RNA)

Nitrogenous Bases

• Pyrimidines: Single-ring structures (cytosine, thymine, uracil)

• Purines: Double-ring structures (adenine, guanine)

Base Pairing and Double Helix

In DNA, bases pair via hydrogen bonds:

• Adenine (A) pairs with Thymine (T)

• Guanine (G) pairs with Cytosine (C)

In RNA, uracil (U) replaces thymine and pairs with adenine.

Central Dogma of Molecular Biology

The flow of genetic information in cells is summarized as:

This process involves transcription (DNA to RNA) and translation (RNA to protein).

Genomics and Proteomics

• Genomics: Study of an organism's entire set of genes (genome)

• Proteomics: Study of the full set of proteins expressed by a genome

• Bioinformatics: Use of computational tools to analyze and interpret biological data

Additional info: The notes infer some context about the central dogma and the importance of protein structure-function relationships, as well as the classification of amino acids, to provide a self-contained study guide.