Proteins: Structure, Function, and Importance in Anatomy & Physiology

Macromolecules: Proteins

Introduction to Biomolecules

Biomolecules are essential organic compounds that form the basis of life. There are four main classes: carbohydrates, lipids, proteins, and nucleic acids. Each class has unique monomers and polymers, with proteins being composed entirely of amino acid monomers.

• Carbohydrates: Monomers are monosaccharides; some form polymers.

• Lipids: Do not form true polymers; composed of fatty acids and glycerol.

• Proteins: Polymers made from amino acid monomers.

• Nucleic Acids: Polymers of nucleotide monomers (covered in a later class).

Proteins

Overview and Importance

Proteins are large, complex molecules that account for approximately 50% of the organic matter in a typical cell. They play a critical role in nearly all biological processes, making them fundamental to anatomy and physiology.

Functions of Proteins

Proteins perform a wide variety of functions in living organisms. The major functional categories include:

• Enzymatic Proteins: Catalyze biochemical reactions, such as digestive enzymes that hydrolyze food molecules.

• Defensive Proteins: Protect against disease; for example, antibodies inactivate and help destroy viruses and bacteria.

• Transport Proteins: Carry substances across cell membranes or throughout the body (e.g., hemoglobin transports oxygen).

• Hormonal Proteins: Coordinate organismal activities (e.g., insulin regulates blood sugar).

• Receptor Proteins: Respond to chemical stimuli (e.g., receptors in nerve cells detect signaling molecules).

• Contractile and Motor Proteins: Enable movement (e.g., actin and myosin in muscles).

• Structural Proteins: Provide support and shape to cells and tissues (e.g., collagen, keratin).

Protein Monomer: Amino Acids

Proteins are polymers composed of repeating units called amino acids. There are 20 different amino acids, each with a central carbon atom bonded to:

• A hydrogen atom (H)

• An amino group (–NH2)

• A carboxyl group (–COOH)

• A variable side chain (R group), which determines the identity and properties of the amino acid

Generalized structure of an amino acid:

• Central (alpha) carbon

• Attached groups: H, amino group, carboxyl group, R group

Additional info: The R group varies among the 20 amino acids, giving each its unique chemical characteristics.

Peptide Bonds and Protein Polymers

Amino acids are linked together by peptide bonds, which are covalent bonds formed through a condensation reaction (also called dehydration synthesis). This reaction releases a molecule of water as the bond forms.

• Peptide bond: Connects the carboxyl group of one amino acid to the amino group of another.

• Polypeptide: A chain of amino acids linked by peptide bonds.

Condensation reaction equation:

$\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$

Hydrolysis reaction (breakdown):

$\text{Dipeptide} + H_2O \rightarrow \text{Amino acid}_1 + \text{Amino acid}_2$

Protein Diversity

With 20 different amino acids, the number of possible protein sequences is enormous. The number of possible polypeptides of length N is:

$20^N$

where N is the number of amino acids in the chain.

Protein Structure and Shape

The unique sequence of amino acids (primary structure) causes the polypeptide to fold into a specific three-dimensional shape, which determines the protein's function. The folding is influenced by interactions among R groups and the surrounding environment.

• Primary structure: Sequence of amino acids

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

• Tertiary structure: Overall 3D shape of a single polypeptide

• Quaternary structure: Association of multiple polypeptide chains

Form fits function: The specific shape of a protein is essential for its biological activity.

Classification: Fibrous vs. Globular Proteins

Proteins are classified by their appearance and function:

• Fibrous Proteins: Long, narrow, and structural (e.g., collagen, keratin, elastin, silk). Found in connective tissues, providing strength and flexibility.

• Globular Proteins: Compact, roughly spherical, and functional (e.g., enzymes, antibodies, hemoglobin). Often soluble in water and involved in dynamic cellular processes.

Protein Structure Determines Function

Even a single change in the amino acid sequence can alter a protein's shape and function. For example, sickle-cell disease results from a single amino acid substitution in the hemoglobin protein, leading to abnormal red blood cell shape and impaired oxygen transport.

Hemoglobin: Structure and Function

Hemoglobin (Hb) is a globular protein found in red blood cells, responsible for transporting oxygen throughout the body.

• Composed of four protein chains, each bound to a heme group.

• Each heme group contains an iron atom that binds one oxygen molecule.

• One hemoglobin molecule can carry up to four oxygen molecules.

Clinical relevance: Deficiency or mutation in hemoglobin reduces the oxygen-carrying capacity of blood, as seen in sickle-cell anemia.

Protein Denaturation

Denaturation is the process by which a protein loses its native shape due to changes in environmental conditions, such as pH, temperature, or exposure to chemicals. Denatured proteins are non-functional.

• Causes: Heat, alcohol, acids/bases, pressure

• Consequences: Loss of structure leads to loss of function

• Examples: Cooking an egg (heat denatures egg proteins), using alcohol as a disinfectant (denatures bacterial proteins)

Reversibility: In some cases, if the denaturing agent is removed, proteins can refold (renaturation), but often the process is irreversible.

Enzyme Function: Lock & Key Model

Enzymes are proteins that catalyze biochemical reactions. The lock & key model describes how enzymes have a specific active site that fits only particular substrate molecules, ensuring specificity in catalysis.

Summary Table: Main Classes of Biomolecules

Class	Polymer?	Monomer
Carbohydrates	SOME	Monosaccharides
Lipids	NO	N/A
Proteins	ALL	Amino Acids
Nucleic Acids	ALL	Nucleotides

Key Learning Objectives

• Recognize that amino acids are the monomers of polypeptides and proteins.

• Describe the basic structure of an amino acid: central carbon, hydrogen, amino group, carboxyl group, and R group.

• Explain condensation and hydrolysis reactions in the formation and breakdown of peptide bonds.

• Understand the importance of protein shape for function and how environmental changes can cause denaturation.

• Describe the structure and function of hemoglobin.

• Understand the lock & key model of enzyme action.