Understanding Proteins: From Structure to Function

Protein Structure, Properties, and Functions

Quaternary structure: The association of multiple polypeptide chains, resulting in a cluster of subunits. Only some proteins have this level. Formed by several polypeptide chains, each unit is called a protomer.

Properties of proteins:

1. Solubility: Proteins have high dispersion and form colloidal solutions.
2. Denaturation: Increased temperature or pH changes alter the links that stabilize the protein structure, causing it to lose its biological activity.
3. Specificity: Each species has its own unique set of proteins, and there is even individual specificity, which can lead to transplant rejections and allergies.
4. Buffer capacity: Proteins, being amphoteric, can help neutralize pH variations.

Classification of proteins:

1. Holoproteins: Proteins formed only by amino acids, adopting globular or fibrous conformations (scleroproteins).
2. Heteroproteins (conjugated proteins): Proteins formed by a polypeptide chain and a non-protein component (glycoproteins, lipoproteins, chromoproteins, nucleoproteins).

Functions of proteins:

1. Reserve: Some proteins are used as nutrients (e.g., casein in milk).
2. Structural: Proteins serve as building blocks for cellular structures (e.g., histones, collagen).
3. Hormonal: Some proteins act as hormones (e.g., growth hormone).
4. Transport: Proteins like hemoglobin and lipoproteins transport molecules (e.g., cholesterol).
5. Defense: Immunoglobulins (antibodies) defend against antigens.
6. Contractile: Proteins like actin and myosin are involved in movement in unicellular organisms and muscle cells.
7. Enzymatic: Enzymes are catalysts that facilitate chemical reactions in living organisms. They are not altered during the reaction. Each enzyme is very specific to the reaction it catalyzes (specificity), recognizing the molecules it acts on (substrates).

Enzymes lower the activation energy of reactions. Each enzyme has an active site that interacts specifically with the substrate.

Models to explain enzyme-substrate interaction:

1. Lock and key model: The active site of the enzyme is complementary to the shape of the substrate.
2. Induced fit model: The active site becomes complementary to the substrate only after interaction.

Factors affecting enzyme activity:

1. pH: pH affects the functional groups of the enzyme, which need the appropriate charge to maintain their three-dimensional structure and function optimally.
2. Temperature: Enzymes function within a specific temperature range; they are thermolabile and can be denatured and inactivated at extreme temperatures.

Coenzymes: Some enzymes require additional substances (coenzymes) for their catalytic activity.

Enzyme nomenclature: Enzymes are named with a prefix indicating the substrate they act on, followed by the suffix '-ase', and then the type of reaction they catalyze. Examples include oxidases, transferases, hydrolases, isomerases, lyases, and ligases (or synthetases).

Enzyme inhibition: A substance that slows down or stops an enzyme-catalyzed reaction. There are two types:

• Irreversible inhibition: The inhibitor binds strongly to the enzyme, making dissociation very slow. Reducing the inhibitor concentration does not eliminate inhibition.
• Reversible inhibition: The inhibitor can easily dissociate from the enzyme. Reducing the inhibitor concentration eliminates inhibition. This includes:
  • Competitive inhibition: The inhibitor and substrate compete for the same active site.
  • Non-competitive inhibition: The inhibitor binds to a different site on the enzyme, modifying its catalytic ability.

Cellular regulation of enzyme activity: Metabolic reactions must change to meet the cell's needs at all times.