Understanding Proteins: Structure and Function

• Proteins are complex molecules containing carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.
• They make up 40% to 70% of the dry weight of living cells.
• Proteins are essential components of all living cells.
• They include enzymes, hormones, and antibodies, which are vital for proper function.
• Proteins are crucial in animal diets for tissue growth and repair.
• Sources of proteins include: meat, fish, eggs, milk and legumes.

Protein basics

• Proteins are made of α-amino acid monomers.
• An amino acid contains both an amino group and a carboxylic group.
• A protein consists of one or more amino acid chains linked by peptide bonds.
• Amino acids are the building blocks of proteins.

L-Form Amino Acid Structure

• An amino acid has a central alpha carbon bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (-H), and a variable side chain (R).
• The "R" group is unique to each amino acid.
• Illustration shows the general structure with examples of Glycine (R=H) and Alanine (R=CH3).

Classification of Proteins

• Simple proteins yield amino acids and sometimes small carbohydrates upon hydrolysis, examples include albumins, globulins, glutelins, albuminoids, histones, and protamines.
• Conjugated proteins consist of simple proteins combined with non-protein material, examples include nucleoproteins, glycoproteins, phosphoproteins, hemoglobins, and lecithoproteins.
• Derived proteins originate from simple or conjugated proteins through physical or chemical processes, examples include denatured proteins and peptides like collagen from fish.

Prosthetic Group

• A prosthetic group is a non-protein component of conjugated proteins that is crucial for biological activity, such as heme, which carries oxygen.
• Prosthetic groups can be organic (vitamin, sugar, lipid) or inorganic (metal ion).
• They are tightly bound to proteins, sometimes through covalent bonds.
• They are often essential for enzyme function.

Apoprotein and Holoprotein

• Apoprotein is a protein without its prosthetic group.
• Holoprotein is a protein with its prosthetic group.
• Prosthetic groups are permanent cofactors, unlike temporary coenzymes.
• In enzymes, prosthetic groups are involved in the active site.
• Example: Heme in hemoglobin is a prosthetic group.
• Active site of an enzyme is the region where the substrate binds and the enzymatic reaction occurs.

Amino Acids

• "R" represents a side chain specific to each amino acid.
• Classified in four groups based on their side chain properties: acidic, basic, hydrophilic (polar), and hydrophobic (nonpolar).
• d-amino acids have both amino and carboxyl groups attached to the same d-carbon.

Classification of Amino Acids

• Essential amino acids cannot be made by the body and therefore must be obtained through diet.
• Nonessential amino acids can be synthesized by the body, not required in the diet.
• Conditionally essential amino acids are normally synthesized, but may need to be obtained from the diet under certain health conditions.
• The eight essential amino acids for humans are tryptophan, lysine, methionine, phenylalanine, threonine, valine, leucine, and isoleucine.
• Histidine and arginine are considered essential only for children.
• PVT TIM HALL: Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine.

Limiting Amino Acid

• The limiting amino acid is the essential amino acid found in the smallest quantity in the food stuff.
• Examples of limiting amino acids in different protein sources:
  • Wheat: Lysine
  • Rice: Lysine, Threonine
  • Maize: Lysine, Tryptophan
  • Pulses/legumes: Methionine
  • Beef: Methionine, cysteine
  • Whey, Milk: None

Amino Acids (AA) - Protein

• Amino Acids are the basic unit.
• Amino acids can be formed through condensation reactions.
• Peptides are an amino acid chain containing two or more amino acids.
• Polypeptides contain fewer than 50 amino acids.
• Proteins contain more than 50 amino acids.

Functional Classification of Proteins:

• Structural proteins provide support and shape to cells, tissues, and organs. Example: collagen
• Catalytic proteins, namely enzymes, speed up biochemical reactions.
• Transport proteins, such as hemoglobin, transport molecules.
• Regulatory proteins, like hormones (e.g., insulin, growth hormone), regulate bodily functions.
• Protective proteins, which include antibodies, defend the body against foreign invaders.

Quiz Questions

• Question 1: Which type of protein provides support and shape to cells, tissues, and organs? -Answer: (b) Structural protein
• Question 2: Under what circumstances might arginine become conditionally essential? -Answer: (b) During periods of trauma, sepsis, or rapid growth
• Question 3: How would you describe a holoprotein? -Answer: (b) A complete protein with its prosthetic group attached
• Question 4: What is an apoprotein? -Answer: (b) A protein missing its prosthetic group
• Question 5: What are prosthetic groups? -Answer: (b) Essential components of proteins permanently attached to them.
• Question 6: How do prosthetic groups differ from coenzymes? -Answer: (a) Prosthetic groups are permanently attached to proteins, while coenzymes are not.
• Question 7: Which of the following describes the active site of a protein? -Answer: (c) The region where the protein interacts with other molecules to carry out its function