Biochemistry of Proteins and Amino Acids

Protein: Amino Acids

• Proteins are made up of amino acids
• Amino acids contain nitrogen atoms, alongside carbon, hydrogen, and oxygen
• These nitrogen atoms give amino acids their name.
• Different side chains distinguish different amino acids
• There are 20 different amino acids.
• Some amino acids are essential, while others are non-essential.

Chapter Outline

• Chemists' view of proteins
• Digestion and absorption of proteins
• Protein in the body
• Protein in foods
• Health effects and recommended intakes of protein

The Chemists' View of Proteins

• Chemically, proteins contain nitrogen atoms, in addition to carbon, hydrogen, and oxygen atoms.
• The nitrogen atoms give amino acids their name.
• Amino acids form links in protein chains.
• Proteins have four structural levels
  • Primary: The sequence of amino acids
  • Secondary: Folding patterns (alpha helix or beta sheet) created by hydrogen bonding
  • Tertiary: 3D structure of the polypeptide chain
  • Quaternary: Structure formed by multiple polypeptide chains

Amino Acids

• All amino acids have the same basic structure
• The side chains vary, contributing to the diversity of proteins
• The side chains distinguish one amino acid from another

Essential Amino Acids

• Nine essential amino acids are not produced by the body.
• They must come from food.
  • Ex: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine

Nonessential Amino Acids

• These are produced by the body
  • Ex: Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine, Proline, Serine, Tyrosine

Conditionally Essential Amino Acids

• Sometimes a non-essential amino acid becomes essential
• This usually occurs when the body cannot produce a required amino acid due to a diet lacking the precursor, or a hereditary condition
• Ex: Tyrosine

Proteins

• Cells link amino acids end-to-end in various sequences, forming thousands of different proteins
• Peptide bonds unite amino acids

Digestion of Proteins

• Dietary protein does not directly become body protein.
• Enzymes break down polypeptides into smaller peptides, then into individual amino acids
  • Ex: Polypeptide chains -> Tripeptides -> Dipeptides -> Individual amino acids

Protein Digestion and Absorption Process

• Digestive enzymes hydrolyze proteins (break them down)

Protein Absorption

• Special carriers transport amino acids and di/tripeptides into intestinal cells
• Amino acids are used for energy or building new proteins

Proteins in the Body

• Protein synthesis occurs in the cytoplasm.
• Genes provide information for creating proteins

Roles of Proteins

• Structural materials
• Enzymes
• Hormones
• Regulators of fluid balance
• Regulators of acid-base balance
• Transporters
• Antibodies
• Source of energy (glucose)

Protein Functions in the Body

• Proteins form part of body tissues
• Facilitate chemical reactions (enzymes)
• Regulate body processes and maintain body fluid
• Function as buffers (Acid-base balance)
• Transport molecules around the body (lipids, vitamins, etc)
• Inactivate foreign invaders
• Provide energy to the body
• Participate in vision
• Involved in blood clotting, scar tissue formation

Protein in Foods

• Protein quality depends on the amino acid composition and digestibility
• Animal protein tends to have high digestibility
• Plant protein, while important, has potentially lower digestibility.
• Two factors that influence Protein Quality are digestibility and amino acid composition.

Two Factors Influence Protein Quality

• Protein digestibility: the measure of amino acids absorbed from a protein source.
• Animal proteins generally have high digestibility (90-99%)
• Plant proteins usually have lower digestibility (70-90%)
• Soy and legumes are notable exceptions

Amino Acid Composition

• To create proteins, a cell needs all the required amino acids
• The liver produces several essential amino acids that are in short supply
• If an essential amino acid is missing, protein synthesis is limited, potentially causing a partial or complete deficiency.
• Dietary protein must contain all essential amino acids, or the diet must contain complimentary food sources to cover the missing amino acids to prevent protein breakdown and synthesis limitations.

High Quality Proteins

• Dietary proteins containing all essential amino acids relatively equally
• Primarily from animal sources, but can also come from soy and legumes

Low Quality Proteins

• Dietary proteins limiting in one or more essential amino acids
• Primarily from plants, but can also come from some animal sources

Complementary Proteins

• Two or more proteins that contain different essential amino acid profiles
• Combining those foods provides the full complement of essential amino acids

Health Effects of Protein

• Protein deficiency can have devastating health consequences
• Deficiencies develop when the diet doesn't supply sufficient or balanced amino acids
• Protein deficiency involves reduced protein synthesis and elevated protein degradation
• Serious deficiency may become life threatening, due to the essential functions proteins serve in the body.
• Protein excess also presents possible risks.

Consequences of Protein Deficiency

• Slowed growth and impaired brain/kidney function
• Weakened immune function and reduced nutrient absorption
• PEM (Protein-energy malnutrition) includes kwashiorkor, and marasmus.
  • Kwashiorkor: Edema, swollen belly, malnutrition, and increased infection risk
  • Marasmus: Muscle and fat loss, starvation, and stunted growth

Overconsumption of Protein

• High-protein diets may present health risks
• Possible links to chronic diseases such as:
  • Heart disease
  • Cancer
  • Osteoporosis
  • Kidney stones

Recommended Intakes of Protein

• RDA (Recommended Dietary Allowance) for protein is 0.8 grams/kg body weight/day
• Protein intake should constitute 10-35% of total daily energy intake

This quiz explores the fundamental aspects of proteins and amino acids, including their chemical distinctions from carbohydrates and lipids. It covers essential amino acids, the role of peptide bonds, and the impact of protein deficiency. Test your knowledge on protein digestion and dietary implications.