Chapter 6: The Proteins and Amino Acids

A. Protein Structure and Basic Chemistry

• Composition:
  • Proteins contain carbon, hydrogen, oxygen, and an extra element—nitrogen.
  • They are built from amino acids (AAs) that share a common backbone (a single carbon with an amine and a carboxyl group) plus a unique side chain.
  • The side chain’s size, shape, and electrical charge give each amino acid its identity and determine protein behavior.
  
  

• Amino Acid Diversity:
  • There are 20 common amino acids in living tissue.
  • The infinite sequence variations (combinations of 20 AAs) produce a vast array of proteins.
  • The sequence is genetically determined and even a single mis-incorporated amino acid (as seen in sickle cell disease) can lead to significant dysfunction.
  
  

B. Essential vs. Nonessential and Conditional Amino Acids

• Essential Amino Acids:
  • Nine amino acids that the body cannot synthesize in sufficient amounts.

  • Without these, the body cannot build the proteins it needs to do its work 

              • They must be obtained from food for proper protein synthesis and function.

• Conditional Essentiality:
  • Under stress or in certain diseases (e.g., PKU), a normally nonessential amino acid (like tyrosine, normally made from phenylalanine) can become essential.

C. Protein Assembly, Folding, and Recycling

• Protein Synthesis:
  • Amino acids join via peptide bonds to form polypeptides (10–50 AAs, sometimes longer).
  • Polypeptides do not remain linear—they fold into unique three-dimensional structures that determine their function.

• Folding & Functional Units:
  • Protein strands may work individually or cluster together (e.g., hemoglobin’s four subunits) and may associate with minerals, vitamins, or carbohydrates.

• Recycling:
  • The body breaks down proteins and reuses amino acids for new protein synthesis.

  • AA from food and body proteins provide the cells with raw materials from which they can build the protein molecules they need

 • By reusing intact AA to build proteins, the body recycles and conserves nitrogen

         • This recycling is crucial during energy shortages or when cells require rapid protein       replacement.

D. Protein Digestion and Absorption

• When a person eats food proteins, the body must 1st break them down into amino acids. 

  • The can be rearranged into a specific human body protein

  • Each protein performs a special task in a particular tissue 

*** Other than being crushed and torn by chewing and moistened with saliva in the month, nothing happens to the protein until it reaches the stomach***

• Digestive Process:
  • Mouth: Chewing and saliva moistening begin the breakdown process (though minimal chemical change occurs here).
  • Stomach:
    – Hydrochloric acid denatures proteins, uncoiling them for enzyme action.
    – Enzymes break peptide bonds into polypeptides and free amino acids.

 • Small Intestine:
  – Pancreatic and intestinal enzymes further cleave polypeptides into dipeptides, tripeptides, and individual amino acids.
  – Absorption occurs at specialized sites on intestinal cells; similar amino acids compete for transport.

        – Alkaline juice from the pancreas neutralizes stomach acid

• Amino acid absorbtion:

  • Absorbed by the small intestine 

    • Larger molecules can enter the bloodstream intact 

      • Acts as hormones to regulate body functions and provide information about the external environment 

      • Might also stimulate an immune response and play a role in food allergy 

    • Cells of the small intestine have separate sites for absorption 

      • Based on types of amino acids - similar amino acids compete for the same absorption site 

      • When a person ingests a large dose of any single amino acid, it may limit the absorption of others if its type 

    • Once amino acids are circulating in the bloodstream, they are carried to the liver 

      • Used or released into the blood for use by other cells 

      • Cells can link amino acids together to build proteins for their own use 

      • When necessary, can be used for energy 

E. Roles of Proteins in the Body

• Structural & Mechanical Functions:
  • Proteins form the backbone of muscle tissue (making up about 40% of body protein) and are critical in the structure of bones, teeth, skin, tendons, and connective tissue (e.g., collagen acts as a glue between cells).

• Enzymes and Hormones:
  • Enzymes are proteins that catalyze biochemical reactions, significantly speeding up metabolic processes.

• • Hormones, many of which are protein-based (like insulin), regulate critical bodily functions such as blood glucose levels.

• Immune and Transport Functions:
  • Antibodies identify and neutralize foreign agents.
  • Hemoglobin carries oxygen; other proteins transport lipids, vitamins, and minerals.

• Fluid, Acid-Base, and Energy Roles:
  • Proteins help regulate fluid and electrolyte balance and act as buffers for pH.
  • They provide energy when other nutrients (carbs and fats) are insufficient.

• Gene Expression:
  • Nutrients (amino acids) influence gene regulation, affecting the synthesis of enzymes and other proteins

• Protein turnover: the process of breakdown, recovery, and synthesis of amino acids

F. Dietary Protein: Quality, Quantity, and Supplementation

• Protein Quality:
  • High-quality proteins contain all essential amino acids.
  • Complementary proteins (e.g., grains with legumes) can together meet amino acid needs.

• Recommended Intake:
  • For healthy adults: approximately 0.8–1.2 g/kg body weight; athletes or those building muscle may require more. • AMDR suggests protein provides 10–35% of total calories.

• Deficiency and Excess:
  • Deficiency may cause slow growth, impaired immunity, and organ dysfunction.
  • Excess protein (especially from animal sources high in saturated fats) may contribute to heart disease, cancer risks, and kidney strain.

Supplement Considerations:
• Protein powders, amino acid supplements, and collagen products are available.
• Table 6–3 (from the slides) highlights groups at risk for harm from supplementation (women of childbearing age, infants, children, the elderly, people with metabolic errors, smokers, low-protein dieters, and those with mental/physical illnesses).