Amino Acids - The building blocks of protein

Structure: every amino acid contains an amino group, and acid group, a hydrogen atom, and a distinctive side group all attached to a central carbon atom

• Non-essential amino acids: the body can synthesize more than half of the amino acids from nitrogen, carbohydrates, and fat
• Essential amino acids (EAAs): There are nine essential amino acids that the body cannot make in sufficient quantities and, therefore, must obtain them from food
• Conditionally-essential amino acids: some normally non-essential amino acids become essential for people with certain conditions (ie: tyrosine for phenylketonuria, and glutamine for some liver diseases) 

  Proteins

  Amino acid chains: condensation reactions create bonds between amino acids to form a peptide bond:

  • dipeptides: two amino acids bonded together
  • tripeptide: three amino acids bonded together
  • oligopeptide: 4-9 amino acids bonded together
  • polypeptide: 10 or more amino acids bonded together

  Amino Acid Sequences: the specific order of amino acids in polypeptide chains varies greatly

  Protein Shapes:

  • the shape depends on the sequence of amino acids, the bonds linking them, and the interactions of the side chains with each other with surrounding molecules
  • the chain folds so that electrically charged (hydrophyllic) side chains are folded outwardly and the neutral (hydrophobic) groups are folded inwardly for stability in bodily fluids

  Protein Functions:

  Formation of hormones, hemoglobin, muscle tissue, tendon structure, transport

  Protein Denaturation:

  Proteins uncoil and lose their functions when exposed to heat, acid, or other conditions that hinder stability  [TOP]

Digestion

In the stomach:

• Hydrochloric acid (HCl):
  • Denatures the protein structure
  • Activates pepsinogen to pepsin
• Pepsin: (a gastric protease):
  • Cleaves proteins to smaller polypeptides and free amino acids
  • Inhibits pepsinogen synthesis

In the small intestine:

• Pancreatic and intestinal proteases hydrolyze proteins further into oligopeptides, tripeptides, dipeptides, and amino acids
• Intestinal dipeptidases and tripeptidases break peptides down further to amino acids
  • Peptidase = digestive enzyme that hydrolyzes peptide bonds
  • Proteases include: enteropeptidase, trypsin, chymotrypsin, carboxypeptidases, elastase, collagenase, aminopeptidases, tripeptidases

Absorption

The cells of the small intestine absorb amino acids and have peptidase enzymes to split the dipeptides and tripeptides into single amino acids  [TOP]

Protein synthesis

• Delivery instructions - messenger RNA (mRNA) determines the needed sequence of amino acids and attaches itself to a ribosome (which makes proteins)
• Lining up the Amino Acids - transfer RNA (tRNA) collects amino acids and carries them to the mRNA in the order dictated by the mRNA
• Sequencing errors - can result in problems like sickle cell anemia where valine is in the place of glutamic acid in hemoglobin (resulting in poor ability to carry oxygen)

Roles of proteins in the body

• Building materials: proteins are needed for the body to manufacture and repair most body structures
  • cell growth, tendons, skin, membranes, muscles, organs, bones
• Enzymes: proteins facilitate chemical reactions
• Hormones: some hormones are made up of protein (ie: insulin)
• Antibodies: inactivate foreign invaders and protect against disease
• Fluid and electrolyte balance: help maintain volume and composition of body fluids
• Acid-base balance: help maintain balance by acting as buffers
• Transportation: transport lipids, vitamins, minerals, and oxygen
• Energy: provide a fuel source for the body  [TOP]

Key Terms:

Protein Turnover
The body's constant degradation and synthesis of endogenous proteins (breakdown and buildup of protein within each cell)

Nitrogen Balance
The balance of nitrogen consumed compared to nitrogen excreted from the body

• Positive nitrogen = more nitrogen consumed than excreted (pregnancy, child growth, illness recovery)
• Negative nitrogen = more nitrogen excreted than consumed (starving, burns, injuries, infections, fever)

Amino acid synthesis
The body can breakdown existing amino acids and proteins to make nonessential amino acids that it needs

Synthesis of other compounds
The body uses amino acids to synthesize neurotransmitters, hormones, enzymes, antibodies, hemoglobin

Energy / fuel source
Can be used to synthesize glucose in the absence of adequate carbohydrates

Deamination
When broken down, amino acids lose the nitrogen-containing amino group
This produces ammonia that is released into the bloodstream and converted to urea by the liver to filtered through the kidneys

Fat synthesis
As a fuel source, too much protein can be converted to fat and stored in adipose tissue. [TOP] 
 

Protein Quality

Limiting Amino Acids
An essential amino acid supplied in food less than the amount needed to support protein synthesis

Complete Protein
Dietary protein that contains all the essential amino acids in the amounts needed by the body
Animal sources tend to be more 'complete' than plant sources which tend to be limiting in one or more amino acid

Complementary Proteins
Two or more proteins whose amino acid arrangements make a more complete protein by supplying the amino acids missing from each other

Mutual supplementation
Eating two different protein sources (usually plant sources) to provide a meeting of complementary proteins (to make a more complete protein).  (example: legumes and bread - together make a complete protein, apart lack different amino acids)
 
Measuring Protein Quality

Amino Acid Scoring
Evaluates protein quality as a comparison of amino acid composition to a reference protein

Biological Value
Measures efficiency of a protein to support the body's needs, based on retention of absorbed nitrogen

Net Protein Utilization
Measures retention of food nitrogen

Protein Efficiency Ratio
Comparison of weight gain to protein intake

Protein digestibility - corrected amino acid scores (PDCAAS)
Compares amino acid score of a food protein with the amino acid requirements of preschool children and corrected for true digestibility. [TOP]