Protein Structure and Function

• Proteins are large, complex molecules in all living cells as dictated by DNA
  • Contain carbon, oxygen, and hydrogen, similar to carbs and fats
  • Also contain nitrogen and sulphur, unlike carbs and fats

Fate of Proteins

• Proteins break down into amino acids
• Amino acids are used for:
  • Structural purposes in muscle and connective tissue
  • Regulatory functions as enzymes and hormones
  • Transport, such as hemoglobin and albumin
• Catabolism of amino acids involves:
  • Carbon skeleton use; glucose/energy/glycogen; acetyl coA/energy/triglyceride
  • Ammonia/ammonium production, leading to the urea cycle
  • Regulation of blood pH

Amino Acids

• Amino acids are the building blocks of proteins
• Proteins are made of a combination of 20 different amino acids
• Amino acids are composed of:
  • Central carbon
  • Amino group (N terminal)
  • Acid group (C terminal)
  • Side chain (R group)

Essential Amino Acids

• There are 9 essential amino acids
  • Must be consumed in the diet
  • Cannot be synthesized by the body in sufficient quantities
• Non-essential amino acids can be synthesized in sufficient quantities

Cysteine (non-essential)

• Non-essential amino acids can be made from essential amino acids
  • Spare use of essential amino acids which we have to get from diet if there is an adequate amount of product (cysteine) in the diet
• Methionine can be spared by having adequate cystine in the diet
  • Methionine can become very limiting in body if inadequate intake and high conversion of methionine to cysteine
    • Limited methionine can have effects on B12 metabolism and DNA
• Sparing protects levels of methionine
• Methionine (essential) converts to cysteine (non-essential)

Conditionally Essential Amino Acids

• Cysteine is non-essential because it can be synthesized (from methionine)
  • But if the (transamination) reaction is impaired, then cysteine becomes conditionally essential
  • May occur in preterm infants
• Conditional refers to the impaired enzymatic reaction or other specific situations
• Arginine can be conditionally essential in preterm infants (enzymes poorly developed) and in cases of illness/trauma
• Glutamine can be quickly depleted during illness because synthesis can't keep up
  • Can be caused by massive infection / sepsis / burn trauma
• Tyrosine can be conditionally essential in phenylketonuria (PKU)
  • Need tyrosine to make neurotransmitters
  • Tyrosine is made from the essential amino acid phenylamine
  • Tested at birth for PKU
• Neurotransmitters made from tyrosine include: dopamine, epinephrine (adrenaline), and norepinephrine

Protein Turnover

• Ingestion of amino acids from food, degradation of cellular proteins, synthesis of new proteins
• High protein turnover occurs in highly metabolically active cells and replacement of dead cells
• There is a pool of amino acids to support renewal, but this is also used for other purposes
  • Protein, glucose, fat, non-protein nitrogen (urea)
• The storage form of protein/amino acids is in muscle, tissue, cells
  • No long term 'inert' storage form of amino acids because we are using them all the time

Protein Organization

• Four potential levels of structure within each protein
  • Primary structure is the sequential order of amino acids
  • Secondary structure involves hydrogen and sulphur bonds that cause the chain to twist and turn
  • Tertiary structure is a unique 3D shape which determines function
  • Quaternary structure requires 2 or more polypeptides to bond together
    • Important because function follows form

Protein Denaturation

• Incorrect protein structure leads to improper function
  • Turns off or turns on constitutively
• Causes an alteration or lost of secondary or above structures
  • Damaging substances include heat, acid, base, heavy metal, and alcohol
  • Denatured enzymes due to fever or out of the normal blood pH range
  • Protein digestion due to low stomach pH

Protein Synthesis

• All essential amino acids must be available
• Limiting amino acid which is missing or is in the shortest supply in food protein
• Low protein synthesis due to inadequate energy consumption
  • Amino acid used for glucose production instead of synthesis

Protein Quality

• Protein quality is based on the ability of food to meet amino acid requirements, which is affected by:
  • Amino acid balance
    • Animal protein tends to reflect human environment
    • Plants usually are deficient in lysine (grains) and sulphur amino acids (SAA) in legumes
  • Digestibility
    • Animal protein is high, plants is variable
  • Presence of toxic factors
    • Plants have toxic factors but not animals
• The amount of protein in 5 oz lean ground beef is equal to 4-5 eggs, equal to white bread slices, equal to peanut butter

Protein Quality – Types

• Incomplete protein (low quality)
  • Insufficient quantity and amounts of essential amino acids
  • Does not support growth and health
  • Mostly plants (lack lysine), legumes (lacking SAA = methionine and cysteine)
• Complete protein (high quality)
  • Sufficient amounts of all 9 essential amino acids
  • Derived from animal sources (ie eggs, poultry, fish, beef, milk), soybean, and quinoa

Addressing Incomplete Protein Sources

• Mutual supplementation is combination of 2 or more incomplete protein sources to make a complete protein
• Complementary proteins are 2 incomplete proteins that when combined provide all 9 essential amino acids
  • One is the process (or mutual) to verb) vs the other is the description (complementary)

Assessing Protein Quality

• Biological Value (BV)
  • Proportion of adsorbed protein from food incorporated into body proteins
• Protein Efficiency Ratio (PER)
  • Weight gain by growing animal compared to animal's protein intake
    • PER = weight gain (g) / protein intake (g)
• Amino Acid or Chemical Score
  • Amount of limiting amino acid in food compared with the amount of the same amino acid in the reference food
• Protein Digestibility Corrected Amino Acid Score (PDCAAS)
  • Use chemical score with a correction factor for digestibility
  • Highest value is 1.0 for most digestible and providing all amino acids
    • Animal protein and soy are highly digestible: 90% absorption
    • Legumes: 70-80%
    • Grains/vegetables: 60-90%

Protein Efficiency Ratio (PER)

• PER is official method in Canada for evaluating protein quality
• Methodology involves:
  • Young rats fed for 4 weeks
  • Diet is adequate in all nutrients except protein
  • Protein is included at 10 % of diet (dry matter basis)
  • Rats weighed at beginning and end of study
  • Food consumption is monitored
  • This is marginal intake of protein, and if anything is wrong with the protein source, growth is limited
    • PER = weight gain (g) / protein intake (g)
• Strengths include it's simple, inexpensive, and very sensitive
  • Includes AA balance, digestibility, and toxic factors
• Weaknesses include rat's, growth not maintenance, and time-consuming

Chemical Score

• Chemical score is based on amino acid analysis of tested protein
  • Hydrolyze (chemically digest) protein to free amino acids
  • Quantify free amino acids by HPLC
  • Compare to composition of whole egg protein (reference)
    • Assume egg if the ideal protein
• Strengths include it's simple, cheap, identifies limiting amino acids
• Weaknesses include that does not account for digestibility, and who said whole egg is ideal protein?

Protein and Digestion

• Protein is crushed and moistened in the mouth
• Protein digestion begins in the stomach
  • Hydrochloric acid (HCl)
    • Denatures protein and activates pepsin from pepsinogen, pH 2
  • Pepsin breaks down protein into short polypeptides and amino acids
    • Pepsinogen = proenzyme/zymogen

Processes of Protein Digestion

• Three processes involved in protein digestion in the stomach
  • Physical mixing
  • Chemical digestion using HCl
  • Enzymatic digestion using pepsin
• Digestion continues in the small intestine
  • Protein digests into oligopeptides, tripeptides, dipeptides, and single amino acids
  • trypsin, chymotrypsin, elastase, and carboxypeptidases complete protein digestion
• Absorption occurs
  • Cells in the intestinal wall absorb amino acids, di, and tripeptides
  • Peptidases in intestinal wall break di, and tripeptides into single amino acids
• Amino acids are transported via portal vein to liver
  • Converted to glucose, fat, protein, and/or used for energy

Functions of Proteins

• Cell growth, repair, and maintenance
• Enzymes and hormones
• Fluid and electrolyte balance
• Acid/base balance
• Supports the immune system
• Energy source
• Nutrient transport and storage

Protein Requirements

• A matter of nitrogen balance determined by assessing the difference between nitrogen intake and nitrogen excretion
• Negative nitrogen balance starvation
• Positive nitrogen balance growth
• Nitrogen balance that is near 0: maintenance

Recommended Daily Allowance for Protein

• RDA = 0.8 g/kg of Body Weight (BW)
  • May be changing to original value 1.0 g/kg BW
  • Recommended percentage of energy from protein is 10-35%
• Protein needs are higher during growth and development
  • 1.5 g/kg (infants)
  • 1.1 g/kg (1-3 yrs)
  • 1.1 g/kg (pregnant or lactating women)

  • 1.2 g/kg (vegetarians)

Optimize Protein Intake

• The number of meals (frequency) is a factor in optimizing protein intake for athletes
• Special population - older adults current recommendations not meeting needs
  • Solution is to increase protein intake, exercise to increase strength

Protein Intake Harms

• High cholesterol and heart disease
  • Animal protein rich foods/diets are associated with high saturated fat and higher blood cholesterol levels
• May contribute to bone loss
  • High protein diets rich in sulphur amino acid
• Kidney disease is an increased risk for susceptible individuals
  • Max 2 g protein / kg Body Weight each day is safe for healthy individuals