Proteins and Amino Acids

• Chapter focuses on proteins and amino acids

Amino Acids - The Building Blocks

• Amino acids serve as the building blocks of proteins.
• Each amino acid has a central carbon atom attached to a hydrogen atom, an amino group, an acid group, and a side chain.
• A total of 20 amino acids exist.
• Of these, 9 are essential amino acids, which the body requires but cannot produce on its own. Therefore, they must be obtained through diet.

Essential vs Nonessential Amino Acids

• Essential amino acids are those that the human body cannot synthesize in sufficient quantities to meet its needs, making it essential to obtain them through diet.
• Non-essential amino acids are those that the human body can synthesize in sufficient amounts to meet its needs.

Essential Amino Acids

• Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine are essential amino acids.
• Isoleucine is found in all animal-based proteins.
• Leucine stimulates muscle protein synthesis.
• Arginine, Cysteine, Glutamine, Glycine, Proline, Serine, and Tyrosine are conditionally essential amino acids under specific circumstances.

Protein Structure: Linking Amino Acids

• Peptide bonds are chemical bonds that join amino acids together.
• Peptide bonds form between the acid group of one amino acid and the nitrogen group of another.
• A dipeptide bond is formed between two amino acids.
• Polypeptides consist of many amino acids bonded together.
• Proteins are composed of one or more polypeptide chains, folded into a three-dimensional structure.
• Energy is required to build and break down these chains. The denser the protein, the more energy is required.

Protein Functions

• Enzymes speed up metabolic reactions.
• Transport proteins move substances in and out of cells.
• Antibodies aid the immune system in fighting off foreign bodies.
• Contractile proteins facilitate muscle movement.
• Hormones, such as insulin and glucagon, act as chemical messengers.
• Proteins help regulate fluid and acid-base balance.

Amino Acid Pool and Protein Turnover

• The amino acid pool encompasses all the amino acids in body tissues and fluids that are available for use.
• Protein turnover is the continuous process of synthesis and breakdown of body proteins.
• These amino acids are in dynamic equilibrium with each other.
• During digestion, proteins are broken down into amino acids in the stomach, pancreas, and small intestine. It is better to not skip breakfast.

Digestive Enzymes

• Salivary amylase, found in the mouth, breaks down starch into smaller carbohydrate molecules.
• Rennin, found in the stomach, curdles milk protein casein.
• Pepsin, found in the stomach, breaks down proteins into polypeptides and amino acids.
• Trypsin and chymotrypsin, found in the pancreas, break down proteins and polypeptides into shorter polypeptides.
• Carboxypeptidase, found in the pancreas, breaks polypeptides into amino acids.
• Pancreatic lipase breaks triglycerides into monoglycerides, fatty acids, and glycerol.
• Pancreatic amylase breaks starch into shorter glucose chains and maltose.
• Carboxypeptidase, aminopeptidase, and dipeptidase, found in the small intestine, break polypeptides into amino acids.
• Lipase breaks monoglycerides into fatty acids and glycerol.
• Sucrase breaks sucrose into glucose and fructose.
• Lactase breaks lactose into glucose and galactose.
• Maltase breaks maltose into glucose.
• Dextrinase breaks short chains of glucose into individual glucose molecules.

Protein Digestion and Absorption

• Chewing in the mouth initiates the mechanical breakdown of protein.
• In the stomach, hydrochloric acid and pepsin begin the chemical digestion of protein.
• In the small intestine, enzymes from the pancreas and the brush border break down polypeptides into amino acids, dipeptides, and tripeptides.
• Transport proteins move the products of protein digestion into the mucosal cell.
• Dipeptides and tripeptides can enter the mucosal cell and are broken down into single amino acids.
• Amino acids pass from the mucosal cell into the blood and travel to the liver, which regulates the distribution of amino acids to the rest of the body
• A small amount of dietary protein is lost in feces.

Protein Synthesis

• In the nucleus, the code for a protein is transcribed from DNA into messenger RNA (mRNA).
• The mRNA carries genetic information from the nucleus to ribosomes in the cytosol, where proteins are made.
• In the cytosol, transfer RNA (tRNA) reads the genetic code and delivers amino acids to the ribosome to form a polypeptide chain.

Muscle Protein Synthesis

• In protein breakdown and synthesis, if you cease consuming protein, the process slows down.

Energy Production From Protein

• Urea, a nitrogen-containing waste product, forms from the breakdown of amino acids and is excreted in urine.
• The body prefers carbs and fat for energy.

Urea Synthesis

• Deamination is the removal of the amino group from an amino acid.
• This process produces ammonia, which is converted to urea.
• Urea is filtered by the kidneys and excreted.
• A high protein diet is not harmful to your kidneys.

Nitrogen Balance

• Nitrogen balance occurs when nitrogen intake is equal to nitrogen output, indicating that total body protein remains constant.
• A negative nitrogen balance occurs when nitrogen intake is less than nitrogen output, leading to a decrease in total body protein. This can be caused by losing protein, stress or surgery.
• Positive nitrogen balance occurs when nitrogen intake exceeds nitrogen output, leading to an increase in total body protein, which we want to achieve.

Determining Protein Requirements

• To determine the required protein intake, first determine body weight; which is weight in pounds / 2.2lbs/kg
• An example calculation is 150 lbs / 2.2lbs/kg = 68 kg
• Then, multiply weight in kilograms by the recommended grams of protein per kilogram for the specific gender and life-stage group.
• Example of a 23 yr old woman weighing 68kg, 0.8 g/kg/day x 68 kg = 54.4 grams of protein/day.

Daily Protein Intake Recommendations

• It is false that the recommended dietary allowance of 0.8 grams per kg of protein per day is all that is needed to maintain health, even for older adults.
• The daily intake range is 1.6-2.2 grams/kg.
• The recommended protein intake per meal for someone under 40 years of age is 0.25 grams/kg. For someone over 40 the recommended protein intake per meal is 0.40 grams/kg.

The Importance of Timing

• Protein timing is an ideal time to recharge, refuel, and recover after exercise.
• Consuming protein before sleep, following resistance training, may have a positive effect on muscle protein synthesis.

Protein Sources

• To support muscle protein synthesis leucine is useful and can be found in most food sources.

Protein Quality

• Leucine has all 20 amino acids.

Protein Pacing

• Frequent intakes of dietary protein appear optimal vs. 1-2 large meals.
• This offsets the potential muscle fullness feeling.
• Aging adults typically consume inadequate protein at breakfast and lunch.
• An even distribution through the day is optimal.

Summary For Protein Intake

• Under 40 years of age, 0.25 grams/kg per meal. Over 40 years of age, 0.4 grams/kg per meal.
• Consume at breakfast, lunch, supper, post-exercise, and before sleep.
• Emphasis should be on complete foods, but protein supplements can be considered.
• Focus on including leucine in food sources.
• Overall intake should be 1.2-2.2 grams/kg/day.
• Those who exercise frequently should take 2.2 grams/kg/day.

Take Home Points

• The amount of protein per serving is important, but overall the total daily amount is what counts.
• Source: the source of protein is not as important as achieving the daily amount
• Frequency: an equal distribution is recommended.
• Timing: specific timing is irrelevant per se.
• Pre-sleep: this may help you achieve the daily protein goal.