Chapter 6: The Proteins and Amino Acids

What elements are proteins composed of?

Proteins contain carbon, hydrogen, oxygen, and an extra element—nitrogen.

What are the building blocks of proteins?

Proteins 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.

What determines the identity of an amino acid and influences protein behavior?

The size, shape, and electrical charge of the amino acid’s unique side chain determine its identity and affect protein behavior.

How many common amino acids are found in living tissue?

There are 20 common amino acids.

How does the sequence variation of amino acids contribute to protein diversity?

The infinite sequence variations (combinations of the 20 amino acids) produce a vast array of proteins.

Why is the genetic determination of amino acid sequence critical?

Because the sequence is genetically determined, even a single mis-incorporated amino acid (as seen in sickle cell disease) can lead to significant dysfunction.

What are essential amino acids?

They are nine amino acids that the body cannot synthesize in sufficient amounts, so they must be obtained from food.

Why are essential amino acids critical for protein synthesis?

Without these amino acids, the body cannot build the proteins it needs to perform its functions.

What is meant by "conditional essentiality" in amino acids?

Under stress or in certain diseases (such as PKU), a normally nonessential amino acid (like tyrosine) can become essential.

How does conditional essentiality affect amino acid requirements?

Normally, the body makes tyrosine from phenylalanine, but under specific conditions or diseases, this conversion may be insufficient, making tyrosine essential.

Non-Essential Amino Acids

• Astonishing = Alanine

• Astronauts = Aspartate

• Create = Cysteine

• Galaxies = Glutamate

• (The body can usually synthesize these in sufficient quantities.)

Conditionally Non-Essential Amino Acids

• Arginine

• Asparagine

• Glutamine

• Glycine

• Proline

• Serine

• Tyrosine

(These can become essential under certain conditions, such as illness or stress.)

Mnemonic for Essential AAs

PVT TIM HALL

• P = Phenylalanine

• V = Valine

• T = Threonine

• T = Tryptophan

• I = Isoleucine

• M = Methionine

• H = Histidine

• A = (Arginine*)

• L = Leucine

• L = Lysine

What must happen to dietary proteins before the body can use them to build specific human proteins?

They must first be broken down into amino acids. These amino acids can then be rearranged into the specific proteins the body needs, each performing specialized tasks in particular tissues.

What happens to proteins in the mouth during digestion?

Proteins are crushed and torn by chewing and moistened with saliva. However, minimal chemical digestion occurs here. Almost no true protein digestion happens until the protein reaches the stomach.

How does the stomach contribute to protein digestion?

• Hydrochloric acid denatures (uncoils) proteins, making them more accessible to enzymes.

• Stomach enzymes then begin breaking peptide bonds, producing polypeptides and some free amino acids.

Describe the role of the small intestine in protein digestion.

• Pancreatic and intestinal enzymes further break down polypeptides into dipeptides, tripeptides, and individual amino acids.

• Alkaline juice from the pancreas neutralizes the acidic chyme from the stomach.

• Absorption occurs through specialized transport sites on the intestinal cells, where similar amino acids compete for uptake.

How are amino acids absorbed in the small intestine?

• Amino acids, dipeptides, and tripeptides are absorbed by specific carriers on the intestinal cells.

• Similar amino acids compete for the same transport sites.

• Large doses of one amino acid can reduce the absorption of others that share the same transporter.

Can larger protein molecules be absorbed intact, and why might this be important?

Yes, some larger protein molecules can enter the bloodstream intact. They may:

• Act as hormones, regulating body functions or signaling changes in the external environment.

• Stimulate an immune response, which can be involved in food allergies.

What happens once amino acids enter the bloodstream?

• They are transported to the liver, where they can be used or released into the blood for other cells.

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

• If necessary, amino acids can be used for energy.

Why might ingesting a large dose of a single amino acid be problematic?

Because similar amino acids compete for the same absorption sites, taking excessive amounts of one amino acid can limit the absorption of others, potentially creating imbalances.

What structural and mechanical roles do proteins play in the body?

• Proteins form the backbone of muscle tissue (making up ~40% of body protein).

• They are critical in the structure of bones, teeth, skin, tendons, and connective tissue.

• Collagen acts like a glue between cells, providing structural support.

How do enzymes function in the body, and why are they important?

• Enzymes are proteins that act as catalysts, speeding up biochemical reactions.

• They lower the activation energy needed, enabling vital metabolic processes to occur efficiently.

Which hormones are protein-based, and what do they regulate?

• Many hormones, such as insulin, are made of protein.

• They regulate critical bodily functions, like blood glucose levels and metabolism.

What immune and transport functions do proteins serve?

• Antibodies (immunoglobulins) help identify and neutralize foreign agents (e.g., pathogens).

• Hemoglobin carries oxygen in red blood cells.

• Other proteins transport lipids, vitamins, and minerals throughout the body.

How do proteins help maintain fluid, electrolyte, and acid-base balance?

• Proteins help regulate fluid and electrolyte balance in body compartments.

• They act as buffers, helping to maintain the body’s pH within a narrow range.

In what circumstances do proteins provide energy for the body?

Proteins can be used for energy when carbohydrates and fats are insufficient (e.g., during prolonged fasting or very low carb intake).

How do amino acids influence gene expression?

• Nutrients (including amino acids) can affect gene regulation.

• This, in turn, influences the synthesis of enzymes and other proteins.

What is protein turnover?

• Protein turnover is the ongoing process of breakdown, recovery, and synthesis of amino acids in the body.

• This process ensures proteins are continually renewed and available for vital functions.

What defines a high-quality protein?

High-quality proteins contain all essential amino acids. They fully meet the body's amino acid needs when consumed.

How can complementary proteins meet amino acid needs?

By combining foods that lack certain essential amino acids (e.g., grains with legumes), complementary proteins together provide a complete amino acid profile.

What is the recommended protein intake for healthy adults?

Healthy adults should aim for approximately 0.8–1.2 g of protein per kilogram of body weight. Athletes or those building muscle might require more.

What does the Acceptable Macronutrient Distribution Range (AMDR) suggest about protein intake?

The AMDR recommends that protein provides 10–35% of total daily calories.

What health issues can arise from protein deficiency?

Protein deficiency may lead to slow growth, impaired immunity, and organ dysfunction.

What risks are associated with excessive protein intake?

Excess protein—especially from animal sources high in saturated fats—may contribute to heart disease, increase cancer risk, and cause kidney strain

What types of protein supplements are available?

Options include protein powders, amino acid supplements, and collagen products.

Which groups are highlighted as being at risk for harm from protein supplementation?

At-risk groups include women of childbearing age, infants, children, the elderly, people with metabolic errors, smokers, those on low-protein diets, and individuals with mental/physical illnesses.