Protein Digestion and Function
Amino Acids
All proteins contain: carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
All 20 amino acids share a common structure:
Carbon backbone with an amino group (NH₂), an acid group (COOH), a hydrogen atom, and a distinctive side chain (R group).
Each amino acid has a unique side chain that determines its properties and function.
Essential vs non-essential:
9 essential amino acids must be obtained from the diet.
11 non-essential amino acids can be synthesized by the body.
Some amino acids can be conditionally essential depending on life stage or health status.
The Chemist’s View of Proteins
There are 20 different amino acids, each with a specific side group that determines its properties.
Protein uniqueness arises from the sequence arrangement of these amino acids.
The human body distinguishes essential vs nonessential amino acids and may deem some conditionally essential under certain conditions.
Conceptual analogy: just as there are many possible words from 26 letters, there are many possible protein structures from 20 amino acids.
Protein Structures and Bonds
Proteins are built from amino acids linked by peptide bonds formed through condensation reactions, which release a molecule of water.
Peptide bond formation is a chemical linkage; the sequence of amino acids is critical for protein function.
Structural hierarchy:
Primary structure: the linear sequence of amino acids.
Secondary structure: local shapes stabilized by weak hydrogen bonds (e.g., alpha helices, beta sheets).
Tertiary structure: the overall 3D folding due to hydrophilic/hydrophobic interactions and other bonds.
Quaternary structure: two or more polypeptide chains interact to form a functional protein.
Peptides vs proteins:
Dipeptide: two amino acids bonded together.
Tripeptide: three amino acids bonded together.
Polypeptide: ten or more amino acids bonded together.
Peptides < 50 amino acids are typically referred to as peptides.
Proteins typically contain > 50 amino acids and commonly range from about 100 to 10,000 amino acids.
Insulin example:
Insulin has 51 amino acids in short polypeptide chains.
Two polypeptide chains are linked by disulfide bridges (–S– bridges).
A disulfide bridge spaces a portion of the chain; proper linkage and sequence are essential for insulin’s biological activity.
Protein Structures: Examples and Implications
Hemoglobin as a quaternary structure example: composed of four polypeptide chains.
If amino acid sequencing is altered, the protein’s shape and function can be disrupted (e.g., oxygen transport by hemoglobin).
Protein Denaturation
Denaturation disrupts or destroys protein structure, uncoiling the chains and destroying function.
Peptide bonds remain intact during denaturation.
Causes of denaturation:
Stomach acid (HCl) denatures proteins during digestion.
High fever can denature proteins (e.g., hemoglobin) and be life-threatening.
Acidity/alkalinity shifts can denature proteins.
Mechanical action (e.g., whipping eggs) can denature proteins.
Heat and certain chemical environments used in cooking also cause denaturation.
Protein Denaturation in Cooking
Cooking often denatures proteins, altering texture and function without breaking peptide bonds.
Digestion and Absorption of Proteins
In the mouth and salivary glands:
Chewing and mixing with saliva begins mechanical breakdown.
No enzymatic digestion of protein occurs in the mouth.
In the stomach:
Pepsinogen is activated to pepsin by hydrochloric acid (HCl).
HCl denatures proteins, facilitating digestion.
Proteins are broken down into shorter polypeptide chains; more digestion occurs in the small intestine (SI).
In the small intestine (SI):
Pancreatic proteases (from the pancreas) and SI proteases continue protein digestion.
Peptidase enzymes from SI villi digest di- and tri-peptides specifically.
Proteases vs SI peptidases:
Proteases target whole proteins.
SI peptidases target di- and tri-peptides.
Protein absorption:
Amino acids are transported to intestinal cells via specific carriers.
Absorption uses active transport and requires ATP.
SI cells may use some amino acids for energy or for synthesis of other SI components.
Unused amino acids are released into the blood and transported to the liver for distribution and metabolism.
Protein Synthesis and Gene Expression
Proteins are encoded by DNA; DNA templates govern protein synthesis.
Transcription: DNA template is used to make messenger RNA (mRNA).
Translation: mRNA carries the code to ribosomes, which, with transfer RNA (tRNA), assemble amino acids in the correct sequence to form a protein.
mRNA specifies amino acid sequence; tRNA delivers amino acids in the correct order to the ribosome.
Sequencing errors can occur and alter protein shape and function (example: sickle cell anemia).
Example: Sickle-cell anemia
A genetic sequencing error replaces valine for glutamic acid in one of the beta chains of hemoglobin.
This single amino acid substitution alters the protein’s shape and function, impairing oxygen transport.
Gene expression and dietary influence:
Cells regulate gene expression to produce the proteins needed for each cell type.
Nutrients can influence gene expression (epigenetics), turning genes on or off.
Disease development can occur even without sequencing errors in the protein chain.
Proteins as Structural Materials, Enzymes, and Hormones
Structural materials:
Collagen matrix provides structural support in connective tissues.
Bones and teeth include mineral-crystal components embedded in a protein matrix.
Proteins replace dead or damaged cells.
Enzymes:
Act as catalysts to speed up chemical reactions, enabling digestion and metabolism.
Hormones:
Serve as messenger molecules (e.g., insulin) that travel in the blood to target tissues.
Regulators of fluid balance and transport:
Proteins help regulate fluid balance and act as transporters for lipids, minerals, vitamins, and oxygen.
Antibodies and immune function:
Antibodies defend the body against disease and provide immune memory.
Other roles:
Energy source and glucose when carbohydrate intake is insufficient (gluconeogenesis).
Blood clotting (fibrin), skin repair (collagen), and vision (opsin) are protein-related functions.
Protein Quality, Digestibility, and Complementation
Digestibility and amino acid composition:
Animal proteins: ~90–99% absorbed.
Plant proteins: ~70–90% absorbed.
For protein synthesis, cells must have all essential amino acids available simultaneously; if any essential amino acid is missing, protein synthesis is limited.
High-quality protein:
Contains all essential amino acids in relatively equal amounts needed by humans.
Plant proteins often miss one or more essential amino acids.
Complementary proteins (for vegetarians):
Plant proteins may have limiting amino acids, but combining different plant sources can provide a complete amino acid profile.
Legumes tend to be high in isoleucine (Ile) and lysine (Lys) but low in methionine (Met) and tryptophan (Trp).
Grains tend to be lower in Lys and Ile but higher in Met and Trp; together with legumes, they complement to provide all essential amino acids.
Examples: Rice + Beans, Corn + Beans, Nuts + Grain.
Protein in Foods: Intake, Labeling, and Supplements
Digestibility and protein quality impact dietary recommendations and food choices.
Food intake guidelines (example framework):
One ounce (serving) of protein provides about 7 g of protein.
Protein foods include meat, fish, eggs, nuts, seeds, and legumes.
Milk and dairy provide roughly 8 g per cup.
Fruits, vegetables, and grains contribute small amounts of protein.
Protein quality and labeling:
Protein is listed on food labels in grams; a daily value (DV) of 50 g is used in labeling, but %DV is not required unless:
There is a protein claim by the manufacturer (≥20% DV is considered “High”).
The product is intended to feed children under age 4.
Labels may refer to both the quantity and quality of protein.
Protein and amino acid supplements:
Building muscle depends on overall training and energy balance; protein supplementation alone does not automatically improve performance.
Protein powders are used by some for athletic performance; effectiveness depends on overall diet and exercise.
Amino acid supplements carry potential risks; benefits are not always clear; lysine and tryptophan are examples of amino acids with particular considerations.
Health Effects and Recommended Intakes of Protein
Health effects of protein:
Protein deficiency can lead to protein-energy malnutrition (PEM), including marasmus and Kwashiorkor, often due to too few calories and insufficient protein.
Heart disease risk can be influenced by saturated fat intake associated with many animal-protein sources; some amino acids (e.g., homocysteine) have been studied for heart disease risk, with arginine potentially influencing blood pressure and clotting (areas still under investigation).
Cancer risk: High intake of processed meat, red meat, and charred meats is associated with increased cancer risk; overall diet quality matters.
Osteoporosis concerns: Higher protein intake can increase calcium excretion; it is important to monitor calcium intake concomitantly.
Weight control: Protein at each meal can promote fullness and satiety; some high-protein/low-carb patterns may support lean mass gains.
Kidney disease: A higher protein load can increase the workload on the kidneys; it may accelerate deterioration in existing kidney disease but does not by itself cause kidney disease.
Recommended intakes:
AMDR (Acceptable Macronutrient Distribution Range) for protein:
Example: for a 2000 kcal/day diet, this corresponds to approximately from protein, i.e., about (since ).
RDA for healthy adults:
Groups with higher needs: infants and children; people in recovery; pregnant women; athletes.
From guidelines to groceries:
Protein foods include meat, fish, eggs, nuts, seeds, and legumes; a typical serving delivers about 7 g per ounce.
Milk and dairy provide roughly 8 g per cup; grains and vegetables contribute smaller amounts.
Nitrogen Balance and Metabolism
Nitrogen balance as a health measure:
Nitrogen intake from dietary protein vs. nitrogen losses from protein breakdown for energy or amino acid use.
Equilibrium (net nitrogen balance): intake = breakdown. Health indicates adequate protein intake.
Positive nitrogen balance: synthesis exceeds breakdown (growth, pregnancy, tissue repair).
Negative nitrogen balance: breakdown exceeds synthesis (illness, starvation, injury).
Turnover and amino acid pool:
Continuous turnover of body proteins; released amino acids contribute to the amino acid pool.
The amino acid pool provides substrates for protein synthesis and can also provide energy or be converted to other substances or stored as fat.
Deamination and nitrogen disposal:
Deamination is the removal of the nitrogen-containing amino group (NH₂) before amino acids can be used for other purposes.
Deamination results in ammonia (NH₃), a toxic substance that must be eliminated.
The liver converts ammonia to urea; urea is released into the blood and excreted by the kidneys in urine.
Deamination increases with higher protein intake that is not used for protein synthesis.
Urea synthesis and removal:
Liver converts ammonia to urea (a less toxic compound).
Kidneys filter urea from the blood and excrete it in urine; adequate water is needed to dilute and excrete urea.
Liver disease can elevate ammonia levels, which can affect mental status; kidney disease can elevate urea levels. Lab values of ammonia and urea help diagnose organ function.
Check Your Knowledge (concepts):
Nitrogen balance concepts help assess protein status and health.
Equilibrium = healthy; positive balance indicates growth; negative balance indicates potential malnutrition or disease.
Practical Considerations: Food Sources, Labeling, and Supplements
Practical protein sources and quality:
Animal proteins generally provide high-quality, highly digestible protein.
Plant proteins are often lower in one or more essential amino acids; complementary combinations can achieve complete protein quality.
Food labeling and protein claims:
Labels report protein grams; a daily value (DV) of 50 g is used on labels.
%DV is not required unless there is a manufacturer protein claim or other specific labeling rules (e.g., foods for children under age 4).
Supplements and practical use:
Protein powders may not automatically enhance performance without proper training and overall diet.
Amino acid supplements can have risks; their benefits are not always clear, and some amino acids (e.g., lysine and tryptophan) require careful consideration.
Summary: Key Takeaways
Proteins have multiple levels of structure (primary, secondary, tertiary, quaternary) determined by amino acid sequence and interactions.
Digestion begins in the stomach with denaturation by HCl and pepsin activity, continues in the small intestine with pancreatic and SI enzymes, and ends with absorption of amino acids via active transport.
Protein synthesis is governed by transcription (DNA to mRNA) and translation (mRNA to protein) with tRNA delivering amino acids; sequencing errors can have serious consequences (e.g., sickle cell anemia).
Protein quality depends on amino acid completeness and digestibility; complementary plant proteins can achieve high-quality protein when combined.
Health considerations include energy balance, disease risk, and organ health; RDA and AMDR provide dietary targets, with higher needs for certain populations.
Practical dietary guidance emphasizes diverse protein sources, awareness of labeling, and the role of protein in satiety, muscle maintenance, and overall health.
Formulas and conversions to remember:
AMDR for protein: of daily energy intake.
Daily protein intake range (example): if daily energy = , then protein energy = , which corresponds to roughly (since ).
RDA for healthy adults: .
Peptide bond formation (conceptual):
Protein density in foods and digestibility vary by source: animal proteins ~0.90–0.99 digestibility; plant proteins ~0.70–0.90 digestibility.
Special note on heredity and protein: DNA → mRNA (transcription) → protein (translation); amino acid sequencing determines protein shape and function; transcription/translation errors can alter function (e.g., sickle cell anemia).