Protein Unit

Essential amino acids

Amino acids that cannot be synthesized by the body and must be obtained from the diet. Examples: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine.

Non-essential amino acids

Amino acids synthesized by the body from other precursors. Examples: Alanine, Arginine, Asparagine, Aspartate, Glutamate, Glutamine, Glycine, Proline, Serine.

Conditionally essential amino acids

Amino acids normally non-essential but required in the diet during stress or illness. Examples: Cysteine, Tyrosine.

Tyrosine in PKU

In Phenylketonuria, a mutation prevents conversion of Phenylalanine to Tyrosine, requiring dietary Tyrosine.

Mnemonic for essential amino acids

PVT TIM HALL: Phenylalanine, Valine, Tryptophan, Threonine, Isoleucine, Methionine, Histidine, Arginine (context-specific), Leucine, Lysine.

Transamination

Transfers nitrogen between amino acid backbones to synthesize non-essential amino acids, forming glutamate and a keto-acid.

Deamination

Removes the amino group from an amino acid (e.g., glutamate), producing ammonia (NH₄⁺) and a keto-acid. Ammonia is converted to urea in the liver.

Ammonia toxicity

Ammonia acts as a solvent (like Windex) and is toxic. It is converted to urea in the liver via the urea cycle and excreted by kidneys.

Fates of amino acid carbon skeletons after deamination

1. Gluconeogenesis (glucose), 2. Ketogenesis (ketone bodies), 3. Energy (TCA cycle), 4. Synthesis (e.g., neurotransmitters).

Role of the keto-acid from deamination

Becomes fuel as glucose (gluconeogenesis) or fat (ketogenesis).

Steps of amino acid metabolism

1. Digestion/absorption (small intestine), 2. Transamination (nitrogen transfer), 3. Deamination (ammonia to urea), 4. Carbon skeleton fates (glucose, ketones, energy, synthesis).

Metabolic homeostasis in proteins

Balancing protein synthesis (anabolic) and breakdown (catabolic) to maintain stable amino acid levels and cellular function.

Protein turnover and homeostasis

Protein turnover (synthesis vs. breakdown) responds to physiological needs (e.g., oxidative damage, adaptation) to maintain homeostasis.

Primary structure of a protein

Sequential order of amino acids (e.g., MADQRKSSCTPGAECN). Determines all higher structures.

Secondary structure of a protein

Local 3D configurations, such as alpha helices and beta sheets, stabilized by hydrogen bonds.

Tertiary structure of a protein

Overall folding of the protein, arranging secondary structures into a full macromolecular shape for function.

Quaternary structure of a protein

Interactions between multiple protein chains (e.g., hemoglobin's four subunits for O₂ transport).

Protein structure and function

Structure determines function; specific shapes enable roles like enzyme catalysis or O₂ transport. Denaturation disrupts these structures, inactivating proteins.

Denaturation

Unfolding of a protein due to heat, acid, alkali, or salts, disrupting secondary, tertiary, and quaternary structures, rendering it inactive.

Nitrogen balance

Nitrogen Balance = (Protein Intake × 0.16) - Urinary Nitrogen - Other Nitrogen Losses (skin/fecal). Measures protein status.

Equilibrium nitrogen balance

Intake = output. Occurs in healthy adults (e.g., 70 kg man needs ~56 g protein/day).

Positive nitrogen balance

Intake > output. Occurs in growth, pregnancy, athletic training, recovery from illness.

Negative nitrogen balance

Intake < output. Occurs in fasting, severe illness, bed rest, poor protein intake.

Example of positive nitrogen balance

Growing child: 60g protein intake (9.6g N) > 8g N output = +1.6g.

Example of negative nitrogen balance

Fasting adult: 40g protein intake (6.4g N) < 10g N output = -3.6g.

Calculating nitrogen balance

For a patient with 80g protein intake, 12g urinary nitrogen, and 2g other losses: N In = 80 × 0.16 = 12.8g; N Out = 12 + 2 = 14g; Balance = 12.8 - 14 = -1.2g (negative).

Protein quality

Ability of a protein to provide essential amino acids in sufficient amounts for protein synthesis.

Limiting amino acid

An essential amino acid in shortest supply in a protein source, preventing protein synthesis unless supplemented.

Complete protein

Contains all essential amino acids in sufficient quantities (e.g., meat, eggs, soy).

Complementary proteins

Two food sources that make up for each other's inadequate essential amino acids (e.g., beans + rice).

Protein Digestibility Corrected Amino Acid Score (PDCAAS)

PDCAAS = chemical score * digestibility. Used for food labeling (e.g., milk/soy = 1, wheat = 0.42).

Biological value (BV) of a protein

Amount of protein nitrogen retained for growth/maintenance, expressed as % of digested nitrogen (e.g., egg = 100%, peanuts = 40%).

Egg white as a reference protein

It has 100% digestibility and high biological value, serving as a standard for protein quality.

All-or-nothing rule for protein synthesis

If any essential amino acid is missing, protein synthesis cannot occur; no partial synthesis is possible.

Excessive protein intake in US diet

Average intake (e.g., 105g/day for men) exceeds RDA (56g for 70 kg man).