2.2 Proteins: Structure, Function, and Metabolism

Amino acids are the building blocks of proteins, and the specific way they're arranged determines what each protein can do. From the essential amino acids you must get from food to the non-essential ones your body can make on its own, these molecules support everything from muscle structure to immune defense.

Proteins serve remarkably diverse roles: building tissues, speeding up chemical reactions, transporting molecules, and more. Understanding how you digest and metabolize proteins is central to grasping their importance in nutrition.

Amino Acids and Protein Structure

Structure and role of amino acids

Every amino acid shares the same basic blueprint. A central carbon atom (called the alpha carbon) bonds to four groups: an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom, and a side chain (R group). The R group is what makes each amino acid unique. Some R groups are hydrophobic, some are polar, and some carry an electrical charge. These differences matter because they influence how the final protein folds and behaves.

Amino acids link together through peptide bonds. This is a condensation reaction: the amino group of one amino acid reacts with the carboxyl group of another, releasing a water molecule. Chain enough amino acids together and you get a polypeptide.

Proteins have four levels of structure, and each level builds on the last:

• Primary structure: the specific sequence of amino acids in the chain. Even one substitution can change how the protein works.
• Secondary structure: local folding patterns like alpha helices and beta sheets, held together by hydrogen bonds along the backbone.
• Tertiary structure: the overall 3D shape of a single polypeptide, stabilized by interactions between R groups (disulfide bonds, hydrophobic interactions, ionic bonds).
• Quaternary structure: two or more polypeptide chains coming together to form a functional protein. Hemoglobin is a classic example, made of four polypeptide subunits.

Essential vs non-essential amino acids

Your body needs 20 amino acids to build proteins, but it can't make all of them.

• Essential amino acids (9 total) must come from your diet because your body cannot synthesize them. These include leucine, isoleucine, and valine (the branched-chain amino acids), along with histidine, lysine, methionine, phenylalanine, threonine, and tryptophan. If even one is missing, your body can't build the proteins that require it.
• Non-essential amino acids are ones your body can produce on its own, so you don't strictly need them from food. Examples include alanine, aspartic acid, and glutamic acid. They're still crucial for protein synthesis; "non-essential" just means your body has the machinery to make them.
• Conditionally essential amino acids are normally non-essential but become essential under certain conditions like illness, severe stress, or rapid growth. Arginine, for instance, becomes essential during wound healing because demand outpaces what the body can produce. Cysteine is another example: premature infants have limited ability to synthesize it.

Protein Functions and Metabolism

Functions of proteins in the body

Proteins do far more than build muscle. Here are the major categories:

• Structural support: Collagen strengthens connective tissue in skin, bones, and tendons. Keratin forms the tough, protective layers in hair, nails, and the outer layer of skin.
• Enzymes: These proteins act as biological catalysts, speeding up chemical reactions without being used up. Digestive enzymes like amylase and lipase are familiar examples, but enzymes regulate nearly every metabolic pathway in your cells.
• Transport: Hemoglobin carries oxygen in red blood cells. Albumin shuttles fatty acids through the blood and helps maintain blood osmotic pressure (which keeps fluid from leaking out of your blood vessels).
• Hormones: Some hormones are proteins. Insulin regulates blood glucose levels, and growth hormone stimulates cell growth and reproduction.
• Immune defense: Antibodies are proteins that recognize and neutralize foreign invaders like bacteria and viruses. Complement proteins assist by enhancing the overall immune response.
• Energy source: Protein provides 4 kcal per gram, but your body prefers to use carbohydrates and fats for fuel. Protein is tapped for energy mainly during prolonged fasting or intense exercise when other fuel sources run low.

Protein digestion and metabolism

Digestion breaks proteins down into individual amino acids your body can absorb. The process works in stages:

1. Stomach: Hydrochloric acid denatures (unfolds) proteins, and the enzyme pepsin begins cleaving peptide bonds.
2. Small intestine: Pancreatic enzymes, mainly trypsin and chymotrypsin, continue breaking polypeptides into smaller peptide fragments.
3. Brush border: Enzymes on the surface of intestinal cells finish the job, splitting small peptides into individual amino acids (and some di- and tripeptides).

Absorption happens in the small intestine. Amino acids are transported into intestinal cells through active transport, then enter the portal vein and travel to the liver. From there, they're distributed to tissues for protein synthesis or, if needed, energy production.

Protein metabolism is a constant balancing act between building proteins and breaking them down:

• Protein turnover is the continuous cycle of synthesis and degradation. Your body is always recycling proteins, not just when you eat.
• Transamination transfers an amino group from one amino acid to a keto acid, creating a different amino acid. This is how your body makes many non-essential amino acids.
• Deamination strips the amino group off an amino acid so the remaining carbon skeleton can be used for energy or converted to glucose.
• Urea cycle: The amino groups removed during deamination contain nitrogen, which is toxic in excess. The liver converts this nitrogen into urea, which the kidneys then excrete in urine.

Protein synthesis builds new proteins from amino acids in three broad steps:

1. Transcription: The DNA code for a protein is copied into messenger RNA (mRNA) in the nucleus.
2. Translation: Ribosomes read the mRNA sequence and assemble the corresponding chain of amino acids.
3. Post-translational modification: The new polypeptide folds into its functional shape and may undergo additional processing (like adding sugar groups or being cleaved into a smaller active form).

Nitrogen balance is a useful way to assess someone's overall protein status. It compares nitrogen intake (from dietary protein) to nitrogen excretion (mainly in urine):

• Positive nitrogen balance: More protein is being built than broken down. This is normal during growth, pregnancy, and recovery from injury.
• Negative nitrogen balance: More protein is being broken down than built. This signals malnutrition, illness, or muscle wasting.
• Equilibrium: Synthesis and breakdown are roughly equal, which is the typical state for healthy adults.