Untitled Quiz

Proteins: Structure, Types & Function

• Proteins are the most abundant and functionally diverse molecules in living systems.
• Virtually every life process relies on proteins.
• Examples include enzymes, polypeptide hormones, contractile proteins (in muscle), collagen, hemoglobin, and immunoglobulins.

Protein Structure

• Proteins are polymers of amino acids.
• Twenty common amino acids are coded for by DNA and found in proteins.
• Amino acids are categorized based on their side chains (R groups):
  • Essential/nonessential: based on the body's ability to synthesize them.
  • Polar/nonpolar: based on their interaction with water.
  • Charged/uncharged: based on their charge at neutral pH.
  • Basic/acidic: based on their net charge at neutral pH.

Biological Functions of Proteins

• Enzymes: Catalysts for biochemical reactions.
• Transport molecules: Examples include hemoglobin and lipoproteins.
• Contractile/motion: Examples include myosin and actin.
• Structural: Examples include collagen and keratin.
• Defense: Antibodies.
• Signaling: Hormones and receptors.
• Toxins: Examples include diphtheria and enterotoxins.

Amino Acid Structure

• Each amino acid has a central carbon atom (α-carbon) bonded to:
  • A carboxyl group (-COOH).
  • An amino group (-NH2).
  • A hydrogen atom.
  • A distinctive side chain (R group).
• Amino acids join together via peptide bonds formed between the carboxyl group of one amino acid and the amino group of another.

Amino Acid Classification: Essential/Nonessential

• Nonessential amino acids can be synthesized by the body.
• Essential amino acids must be obtained from the diet.

Nonpolar Amino Acids

• These amino acids have nonpolar side chains.
• They tend to cluster together in the interior of proteins in aqueous solutions due to hydrophobic interactions.
• Proline, glycine, alanine, phenylalanine, valine, leucine, isoleucine, tryptophan, and methionine are examples.

Nonpolar Amino Acid: Proline

• Proline's side chain and α-amino nitrogen form a rigid, five-membered ring structure.
• It's a secondary amino acid (rather than a primary).
• It often interrupts a-helices in globular proteins, contributing to protein structure.
• It is a nonpolar, nonaromatic, nonessential amino acid with a high concentration in collagen.
• Its unique geometry in proteins affects how the rest of the protein structure is built.

Nonpolar Amino Acid: Glycine

• Glycine is a nonpolar, nonessential amino acid.
• It dissolves in water and other organic solvents due to both hydrophobic and hydrophilic properties.
• It's a precursor to heme and the purine ring.

Nonpolar Amino Acid: Alanine

• Alanine is a nonpolar, nonessential amino acid.
• It is released from muscle during conditions like starvation, exercise, or high-carbohydrate meals.

Nonpolar Amino Acid: Phenylalanine

• Nonpolar, aromatic, and essential amino acid.
• It's a precursor to tyrosine, dopamine, norepinephrine, epinephrine, thyroxine, and melanin.
• Used in treating depression, hyperactivity disorder, and Parkinson's disease.

Nonpolar Amino Acid: Valine

• Nonpolar and essential amino acid, involved in nerve myelin sheath function and various bodily functions.

Nonpolar Amino Acid: Leucine

• Branched chain, essential, nonpolar amino acid.
• High concentration in globulins and albumins (hemoglobin).
• Acts as a component in various proteins.
• Similar to valine, differing only by one additional methylene group in the side chain.

Nonpolar Amino Acid: Isoleucine

• Nonpolar and essential amino acid, isomer of leucine.

Nonpolar Amino Acid: Tryptophan

• Nonpolar, aromatic, and essential amino acid.
• Precursor for serotonin, melatonin, and niacin (vitamin B3).
• Deficiency can lead to various symptoms, including anxiety, depression, low mood, weight loss, and impaired growth.

Nonpolar Amino Acid: Methionine

• Nonpolar, essential, sulfur-containing amino acid.
• Precursor to cysteine, involved in liver detoxification and other bodily functions.

Uncharged Polar Amino Acids

• These amino acids have polar, uncharged side chains.
• They can form hydrogen bonds with other molecules.
• Examples: Serine, threonine, tyrosine, asparagine, glutamine, and cysteine.

Amino Acids with Acidic Side Chains

• These amino acids have acidic side chains that lose protons readily.
• At physiological pH (around 7.4), they carry a net negative charge.
• As examples, there is aspartic acid and glutamic acid.

Amino Acids with Basic Side Chains

• These amino acids have basic side chains that can accept protons.
• Commonly, they carry a net positive charge.
• Histidine, lysine, and arginine are examples.

Protein Folding

• Protein folding determines biological function.
• Chaperone proteins assist in the process.
• Improper folding can lead to disease.

Protein Denaturation

• Denaturation is the process of unfolding and disorganizing the protein's secondary and tertiary structures.
• Various factors, like heat, strong acids/bases, and detergents, can cause denaturation.
• Denaturation is usually not accompanied by hydrolysis of peptide bonds.

Protein Classification: Globular vs. Fibrous

• Globular proteins: Compact, roughly spherical in shape, often water-soluble; involved in metabolic processes
• Fibrous proteins: Extended, long, rope-like structures; insoluble in water; provide structural support.

Collagen

• Abundant structural protein, forming an important part of connective tissue.
• Characteristic triple-helix structure with significant proline, glycine, and hydroxyproline content.

Elastin

• Insoluble protein crucial for elasticity in connective tissues.
• Rich in proline and lysine, lacks hydroxyproline.
• Provides elasticity to tissues like lungs and arteries.

Hemoglobin

• A globular protein in red blood cells, responsible for oxygen transport.
• Composed of four polypeptide subunits, each containing a heme group.
• Exhibits cooperative oxygen binding.
• Its oxygen-binding properties are regulated by various factors: partial pressure of oxygen (pO2), pH, and the presence of 2,3-bisphosphoglycerate (2,3-BPG).

Diseases caused by Protein Abnormality

• Amyloid disease: Caused by misfolding of proteins, forming aberrant assemblies.
• Prion disease: Caused by abnormal folding of prion protein (PrP), leading to misfolded aggregates.

Other Important Topics Covered

• Amino acid abbreviations and symbols.
• Optical properties of amino acids (L-configuration).
• Buffering properties of amino acids.
• Drug absorption considerations based on pH.
• Cooperative binding in hemoglobin(binding of oxygen).
• Differences between oxyhemoglobin and deoxyhemoglobin.
• Allosteric effects and various factors affecting oxygen binding affinity of hemoglobin.
• Hemoglobin variants (minor hemoglobins, hemoglobinopathies).
• Denaturation and renaturation of proteins.
• Role of chaperone proteins in protein folding.