The Chemistry of Living Systems CHAPTER 18 The Chemistry of Living Systems 18.3 Proteins

Cellular respiration Step 1: Glycolysis Step 2: Krebs cycle C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) Step 1: Glycolysis Step 2: Krebs cycle Step 3: Electron transport chain

Cellular respiration What are proteins? What do they do? C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l) Remember that this step required special proteins called cytochromes to transfer electrons. What are proteins? What do they do? Step 3: Electron transport chain

Proteins play many important roles Movement Muscles are primarily made of proteins Structure Tendons, skin, bones, claws, and fibers such as wool and hair Catalysis Enzymes that catalyze chemical reactions Reaction: A + B → C + D ∆H < 0 Enzymes lower Ea

Proteins play many important roles Hemoglobin is a bundle of four proteins in red blood cells Transport Hemoglobin, for example, transports oxygen to our tissues Storage Proteins store minerals needed by the body Protection Blood-clotting proteins which keep us from bleeding too much; antibodies from our immune system protect us from infections Energy Cytochromes transfer electrons through a series of redox transfer reactions

Proteins are polymers Proteins are made up of monomers called amino acids When you eat proteins, they are broken down into amino acids, which are then used to build new proteins in your body. Some amino acids cannot be produced by the body and must be obtained through food. They are called essential amino acids.

Proteins are polymers Proteins are made up of monomers called amino acids When you eat proteins, they are broken down into amino acids, which are then used to build new proteins in your body. Each amino acid has a similar composition: - an amino group (basic) - a carboxyl group (acidic) - an R group

Amino acids There are 20 naturally occurring amino acids, each with a different R group with different properties.

Amino acids There are 20 naturally occurring amino acids, each with a different R group with different properties. Serine Alanine The R-group can be polar or nonpolar

Amino acids Glutamate Arginine The R-group can be acidic or basic

Chirality in nature Nature tends to favor one isomer (or “handedness”) over the other. Most seashells twist to the right. Amino acids are almost always left-handed (in the L-form). Optical isomers chiral carbons

How do two monomers become linked? Primary structure one amino acid another amino acid How do two monomers become linked?

Primary structure together form H2O Two amino acid become linked through a condensation polymerization reaction

Primary structure a bond forms between C and N The carbon from COOH and nitrogen from NH2 form a peptide bond

Primary structure one amino acid another amino acid peptide bond water

If there is a mistake in the sequence, the consequences can be severe. Primary structure peptide bond peptide bond peptide bond peptide bond Alanine Threonine Glycine Glycine Serine Proteins have more than 100 amino acids linked together. This sequence is called the primary structure of the protein. If there is a mistake in the sequence, the consequences can be severe.

Secondary structures Alanine Threonine Glycine Serine Based on the sequence, the protein folds into a specific shape that allows it to perform its function

Secondary structures a helix There are two regular arrangements of amino acids. They are called secondary structures. a helix Hydrogen bonds every four amino acids give the chain the shape of a spiral

Secondary structures a helix pleated sheet (b sheet) There are two regular arrangements of amino acids. They are called secondary structures. a helix pleated sheet (b sheet) Hydrogen bonds every four amino acids give the chain the shape of a spiral Hydrogen bonds hold amino acids across from each other into the shape of a folded paper fan

Tertiary structures The secondary structures are organized further into a tertiary structure. a helix pleated sheet Lysozyme is an enzyme found in tears. It breaks down bacterial cell walls and protects us from infections

Tertiary structures Our body is mostly an aqueous environment hydrophobic amino acids tend to be buried inside Our body is mostly an aqueous environment hydrophilic amino acids tend to be on the surface Most proteins fold into a globular shape. Lysozyme for instance, is a globular protein.

Protein folding is still an active field of research. Tertiary structures Many intermolecular forces are involved in the folding of proteins. Each protein much reach a very specific tertiary structure in order to function properly. Protein folding is still an active field of research.

Tertiary structures Many intermolecular forces are involved in the folding of proteins. cysteine cysteine Keratin on our hair contains cysteine amino acids that, together, form disulfide bonds. When we get our hair “permed,” those disulfide bonds are broken and new ones are formed that hold a curl.

Enzymes Movement Muscles are primarily made of proteins Structure Tendons, skin, bones, claws, and fibers such as wool and hair Catalysis Enzymes that catalyze chemical reactions Transport Hemoglobin, for example, transports oxygen to our tissues Storage Proteins store minerals needed by the body Protection Blood-clotting proteins which keep us from bleeding too much; antibodies from our immune system protect us from infections Energy Cytochromes transfer electrons through a series of redox transfer reactions

Enzymes Reaction: A + B → C + D ∆H < 0 We have seen in Chapter 12 that enzymes are catalysts that lower the activation energy of chemical reactions Enzymes lower Ea

Enzymes Reaction: A + B → C + D ∆H < 0 We have seen in Chapter 12 that enzymes are catalysts that lower the activation energy of chemical reactions Enzymes lower Ea Also remember that a catalyst remains unchanged during the chemical reaction and can be used again.

Enzymes 1) A substrate (or reactant) binds to the active site of the enzyme. 2) The chemical reaction takes place, and the product is released. 3) The enzyme with an empty active site is ready to accept a new substrate. Also remember that a catalyst remains unchanged during the chemical reaction and can be used again.

Enzymes Some enzymes are very specific and can only accept one kind of substrate. Other enzymes, especially those in the liver, are less specific and can break down many different types of substrates. Also remember that a catalysts remains unchanged during the chemical reaction and can be used again.

Cytochrome proteins Movement Muscles are primarily made of proteins Structure Tendons, skin, bones, claws, and fibers such as wool and hair Catalysis Enzymes that catalyze chemical reactions Transport Hemoglobin, for example, transports oxygen to our tissues Storage Proteins store minerals needed by the body Protection Blood-clotting proteins which keep us from bleeding too much; antibodies from our immune system protect us from infections Energy Cytochromes transfer electrons through a series of redox transfer reactions

Cytochrome proteins The last step of cellular respiration requires cytochrome proteins to transfer high-energy electrons Step 3: Electron transport chain

Cytochrome proteins Step 3: Electron transport chain Cytochrome proteins all fold around a special porphoryn molecule or heme group that holds an iron (Fe) atom. Step 3: Electron transport chain

Cytochrome proteins Reduction: Fe3+ → Fe2+ + e– heme group Oxidation: Fe2+ + e– → Fe3+ Electrons are passed around through an oxidation-reduction cycle of iron

Proteins play many important roles in the body Composition of an amino acid Proteins are made of amino acids. The R group can be polar or nonpolar, and have acidic and basic properties. These groups play a crucial role in the structure and function of the protein. Movement Structure Catalysis Energy transfer Transport Storage Protection Proteins play many important roles in the body

Primary structure Specific sequence of amino acids a helix Primary structure Specific sequence of amino acids Secondary structure Amino acids are arranged into specific folds such as an a helix or a pleated sheet Tertiary structure The secondary structures are organized into a structure that allows the protein to function properly pleated sheet tertiary structure of lysozyme