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

2. 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

3. 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

4. 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

5. 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

6. 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.

7. 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

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

9. 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

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

11. 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

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

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

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

15. Primary structure
one amino acid
another amino acid
peptide bond
water

16. 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.

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

18. 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

19. 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

20. 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

21. 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.

22. 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.

23. 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.

24. 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

25. 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

26. 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.

27. 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.

28. 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.

29. 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

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

31. 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

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

33. 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

34. 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