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Proteins chemistry. Proteins What are the many functions of proteins? What are amino acids? What are zwitterions? What determines the characteristics.

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Presentation on theme: "Proteins chemistry. Proteins What are the many functions of proteins? What are amino acids? What are zwitterions? What determines the characteristics."— Presentation transcript:

1 Proteins chemistry

2 Proteins What are the many functions of proteins? What are amino acids? What are zwitterions? What determines the characteristics of amino acids? What are uncommon amino acids? How do amino acids combine to form proteins?

3 Function of proteins Proteins serve many functions, including the following. Given are examples of each. Structure: collagen and keratin are the chief constituents of skin, bone, hair, and nails. Catalysts: virtually all reactions in living systems are catalyzed by proteins called enzymes. Movement: muscles are made up of proteins called myosin and actin. Transport: hemoglobin transports oxygen from the lungs to cells; other proteins transport molecules across cell membranes. Hormones: many hormones are proteins, among them insulin, oxytocin, and human growth hormone.

4 Function of proteins (cont’d) Protection: blood clotting involves the protein fibrinogen; the body used proteins called antibodies to fight disease. Storage: casein in milk and ovalbumin in eggs store nutrients for newborn infants and birds; ferritin, a protein in the liver, stores iron. Regulation: certain proteins not only control the expression of genes, but also control when gene expression takes place.

5 Amino acids Amino acid: Amino acid: a compound that contains both an amino group and a carboxyl group –  -Amino acid has –  -Amino acid has an amino group attached to the carbon adjacent to the carboxyl group –  -carbon also bound to side chain group, R – R gives identity to amino acid – Two steroisomers of amino acids are designated L- or D-. Based on similarity to glyceraldehdye

6 Amino Acid Structure and Properties With the exception of glycine, all protein- derived amino acids have at least one stereocenter (the  - carbon) and are chiral (stereoisomers) – the vast majority of  - amino acids have the L- configuration at the  - carbon (Proline is usually D)

7 Amino Acid Structure and Properties (cont’d) A comparison of the stereochemistry of L-alanine and D-glyceraldehyde (as Fischer projections):

8 Amino acids classification Amino acids are classified according to two major criteria: – The polarity of the side chains – The presence of an acidic or basic group in the side chain

9 Individual Amino Acids Group A:Nonpolar side chains Ala, Val, Leu, Ile, Pro. Phe, Trp, Met. 1.Ala, Val, Leu, Ile, Pro- contain aliphatic hydrocarbon group. 2.Pro has cyclic structure. 3.Phe- hydrocarbon aromatic ring. 4.Trp- Indole ring side chain, aromatic. 5. Met- Sulfur atom in side chain.

10 Nonpolar side chains (at pH 7.0)

11 Amino acids (cont’d) Group B: Neutral Polar side chains Thr, Tyr, Cys, Gln, Asn and Ser 1.Ser, Thr- Side chain is polar hydroxyl group 2.Tyr- hydroxyl group bonded to aromatic hydrocarbon group 3.Cys- Side chain contains thiol group (-SH) 4.Gln, Asn- contain amide bonds in side chain

12 Polar side chains (at pH 7.0)

13 Amino acids (cont’d) Group C: Acidic Side Chains Glu, Asp 1.Both have a carboxyl group in side chain 2.Can lose a proton, forming a carboxylate ion 3.These amino acids are negatively charged at neutral pH

14 Acidic side chain

15 Amino acids (cont’d) Group D: Basic side chains His, Lys, Arg 1.Side chains are positively charged at pH 7 2.Arg-side chain is a guanidino group 3.His-side chain is an imidazole group 4.Lys-side chain NH 3 group is attached to an aliphatic hydrocarbon chain

16 Basic side chain

17 Amino acids summary Important structural features: 1.All 20 are  -amino acids 2.For 19 of the 20, the  -amino group is primary; for proline, it is secondary 3.With the exception of glycine, the  -carbon of each is a stereocenter 4.Isoleucine and threonine contain a second stereocenter 5.Amino acids can be referred to by three-letter or one- letter codes in.

18 Practice 1 Identify the amino acids with nonpolar side chains and those with basic side chains in the following group: 1.Alanine 2.Serine 3.arginine 4.lysine 5.leucine 6.phenylalanine

19

20 Solution 1 Nonpolar Alanine leucine phenylalanine Basic Arginine lysine Polar Serine

21 Uncommon Amino Acids Each derived from a common amino acid by a modification – hydroxylysine and hydroxyproline are found only in a few connective tissues such as collagen – thyroxine is found only in the thyroid gland

22 Ionization of Amino Acids In amino acid, the carboxyl group and the amino group are charged at neutral pH. The carboxylate portion (-) The amino group (+) In free amino acids  -carboxyl, and α - amino groups have titratable protons. Some side chains do as well

23 Ionization of Amino Acids The net charge on an amino acid depends on the pH of the solution in which it is dissolved. – If we dissolve an amino acid in water, it is present in the aqueous solution as its zwitterion. – If we now add a strong acid such as HCl to bring the pH of the solution to 2.0 or lower, the strong acid donates a proton to the -COO - of the amino acid turning the zwitterion into a positive ion. – If we add a strong base such as NaOH to the solution and bring its pH to 10.0 or higher, a proton is transferred from the NH 3 + group to the base turning the zwitterion into a negative ion.

24 Ionization of amino acids

25 To summarize: R emember, amino acids without charged groups on side chain exist in neutral solution as zwitterions with no net charge

26 Ionization of amino acids Isoelectric pH, pI: Isoelectric pH, pI: the pH at which the majority of molecules of a compound in solution have no net charge Electrophoresis: the process of separating compounds on the basis of their electric charge Electrophoresis: the process of separating compounds on the basis of their electric charge

27 Peptides Individual amino acids can be linked by forming covalent bonds. Peptide bond: Peptide bond: the special name given to the amide bond between the  -carboxyl group of one amino acid and the  -amino group of another amino acid

28 Peptide bond:

29 Peptides peptide peptide: the name given to a short polymer of amino acids joined by peptide bonds; they are classified by the number of amino acids in the chain dipeptide dipeptide: a molecule containing two amino acids joined by a peptide bond tripeptide tripeptide: a molecule containing three amino acids joined by peptide bonds polypeptide polypeptide: a macromolecule containing many amino acids joined by peptide bonds protein protein: a biological macromolecule of molecular weight 5000 g/mol or greater, consisting of one or more polypeptide chains

30 Peptides By convention, peptides are written from the left, beginning with the free -NH3+ group and ending with the free -COO- group on the right. C-terminal amino acid: the amino acid at the end of the chain having the free -COO- group. N-terminal amino acid: the amino acid at the end of the chain having the free -NH3+ group.

31 Practice 2 Write an equation with structures for the formation of a dipeptide when alanine reacts with glycine to form a peptide bond? Is there more than one possible product for this reaction?

32 Peptides with Physiological Activity Carnosine: – is a dipeptide – found in muscle tissue Glutathione: – It is tripeptide – It is a scavenger for oxidizing agents – Harmful (cancer) Enkephalins – Pentapeptides – Pain relievers

33 Peptides with Physiological Activity Oxytocin – induce labor in pregnant – Stimulate the flow of milk in a nursing mother Vasopressin – Control blood pressure – Antidiuretic effect

34 HW Name an amino acid in which the R group contains the following: – An aromatic ring – An amide group – An indole ring – A second chiral carbon atom – A hydroxy group – A sulfur atom – An amino group – An acid group – A guanidino group


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