Nursing First semester 2021 General structure Proteins are polymers of αamino acids or amino acids An amino acid consists of a central carbon atom called the carbon linked to four groups ID: 908001
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Slide1
Amino acids
Prof. Mamoun Ahram
Nursing
First semester, 2021
Slide2General structure
Proteins are polymers of
α-amino acids (or amino acids).
An amino acid consists of a central carbon atom, called the carbon, linked to four groupsan amino group (-NH2), a carboxylic acid group (-COOH), a hydrogen atom, and a specific R group (the side chain)
Slide3L and D isomers
With four different groups connected to the tetrahedral
α
-carbon atom, amino acids can be present in two forms that are mirror-images of each other (they are enantiomers).They are called L isomer and D isomer.Amino acids with their two isomers are said to be chiral (when a central carbon is bonded to four different groups).The presence of one chiral carbon atom always produces a chiral molecule that exists in mirror-image forms.
Only L amino acids naturally make up proteins.
Slide4Slide5Types of amino acids
There are twenty kinds of amino acids depending on the side chains varying in:
Size
ShapeChargehydrogen-bonding capacityhydrophobic characterchemical reactivity
Slide6Negatively-charged
Positively-charged
Polar
Non-polar, aliphatic
Glutamate
Lysine
Serine
Phenylalanine
Glycine
Aspartate
Arginine
Threoeine
Tryptophan
Alanine
Histidine
Glutamine
Valine
Asparagine
Leucine
Cysteine
Isoleucine
Tyrosine
Methionine
Proline
Slide7Glycine
The simplest one is glycine, which has just a hydrogen atom as its side chain.
With two hydrogen atoms bonded to the
-carbon atom, glycine is unique in being achiral (not chiral).
Slide8Non-polar, aliphatic amino acids
Slide9Alanine
Alanine, the next simplest amino acid, has a methyl group (-CH3) as its side chain.
Slide10Valine, leucine, and isoleucine
Larger hydrocarbon side chains are found in valine, leucine, and isoleucine.
Slide11Methionine
Methionine contains an aliphatic side chain that includes a thioether (-S-) group.
Ether
Thioether
Slide12Proline
Proline also has an aliphatic side chain, but is bonded to both the nitrogen and the
-carbon atoms.The ring structure of proline makes it more rigid than the other amino acids.
-nitrogen
Slide13Phenylalanine and Tryptophan
Phenylalanine contains a phenyl ring.
Tryptophan has an indole ring; the indole group consists of two fused rings and an NH group.
Slide14Positively-charged amino acids
Slide15Lysine and arginine
Lysine and arginine have relatively long side chains that terminate with groups that are positively charged at neutral pH.
Lysine ends with a primary amino group and arginine by a guanidinium group.
Slide16Histidine
Histidine contains an imidazole group, an aromatic ring that also can be positively charged.
Slide17Negatively-charged amino acids
Slide18Aspartic acid and glutamic acid
Two amino acids contain acidic side chains: aspartic acid and glutamic acid.
These amino acids are often called aspartate and glutamate when they are charged.
Slide19Polar amino acids
Slide20Serine and threonine
Serine and threonine, contain aliphatic hydroxyl groups.
The hydroxyl groups on serine and threonine make them hydrophilic and reactive.
Slide21Cysteine
Cysteine contains a sulfhydryl or thiol (-SH), group. The sulfhydryl group is reactive.
Slide22Asparagine and glutamine
Asparagine and glutamine are uncharged derivatives of aspartate and glutamate.
Each contains a terminal carboxamide in place of a carboxylic acid.
Slide23Tyrosine
The aromatic ring of tyrosine contains a hydroxyl group. This hydroxyl group is reactive.
Slide24Three-letter abbreviation
Amino acid
Ala
Alanine
Arg
Arginine
Asn
Asparagine
Asp
Aspartic Acid
Cys
Cysteine
Gln
Glutamine
Glu
Glutamic Acid
Gly
Glycine
His
Histidine
Ile
Isoleucine
Leu
Leucine
Lys
Lysine
Met
Methionine
Phe
Phenylalanine
Pro
Proline
Ser
Serine
Thr
Threonine
Trp
Tryptophan
Tyr
Tyrosine
Val
Valine
Amino acids are often designated by either a three-letter abbreviation.
Slide25Ionization of amino acids
Slide26Why do amino acids get ionized?
Amino acids can become ionized since the carboxyl group and amino group can become protonated (gain a proton) and unprotonated (lose a proton).
Therefore, they can act as acids or bases. Such molecules are said to be amphoteric.
Slide27REMEMBER!!
As pH decreases, [H+] increases, and ionizable groups become protonotaed.
Protonation of the amino group (NH2) makes the group positively charged (NH3+).
Protonation of the carboxylic group (COOH) makes the group uncharged (COOH).As pH increases, [H+] decreases, and ionizable groups become unprotonotaed (they lose their proton).Protonation of the amino group (NH2) makes the group positively charged (NH3+).Protonation of the carboxylic group (COOH) makes the group uncharged (COOH).
Slide28Effect of pH
The ionization state of an amino acid varies with pH since each group has its own pKa.
Amino acids at physiological pH (pH 7.4) exist as dipolar ions where the carboxyl group is unprotonated (-COO-) and the amino group is protonated (-NH3+).
In acid solution (e.g., pH 1), the amino group is protonated (-NH3+) and the carboxyl group is not (-COOH).As the pH is raised, the carboxylic acid gives up a proton.The dipolar form persists until the pH approaches 9, when the protonated amino group loses a proton.
Slide29Zwitterion and isoelectric point
Even though this amino acid is charged, it is electrically neutral.
Such a molecule with two opposite charges and a net charge of zero is termed a zwitterion.
The pH where the net charge of a molecules such as an amino acid or protein is zero is known as isoelectric point or pI.
Slide30Ionization of side chains
Nine of the 20 amino acids have ionizable side chains.
These amino acids are tyrosine, cysteine, arginine, lysine, histidine, serine, threonine, aspartic and glutamic acids.
Each side chain has its own pKa values for ionization of the side chains.At neutral pHaspartic acid and glutamic acid are negatively charged.Arginine and lysine are positively charged.
Slide31Histidine
An important amino acid in the function of many proteins and enzymes in terms of its pKa is histidine.
With a pKa value near 6, the imidazole group can be uncharged or positively charged near neutral pH.
Slide32Glutamate (same for Asp)
pH < 2
pH = 3
9> pH > 4
pH > 10
+1
0
-1
-2
Total charges:
Slide33Lysine (similar to arginine)
pH < 2
9> pH > 3
pH > 10
pH > 11
+2
+1
0
-1
Total charges:
Slide34Note
You need to know the names of amino acids, the special structural features of amino acids, their abbreviations or designations, the pKa of groups (not exact numbers, but which ones are acidic, basic, or near neutral).
Slide35Essential amino acids
There are nine amino acids that are essential.
Essential nutrients are those not made by the human body in significant amounts and must be derived from diet
These are: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine.The other 11 amino acids are non-essential amino acids.