chapter 1: BIOLOGICAL MOLECULES: proteins! Flashcards

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1
Q

the four different groups of atoms bonded to an α-carbon

what is the structure of amino acids?

A
  • an amino acid consists of a central alpha carbon (α-carbon) atom
    > known as an asymmetrical carbon

four diff groups of atoms bonded to α-carbon are:
- hydrogen atom
- amino/amine group (-NH2)
- carboxyl/ carboxylic acid group (-COOH)
- variable R group known as a side chain
> R group differs with each amino acid

  • the R group/ side chain of each amino acid determines its chemical properties
  • the simplest R group/ side chain is just a hydrogen atom
    > the amino acid with this R group is glycine
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2
Q

how many common amino acids are there and what are the 4 main groups they are classified under?

A
  • 20 naturally occuring amino acids are used to synthesise proteins

groups that amino acids are classified into:
* non-polar amino acids
* polar amino acids
* acdic amino acids (electrically charged)
* basic amino acids (electrically charged)

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3
Q

what are the properties of the side chain (R group) in non-polar amino acids?

A
  • have a non-polar side chain
  • contains hydrocarbon chain or carbon ring
  • and are therefore hydrophobic
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4
Q

what are the properties of the side chain (R group) in polar amino acids?

A
  • have a polar side chain
  • usually contains functional groups with oxygen, nitrogen and sulfur in the R group
  • hydrophilic
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5
Q

what are the properties of the side chain (R group) in acidic amino acids?

electrically charged amino acids

A
  • have a side chain with a carboxyl group which is acidic and negativly charged at cellular pH
  • hydrophilic
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6
Q

what are the properties of the side chain (R group) in basic amino acids?

electrically charged amino acids

A
  • have a side chain with an amino group which is basic and posiitvely-charged at cellular pH
  • hydrophilic
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7
Q

properties of amino acids

are amino acids soluble?

A
  • they are soluble/ able to dissolve in water and other aqeous solution where they form ions
  • generally insoluble in organic solvents
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8
Q

properties of amino acids

what is the zwitterion formation?

A
  • in aqueous solution, amino acids exists as ions having both a positive and a negative charge
    > also known as dipolar ions
    > zwitterion

ions formed by:
- the loss of a hydrogen ion ( H+) from the carboxyl group (COOH), making it negatively charged (-COO^-)
- and the gain of a hydrogen ion (H+) by the amino group (-NH2), making it positively charged (-NH3^+)

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9
Q

properties of amino acids

what are the buffering capabilities of amino acids?

A
  • due to the presence of both the amino group and the carboxyl group
    > amino acids possess both acidic and basic properties
    > amphoteric
  • aqueous solutions of amino acids can function as buffer solutions which can resist small changes in pH
    > when small amounts of acids or alkalis are added to it
  • in an acidic solution
    > hydrogen ions conc is high and the pH is decreased
    > the COO- group of zwitter ion accepts the excess H+ ions to form COOH
    > overall molecule becomes positively charged
  • in an alkkaline solution
    > (OH- conc is high and pH is increased)
    > the NH3+ group of zwitter ion donates H+ ions to form -NH2 and the overall molecule becomes negatively charged

-the ability of amino acids to function as buffers is essential in biological systems
> function to prevent any sudden changes in cellular pH that could adversely affect the activity of enzymes and the functions of other proteins

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10
Q

describe the formation of peptide bonds

A
  • a condensation reaction occurs between the carboxyl group of one amino acid and the amino group of another amino acid to form a covalent peptide bond
  • (CONH)
    > via the removal of one water molecule
    > the resulting compound is called a dipeptide

-in cells, peptide bonds are synthesised by ribosomes during the process of translation
- peptidyl transferase, an enzyme, catalyses the formation of peptide bonds between amino acids in cells
-

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11
Q

describe the breaking of peptide bonds

A

-the peptide bond between adjacent amino acids residues can be broken by hydrolysis to yield amino acid monomers
- the addition of a water molecule is required to break a peptide bond
> proteases catalyses the hydrolysis reaction

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12
Q

what is the structure of a polypeptide chain?

A
  • a polypeptide chain has two different ends- amino end and the carboxyl end
  • at the start of the polypeptide chain is a free amino group
    >not included in the formation of the peptide bond
    >called the N terminus
  • the opposite end has a free carboxyl group and is called the carboxyl end (C-terminus)
  • the repeating sequence of atoms (-NCC-) is called the polypeptide backbone
    > extending from the backbone are the side chains/ R groups of amino acids
  • each specific polypeptide has a unique linear sequence of amino acids
    > a great variety of polypeptide chains can be synthesised from just a limited set of monomers
  • polypeptide chains fold proteins into specific shapes held in place with 4 different types of bonds and itnerations
    > different linear sequences determine the specific three-dimensional shape/ configuration/ conformation of proteins
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13
Q

what are the bonds stabilising protein in its unique structure?

A
  • after amino acids are joined by peptide bonds to form a polypeptide
    > the polypeptide is then folded to form a specific shape/ three dimensional conformation
    > polypeptides are folded in the lumen of the rough endoplasmic reticulum in a eukaryotic cell

four types of bonds and interaction maintain the shape of each protein:
- hydrogen
- ionic bond
- hydrophobic interations
- disulfide bonds

bonds and interactions can occur between amino acid residues in:
- different parts of the same polypeptide ( INTRAmolecular or INTRA-chain)
- between different polypeptides( INTERmolecular or INTER-chain)

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14
Q

how do hydrogen bonds help stabilise protein in its unique structure?

A

hydrogen bonds may form between
- the C=O and N-H of amino acid residues on the polypeptide backbone in secondary structures of proteins
OR
- polar R groups of amino acid residues in the tertiary and quatenary structure of proteins

  • an individual hydrogen bond is a weak bond
    > if it occurs frequently in the structure of a protein, collectively, multiple hydrogen bonds are strong and are able to maintain the conformation and stability of the protein
  • hydrogen bonds are disrupted by a change in temperature and pH of the surrounding medium
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15
Q

how do ionic bonds help stabilise protein in its unique structure?

A
  • ionic bond is an electrostatic attraction/interaction formed between a negatively charged acidic R group and a positively chargen basic R group
    > in the tertiary and quaternary structure of proteins
  • ionic bonds can be disrupted by a change in temperature and the pH of the surrounding medium
    > pH changes cuases the charges on the acidic and basic R groups to be altered potentially disrupting ionic bonds
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16
Q

how do disulfide bonds/disulfide bridges help stabilise protein in its uniquee structure?

A
  • a disulfide bond is formed between two cysteine amino acids residues with sulfhydryl groups (-SH) on their R groups in the tertiary and quaternary structure of proteins
  • disulfide bonds are strong covalent bonds that can be broken only under high temperatures
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17
Q

how do hydrophobic interactions help stabilise protein in its unique structure?

A
  • hydrophobic interactions are formed between amino acids residues with R groups that are non-polar and are therefore hydrophobic
  • these interactions form in the tertiary and quaternary structure of proteins
  • they can form more stable interactions with each other than with water molecules
    > thus, they tend to group together to avoid interacting with water in an aqueous environment
  • if a polypeptide chain contains a number of these groups and is in aqueous environment
    > the polypeptide chain will tend to fold such that the maximum number of hydrophobic R groups come into close contact with each other in the interior of protein and are shielded from water
    > and the hydrophilicc R groups would be exposed to the environment
    > found on the exteiror of the protein surface
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18
Q

what is protein denaturation?

A

denaturation is the unfolding of the three-dimensionsla conformation of a protein due to:
- increase in temperature away from optimum temperature or
- change (increase or decrease) in pH away from the optimum pH

  • when a protein molecule denatures, it loses its quaternary, tertiary and secondary structure but retains its primary structure
  • since the function of a protein depends on its folded 3D shape, denaturation leads to the loss of its function
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19
Q

how does the increase in temperature cause proteins to denature?

A
  • increasing temperature increases thermal agitation/ vibrations of protein molecules
    > causes R group interactions such as hydrogen bonds, ionic bonds and hydrophobic interactions in the tertiary and quaternary structures to be disrupted/ break
  • hydrogen bonds in the secondary structures may also be disrupted
    ( *the number of H bonds broken in the secondary structures increases with increasing temperatures > causes the extent of unfolding of secondary structures to increase with increasing temperatures) *
  • the protein is denatured as it unfolds and loses its 3D structure
    *
  • disulfide bonds are strong covalent bonds
    > usually broken only at very high temperatures by high heat energy but protein denaturation usually occures before it can happen
    > enzymes with very high optimum temperatures usually cconsist of many disulfide bonds in their tertiary and quaternary structures
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20
Q

how does the changes in pH cause protein denaturation?

A
  • pH changes ( can either be and increase or decrease in H+ concentration) can result in changes in the charges of the R groups such as:
    > alteration of charges on acidic and basic R grouos of amino acid residues
    > an unchanged polar R group becoming charged

> hence changes in pH results in the disruption of and ionic bonds and hydrogen bonds respectively
hydrophobic interations and disulfide bonds remain unaffected

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21
Q

difference in structure & property between globular and fibrous protein

what is the difference in their shape?

A

globular protein: polypeptide chain is tightly folded to form a spherical shape
fibrous protein: polypeptide chains are cross-linked at intervals to form long, rope-like fibres or sheets

22
Q

difference in structure & property between globular and fibrous protein

what is the difference in the regularity of amino acid sequence?

A

globular protein: rarely exhibit regular, repetitive amino acid sequences

fibrous protein: regular and repetitive sequence of amino acids
eg in collagen,
- every third residue in collagen is glycine
- tripeptide sequence glycine-proline-hydroxyproline recurs frequently

23
Q

difference in structure & property between globular and fibrous protein

what is the difference in the main bonds and interactions in quaternary structure?

A

globular proteins: quaternary structure is established mainly by hydrophobic interactions, ionic bonds and hydrogen bonds between polypeptide chains (weak)

fibrous proteins: quaternary structure is stablished mainly by strong, extensive network of hydrogen bonds and covalent cross-links between polypeptide chains

24
Q

difference in structure & property between globular and fibrous protein

what is the difference in their level of protein structure of greatest importance?

A

globular proteins: tertiary structure most important
fibrous protein: secondary structure most important

25
Q

difference in structure & property between globular and fibrous protein

what is the difference in their solubility?

A

globular proteins: generally more soluble than fibrous proteins in aqueous medium
- hydrophilic R groups of amino acids residues on protein exterior
- hydrophobic R groups amino acids buried in interior

fibrous protein: generally insoluble in aqueous medium
- large number oh hydrophobic R groups of amino acid residues on protein exteiror

26
Q

difference in structure & property between globular and fibrous protein

what is the difference in their functions?

A

globular protein: metabolic functions
eg. haemoglobin, enzymes, hormones, antibodies

fibrous protein: structural support
eg. collagen, keratin

27
Q

globular protein

what is the function of haemoglobin?

A
  • haemoglobin is a globular protein ( with a non-protein compopenent) found in the red blood cells and functions in transporting oxygen in the blood
  • it also acts as a pH buffer to maintain pH at optimum level of blood
28
Q

globular protein

what is the structural organisation of haemoglobin like?

A

primary structure: 4 individual polypeptide chains
- 2 alpha chains (subunits)
- 2 beta chains (subunits)

secondary structure: parts of each chain are folded into alpha helices, stabilised by hydrogen bonds formed between atoms of the polypeptide backbone

tertiary structure: each chain is folded into a globular structure, stabilised by bonds like
> hydrogen bonds, disulfide bonds, ionic bonds and hydrophobic interactions formed between R groups of amino acid residues

quaternary structure:
- the 4 folded chains assemble together via R group interactions

  • each subunit contains a haem group ( so in total there are 4 haem groups per haemoglobin molecule
  • the haem group is a prosthetic group which is a non-protein component of haemoglobin required for its function
    > ocnsists of a porphyrin ring and an iron ion (Fe2+)
  • O2 binds to the Fe2+ of the haem group
29
Q

relating haemoglobin’s structural features to its properties & functions

structure: globular shape

A

property: compact
function: allows many haemoglobing molecules to be packed in a red blood cell

30
Q

relating haemoglobin’s structural features to its properties & functions

structure: has a quaternary structure
consists of 4 subunits
(2 α- chains and two β chains)

A

function: haemoglobin molecule can carry 4 oxygen molecules
> increases capacity for transport of oxygen, making haemoglobin an efficient oxygen carrier

31
Q

relating haemoglobin’s structural features to its properties & functions

structure: each subunit has a haem prosthetic group that contains a porphyrin ring bound to an Fe2+ ion

A

property: Fe2+ of a haem group in each subunit can bind to one oxygen molecule
function: this enables haemoglobin to transport oxygen

property: Fe2+ can combine reversibly with oxygen
function: this allows release of oxygen at metabolically active tissues

32
Q

relating haemoglobin’s structural features to its properties & functions

the haem group is orientated such that its Fe2+ is complexed to an amino acid residue on one face, leaving the other face accessible to bind oxygen

A

function: this enables haemoglobing to transport oxygen

33
Q

relating haemoglobin’s structural features to its properties & functions

structure: each subunit has a deep hydrophobic cleft, lined with amino acid residues with non-polar, hydrophobic R groups

A

function: provides a binding site for haem group, which is hydrophobic

34
Q

relating haemoglobin’s structural features to its properties & functions

structure: each subunit is folded into a tertiary structure such that most of its amino acid residues with hydrophilic (acidic, basic and polar) R groups are on the exterior surface while amino acids residues with hydrophobic R groups are buried in the interior

A

property: soluble in aqueous medium
function: suitable for transport in aqueous cytosol of red blood cells

35
Q

relating haemoglobin’s structural features to its properties & functions

structure: 4 subunits are held together by non-covalent R group interactions
(hydrophobic itneractions, hydrogen and ionic bonds in the quaternary structure)

A

function: binding of oxygen to haem group of one subunit causes its tertiary structure to change
> this then causes the tertiary structure of other subunits change
> all subunits then gain an increased affinity for oxygen

this phenomenon is known as cooperative binding of oxygen, for faster loading and unloading of oxygen from haemoglobin molecules

36
Q

what is the function of collagen?

A
  • collagen is a fibrous protein
  • collagen is usually deposited outside the cells in the extracellular matric and has a structural support function
    > supports tissues and gives cells a structure
    > forms the major components in bone, connective tissues , tendons, ligaments and skin
37
Q

protein with quaternary structure

what is collagen?

A
  • collagen is a fibrous protein that is insoluble and has high tensile strength
38
Q

describe the primary structure of collagen

A
  • each polypeptide chain is about 1000-1400 amino acid residues long

each chain consists of a repetitive sequence of three amino acid residues
- G-X-Y
- which gives the polypeptide chain a regular helical structure
> where G is glycine
> X is often proline
> Y is proline or hydroxyproline
> this allows the three chains to coil together to form a triple helix

  • every third amino acid in the polypeptide chain is a glycine
    > glycine has the smallest R group (H atom) which allos it to fit into the centre of the triple helix
    > this enables tight packing of the three polypeptide chains in the triple helix
39
Q

describe the quaternary structure of collagen

A
  • a tropocollagen molecule consists of three polypeptide chains wound around one another to form a right-handed triple helix
  • with tropocollagen, many inter-chain hydrogen bonds are formed between the polypeptide backbones of the three polypeptide chains, resulting in high tensile strength
    > hydrogen bonds are formed between the O atom of CO group of an amino acids residue and H atom of NH group of another amino acid residue on another chain
40
Q

describe the beyong quaternary structure of collagen

A
  • the N terminus of one tropocollagen is covalently cross-linked to the C terminus of a neighbouring tropocollagen
  • bundling of many covalently cross-linked tropocollagen molecules that are parallel to each other to form a collagen fibril
    > collagen fibrils then bundle to form collagen fibres, further increasing tensile strength
  • parallel, staggered arrangement of tropocollagen molecules in bundles ensure that there is no weak spot along the length of the fibrils
41
Q

relating collagen’s structural features to its properties

what are the structural features that causes collagen to be insoluble in water (property) and provide structural support in connective tissues (function)?

A
  1. large molecular size of the tropocollagen molecule- each polypeptide chain has about 1000 to 1400 amino acid residues
    > long fibrous structure
  2. large number of hydrophobic R-groups of proline residues on the exteriour of the triple helix/ tropocollagen
42
Q

relating collagen’s structural features to its properties

what are the structural features that cause collagen to have ** high tensile strength** (property)** and provide structural suppose in connective tissues** (function)?

A
  1. every third amino acid of each polypeptide chain is glycine, which has a small R group, a H atom
    > its small size allows it to fit inot the centre of the triple helix
    > allows tight packing of the three polypeptide chains
  2. repetitive amino acid sequence (glycine, often proline and hydroxyproline) gives each polypeptide chain a regular helical shape
    > aids the formation of numerous inter-chain hydrogen bonds between the three polypeptide chains in tropocollagen
    > this extensive H bonding stabilises the fibrous structure of collagen
  3. many tropocollagen molecules are covalently cross-linked with neighbouring tropocollagen molecules running parallel to them to form a collagen fibril
    > fibrils then bundle to form collagen fibre
43
Q

relating collagen’s structural features to its properties

what structural feature of collagen ensures that ther is no weak spot running through collagen fibril ( property) and enables collagen to** provide structural support in connective tissues** ( function)?

A
  • parallel, staggered arrangement of tropocollagen molecules in bundles to form collagen fibril (longitudinal displacemnt of tropocollagen molecules
44
Q

what does the primary structure of polypeptides refer to?

A
  • refers to the specific type, number and sequence of amino acid residues that make up a polypeptide chain
  • each protein’s polypeptide chain is unique in its type, number and sequence of amino acids
    > which is dictated by the unique DNA base sequence of the gene
  • the sequence of amino acids determines the overall shape, which also determines the function of the protein
  • at the primary structure level, only peptide bonds are present, holding the amino acid residues together to form the polypeptide chain
    • ## the polypeptide is not folded yet at the primary structure level
45
Q

what does the secondary structure of proteins refer to?

A
  • it refers to the regular structures formed by coiling and folding of polypeptide chain into alpha helix or beta -pleated sheet
  • these coils and folds are maintained by hydrogen bonds
    > formed between the O atom of the C=O group and the H atom of the N-H group of the neighboring amino acid residues on the polypeptide backbone
  • R-groups are not involved in the formation of these hydrogen bonds
46
Q

secondary structure of proteins

what is an alpha-helix?

A
  • the α-helix is made when a part of a polypeptide chain is coiled into a helical form
  • it is stabilised by the hydrogen bonds formed between the O atom of the C=O group of one amino acid and the H of the -NH group of another amino acid, that is 4 residues away in the polypeptide backbone
  • bonds are formed within the same polypeptide chain, hence also known as ‘ intra-chain hydrogen bonds’
  • there are 3.6 amino acid residues in every turn of the α- helix
47
Q

secondary structure of proteins

what is a Beta-pleated sheet?

A
  • a β-pleated sheet is formed when segments of the same polypeptide chain is folded back and forth into a zigzag, sheet-like shape/ conformation
  • hydrogen bonds are formed between O atom of the CO group of one amino acid residue and the H atom of the NH group of another amino acid residue of the same polypeptide backbone
  • the H bonds are formed between more than one segment of the same polypeptide chain, lying side by side
  • ## the polypeptide chain may either run in the same direction( parallel β- pleated sheets) or in opposite directions ( anti-parallel β-pleated sheets)
48
Q

what does the tertiary structure of a protein refer to?

A
  • it refers to compact, globular, 3D conformation of a protein formed by further folding and bending of the polypeptide chain
  • R group interactions maintain the tertiary structure of a folded protein > 4 types of interactions
  • hydrophobic interactions, ionic bonds and hydrogen bonds are weak interctions but their cummulatice effects help to maintain the protein’s unique 3d shape
    ionic bonds: formed between R groups of negatively charged acidic and positively charged basic amino resides
    hydrophobic interactions: formed between the R groups of the non-polar amino acid residues
    hydrogen bonds: formed between the R groups of polar amino acid residues
  • strong covalent bonds reinforce the protein’s 3D shape
    disulfide bonds: formed between R groups of cysteine residues
49
Q

what does the quaternary structure of proteins refer to?

A
  • many proteins are made up of a single folded polypeptide chain and have only three levels of structure
  • but some proteins are made of 2 or more folded polypeptide chains known as subunits which are already folded into their tertiary structures
    > subunits associate to form one functional complex protein molecule, giving the protein its quaternary structure
  • the subunits are held together by R group interactions such as ionic bonds, hydrogen bonds, hydrophobic interactions and sometimes disulfide bond to form a
    > multi-subunit/ multimetric protein
    >
50
Q
A
51
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