Chapter 3: Biologically Important Molecules Flashcards
1
Q
Name seven examples of protein function/purpose in the body?
A
- Enzymes
- Hormones
- Receptors
- Channels
- Transporters
- Support structures inside and outside cells
- Antibodies
2
Q
What is responsible for protein uniqueness in role and structure?
A
The composition and sequence of amino acids in the polypeptide chain
3
Q
What are the building blocks of proteins?
A
Amino Acids
4
Q
What are the four main components of the Amino Acid Structure?
A
α-Amino Group (H2N)
Tetrahedral α-carbon
Variable R Group
α- carboxyl group (COOH)
5
Q
What’s a Carboxyl Group?
A
A combination of two functional groups attached to a single carbon atom
Hydroxyl (single bonded OH) + Carbonyl (double bonded O) = carboxyl group
Includes Carboxylic Acids and Amino Acids
6
Q
What is Hydroxyl?
A
A functional group -OH that consists of one atom of hydrogen covalently bonded to one oxygen
Is neutral or negatively charged
7
Q
What’s a Carbonyl ?
A
Functional group composed of a carbon atom double bonded to an oxygen atom
C=O
8
Q
What is Carboxylic Acid ?
A
An organic acid containing a carboxyl group (C(=O)OH) attached to an R group
General formula R-COOH or R-CO2H
9
Q
Define Organic Acid
A
An organic compound with acidic properties
The most common organic acids are carboxylic acids, whose acidity comes from the carboxyl group
10
Q
What is a functional group in chemistry?
A
A functional group or moiety is a specific group of atoms within a molecule that is responsible for characteristic chemical reactions of that molecule
11
Q
Which part of the structure do amino acids share, and which is unique?
A
Amino acids share the same Nitrogen- Carbon- Carbon backbone
The unique feature is the side chain/ Variable R-group which give it the physical and chemical properties that distinguish it
12
Q
What are the 4 important properties of amino acid side chains that affect an amino acids ability to act as acids or bases?
A
- Shape
- Charge
- Ability to Hydrogen Bond
- Ability to act as acids or bases
13
Q
What are the two common structures of Hydrophobic (Nonpolar) Amino Acids?
A
Aliphatic (straight chain) or Aromatic (ring) side chains
14
Q
Which hydrophobic non-polar amino acids are aliphatic?
A
- Glycine
- Alanine
- Valine
- Leucine
- Isoleucine
15
Q
Which hydrophobic non-polar amino acids are aromatic?
A
- Phenylalanine
- Tyrosine (neutral, polar)
- Tryptophan
16
Q
Where are hydrophobic non-polar amino acids found?
A
They are found on the interior of folded globular proteins, because they are being repelled away from water
Hydrophobic residues associate more with each other than with water
17
Q
The larger the hydrophobic group, the _____ (greater or less) the hydrophobic force
A
Greater
18
Q
List the Hydrophobic non-polar amino acids
A
Aliphatic: Glycine, Alanine, Valine, Leucine, Isoleucine
Aromatic: Phenylalanine, Tryptophan
Methionine, Proline
19
Q
What are the characteristics of the R group in Polar amino acids?
A
- polar enough to form hydrogen bonds with water
- NOT polar enough to act as an acid or base
- hydrophillic, water and R group are attracted to each other
20
Q
Which 3 Polar amino acids stand out from the rest and why?
A
- Serine
- Threonine
- Tyrosine
- They have an attachment of a phosphate group by the regulatory enzyme kinase. The attached phosphate group is very hydrophilic.
21
Q
Name the polar, Neutral Amino Acids
A
Serine
Threonine
Tyrosine
Asparagine
Glutamine
Cysteine
22
Q
Which Amino Acids are Polar Acidic?
A
Aspartic Acid (Aspartate: anionic unprotonated form)
Glutamic Acid ( Glutamate: “””)
23
Q
What makes polar Acidic amino acids Acidic?
A
They have Carboxylic Acid Functional groups (pKa ≈ 4) in their side chains –> acidic
24
Q
Which 3 functional groups in Acidic Amino acids may act as acids or bases?
A
The two backbone groups and the R group
25
What makes Basic Amino Acids Basic?
They have basic R-group Side chains
26
What are the Polar Basic Amino Acids and their pKa's ?
Lysine. pKa= 10
Arginine. pKa= 12
Histidine. pKa= 6.5
27
Which of the polar basic amino acids are unique and why?
Histidine
- Unique because it's close to physiological pH
- at pH 7.5 it can be either pronated or depronated
27
Which of the polar basic amino acids are unique and why?
Histidine
- Unique because it's close to physiological pH
- at pH 7.5 it can be either protonated or de-protonated
- It is classified as basic but can often act as acid too (is a readily available proton acceptor or donor)
- very prevalent at protein active sites
HIS GOES BOTH WAYS
28
a) Amino acids containing _____ are ALWAYS anionic at physiological pH
b) Amino Acids containing _____ are ALWAYS cationic at physiological pH
a) --COOH (RCOO-)
b) --NH2 side chains (RNH3+)
29
What are the two sulfur containing amino acids? How do their structures differ from the rest?
a) Cysteine: Contains a thiol/ sulfhydryl (an alcohol with an S atom instead of O) and is fairly polar
b) Methionine: Contains a thioether (an ether with an S atom instead of O) and is fairly NON-polar
30
What is Proline and why is it unique in its group?
It is an amino acid: categorized as non-polar hydrophobic
It is unique because its amino group is bound covalently to a part of the side chain creating a secondary a-amino group and a distinctive ring structure
This effects protein folding
31
What are the 8 essential amino acids? What does this mean?
1. Valine
2. Leucine
3. Isoleucine
4. Phenylalanine
5. Tryptophan
6. Methionine
7. Threonine
8. Lysine
These are amino acids that cannot be synthesized by adult humans and must be obtained from diet
32
What are the two common covalent bonds between amino acids in proteins?
Peptide bonds and Disulfide Bridges
33
What are Peptide Bonds?
Link amino acids together into polypeptide chains
Formed between the carboxyl group of one amino acid and the a-amino group of another amino acid with the loss of water
34
What are Disulfide Bridges ?
Exist Between Cysteine R groups
A covalent sulfur- sulfur bond
35
Amino Acids linked with peptide bonds form what?
polypeptides
36
What is the polypeptide backbone?
The N-C-C-N-C-C chain pattern
37
What is an individual amino acid called when its part of a polypeptide?
Residue
38
What is the amino terminus?
The first end made during polypeptide synthesis
The amino terminal residue is always written first
39
What is the carboxy Terminus?
The last end made during polypeptide synthesis
40
Phe- Glu- Gly- Ser- Ala
a) What is the Amino Terminus?
b) What is the Carboxy Terminus?
a) Phe
b) Ala
41
What is thermodynamically favoured but kinetically slow?
Hydrolysis of peptide bonds
42
Define Proteolysis/ Proteolytic Cleavage
Hydrolysis of a protein by another protein
43
Describe how peptide bonds are formed and maintained between amino acids.
During protein synthesis, stored energy is used to force the formation of peptide bonds
Once formed, destruction of peptide bonds is thermodynamically favoured. However, the bond remains stable because the activation energy for hydrolysis is too high.
Hydrolysis is thermodynamically favoured but kinetically slow.
44
Describe how proteolysis or proteolytic cleavage works?
The protein that cuts the bond is called the proteolytic enzyme, or protease
Some enzymes only cleave peptide bonds adjacent to specific AAs
45
Protease
Also known as proteolytic enzyme
Is the protein that cuts the peptide bonds of proteins in proteolysis (the hydrolysis of a protein)
46
How does Disulfide Bonding work?
Occurs in amino acids [cysteine] with a reactive thiol (sulfhydryl, SH)
Thiol of one cysteine reacts with the thiol of another to produce bond
This bonding can occur within the same, or different polypeptide chains
47
Why are disulfide bonds important?
They stabilize tertiary protein structure
48
What do we call cysteine once it becomes disulfide bonded?
Cystine
49
What is Cystine ?
This is what we call cysteine once it becomes disulfide bonded
50
Would the sulfur in cystine be more or less oxidized than cysteine ?
The sulfur in cystine is more oxidized because it is bonded to a carbon and a sulfur, whereas cysteine is bonded to a hydrogen and a carbon
SO. When a carbon is bound to a heteroatom (any atom that is not carbon or hydrogen), this increases oxidization.
" Heteroatoms such as oxygen and nitrogen are more electronegative than carbon, so when a carbon atom gains a bond to a heteroatom, it loses electron density and is thus being oxidized. Conversely, hydrogen is less electronegative than carbon, so when a carbon gains a bond to a hydrogen, it is gaining electron density, and thus being reduced."
Therefore, in cystine, the sulfur will be more oxidized because its bonded to a carbon and a heteroatom. But in cysteine, its bonded to a carbon and hydrogen thus decreasing oxidization.
51
Where in the body are Disulfide Bridges found?
These bridges are only found in extracellular polypeptides
In other environments, more reducing environments, the S-S group is reduced to two S-H groups. In extracellular environments the S-S group wont be reduced.
52
What two important proteins in the body are held together by disulfide bridges?
Antibodies and the hormone insulin
53
What can cause a protein to become nonfunctional?
Denaturation or if they are improperly folded
54
How many levels of protein folding are there? What is each level of structure dependent on?
4 levels
Each level is dependent on a specific type of bond
55
Define Denaturation
Disruption of a proteins shape without breaking peptide bonds
56
What causes denaturation of proteins?
Proteins are denatured by urea (disrupts H bonding interactions) through 3 acts:
1. Extremes of pH
2. Extremes of temperature
3. Changes in salt concentration (Tonicity)
57
Define tonicity
Tonicity is a measure of the effective osmotic pressure gradient
The water potential of two solutions separated by a partially-permeable cell membrane
58
What is the simplest level of protein structure?
Primary Structure, The Amino Acid Sequence
59
What is the primary structure of Proteins?
The linear ordering of amino acid residues
The order of amino acids are bonded in the polypeptide chain
The peptide bond is the characteristic bond that determines primary structure as it links amino acids
60
What is secondary protein structuring?
Hydrogen Bonds between backbone groups
The Initial folding of polypeptide chains into shapes stabilized by hydrogen bonds between backbone NH and CO groups
2 common structures: a-helix and B pleated sheet (parallel and antiparallel)
61
What are the two common secondary structures of proteins?
1. alpha helix
2. Beta pleated sheets
62
What are the two subtypes of Beta pleated sheets? How do they differ?
a) Parallel Beta pleated sheet
- Adjacent polypeptide strands run in the SAME direction
b) Anti-parallel Beta pleated sheet
- Polypeptide strands run in OPPOSITE directions
63
What is tertiary structure of proteins?
Hydrophobic/ Hydrophilic interactions (depending on the amino acid)
Concerns interactions between amino acid residues located more distantly from each other in the polypeptide chain
The folding of secondary structures into 3D structures
Driven by interactions of R-groups with each other and with solvent (Water)
64
How do amino acids with hydrophobic R groups fold in tertiary protein structuring?
Often fold into the interior of the protein, away from the solvent
65
How do amino acids with hydrophilic R groups fold in tertiary protein structuring?
These tend to be exposed to water on the surface of the protein
66
What determines how a protein folds in tertiary structure?
The forces that drive hydrophobic avoidance and hydrogen bonding fold polypeptides spontaneously into the correct conformation, the lowest energy conformation
67
What is Quaternary Protein Structure?
Various Bonds between Separate Chains; The highest level of protein structure
The arrangement of subunits (a single polypeptide chain as part of a large complex containing many subunits) in a multi-subunit complex
68
What forces stabilize Quaternary and generally are the same as those stabilizing Tertiary?
Non Covalent interactions
Van der Waals forces
H bonds
Disulfide bonds
Electrostatic interactions
69
What bond is NOT involved in quaternary protein structure, but is key in primary structures?
Peptide bonds
70
Whats the difference between a disulfide bridge involved in quaternary structure and one involved in tertiary structure?
In quaternary structure, bonds form between chains that are separate, they arent linked by peptide bonds
In tertiary structure, bonds occur between residues in the same polypeptide
71
Oxidation of carbohydrates means what? What is the result?
The combustion or burning to break down carbohydrates into CO2
The process releases large amounts of energy
72
What is the principle energy source for cellular metabolism?
carbohydrates
73
What is a monosaccharide?
A single carbohydrate molecule AKA a simple sugar
"a single sweet unit"
74
What is the general chemical formula for a monosaccharide?
CnH2nOn
75
3 examples of monosaccharides
Fructose
Glucose
Ribose
76
What are Disaccharides?
Two monosaccharides bonded together
77
What is an Oligosaccharide?
A few monosaccharides bonded together
78
What is a polysaccharide?
Many monosaccharides bonded together
79
How are 2 sugar molecules bonded together?
Glycosidic linkage: a covalent bond formed in a dehydration reaction that requires enzymatic catalysis
79
How are 2 sugar molecules bonded together?
Glycosidic linkage: a covalent bond formed in a dehydration reaction that requires enzymatic catalysis
80
What is Glycosidic linkage?
The bond between 2 sugar molecules
Covalent bond formed in a dehydration reaction that requires enzymatic catalysis
81
Glucose + Fructose =?
Sucrose
82
Galactose + Glucose = ?
Lactose
83
What 2 monosaccharides form Sucrose?
Glucose and Fructose
84
What 2 monosaccharides form Lactose?
Galactose and Glucose
85
Name 4 common disaccharides
1. Sucrose
2. Lactose
3. Maltose
4. Cellobiose
86
Define polymer
A polymer is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits
87
Together, what do disaccharides form? Give broad term and examples.
Disaccharides form important polymers
- Glycogen
- Starch
- Cellulose
88
What is glycogen?
energy storage carbohydrate composed of thousands of glucose units in animals
89
What is Starch?
The same as glycogen, an energy storage carbohydrate composed of thousands of glucose units in animals. Starch however, serves the same purpose in plants
90
What is cellulose?
A polymer of Cellobiose (a disaccharide)
Cellobiose does not exist freely in nature, it only exists in its polymerized cellulose form.
You would have to obtain it through enzymatic or acidic hydrolysis of cellulose.
91
What are lipids?
Oily/ fatty substances
92
What 3 physiologic roles do lipids play?
1) In adipose cells, triglycerides (fats) store energy
2) In cellular membranes, phospholipids constitute a barrier between intracellular and extracellular environments
3) Cholesterol is a special lipid that serves as the building block for hydrophobic steroid hormones
93
How does polarity effect reaction with water?
Water is very polar. Polar substances dissolve well in water. These polar substances are "water loving" or "hydrophilic"
94
How does non-polarity effect reaction with water?
C-C bonds and C-H bonds are non polar
They do not dissolve well in water and are therefore hydrophobic
95
How do hydrophobic substances react with lipids?
Hydrophobic= lipophilic (lipid loving )
96
How do hydrophilic substances react with lipids?
Hydrophilic= lipophobic (lipid hating)
97
Define Alkanes
Any series of saturated hydrocarbons including methane, ethane, and propane
98
What is the fatty acid structure?
Composed of long unsubstituted alkanes ending in a carboxylic acid
Chains are usually 14-18 carbons long and always even numbered in human cells
99
Describe the structure of saturated fatty acids.
A fatty acid with no C-C double bonds, because every carbon is covalently bound to the maximum number of Hydrogen
100
Describe the structure of an unsaturated fatty acid.
Contain one or more double bonds in the tail. These double bonds are almost always (Z) (or cis)
Unsaturated fatty acids are bent or "kinked" at the Cis double bond
101
How do hydrophobic chains react to water?
Long fatty acid hydrophobic chains will interact with each other to minimize contact with water, exposing their charged carboxyl group to the aqueous environment
When interacting in aqueous solutions, fatty acids form structures called micelles. A force called the hydrophobic interaction drives the tails into the centre of the micelle.
102
Define Micelle
The structure formed by fatty acids when they interact with aqueous solutions
103
Define hydrophobic interaction forces
The force that drives the tails of fatty acids into the centre of the micelle, exposing the carboxyl groups to the aqueous environment
104
How are fatty acids stored?
As fats, or Triacyglycerol/ triglycerides
105
What are triglycerides?
They are fats that fatty acids are stored as
106
How are triglycerides formed?
3 fatty acids esterified [process of combining an organic acid (RCOOH) to an alcohol (ROH)] to a glycerol molecule (a simple triol compound , meaning 3 -OH compounds)
107
Define Triol
A chemical compound containing 3 hydroxyl groups
108
What is glycerol? How is it formed?
glycerol is a simple triol compound.
A 3 carbon triol, formula HOCH2-CHOH-CH2OH
- has 3 hydroxyl groups (R-O-H) esterified to fatty acids
glycerol forms the background of lipids known as glycerides
109
Why do we need to store fatty acids in relatively inert forms?
Because free fatty acids are reactive chemicals
110
Define Esterification
Compound production by reaction between acids and alcohols with the elimination of water
111
Define lipases. What is their function
Lipases are enzymes that hydrolyze fats
Necessary because triacyglycerols are stored in fat cells as an energy source
112
Which is the more efficient energy storage molecule? Carbs, or Fats? Why? (2 reasons)
Fats
1. Packing: fat hydrophobicity lets them pack together closer than carbohydrates
- Carbs carry large amounts of water-of-solvation (water molecules H-bonded to their hydroxyl groups)
2. Energy Content: Fat molecules also store more energy than carbs
- No matter what its dissolved in, fat has more energy per carbon than a carb
113
What are membrane lipids? What are they composed of?
Membrane lipids are lipids involved in forming the structure of biological membranes
Consist primarily of phospholipids, glycolipids, and cholesterol derived from diacylglycerol phosphate or DG-P (a second messenger signaling lipid)
114
What is a phospholipid?
A class of lipids whose molecule has a hydrophilic head containing a phosphate group and 2 hydrophobic tails derived from fatty acids joined by an alcohol residue
115
What happens when phospholipids interact with water?
They minimize their interactions with water by forming a structure : a lipid bilayer
116
Define lipid bilayer
A thin polar membrane made of two layers of lipid molecules
These membranes are flat sheets that form a continuous barrier around all cells
The lipid bilayer is self-assembled and stabilized by the so called hydrophobic effect (lipid molecules unable to hydrogen bond with water aggregate to prevent their hydrophobic portions from being exposed to water
117
What stabilizes the lipid bilayer?
Van der Waals forces between the long tails
118
Would a saturated or unsaturated fatty acid residue have more Van der Waals interactions with neighbouring alkyl chains in a bilayer membrane ?
Phospholipids composed of saturated fatty acids make the membrane more solid
The bent shape of unsaturated fatty acids means it doesn't fit in as well and has less contact with neighbouring groups to form van der waals interactions
Double bonds in unsaturated fatty acids tend to increase membrane fluidity and prevent membrane from solidifying by disrupting the orderly packing of hydrophobic lipid tails (The right amount of fluidity is essential for function)
119
What affects membrane fluidity in the lipid bilayer and how?
Double bonds (unsaturation) in fatty acids increase membrane fluidity and prevent membrane from solidifying by disrupting the orderly packing of hydrophobic lipid tails (The right amount of fluidity is essential for function)
Decreasing length of fatty acids also increases fluidity
Cholesterol: Cholesterol is known as membrane antifreeze. At low temperatures it increases fluidity in the same way as kinks in fatty acid tails. At high temperatures, cholesterol attenuates (reduces) membrane fluidity
120
What is cholesterol's role in phospholipid bilayer membrane fluidity?
Cholesterol is known as membrane antifreeze
At low temperatures it increases fluidity in the same way as kinks in fatty acid tails. At high temperatures, cholesterol attenuates (reduces) membrane fluidity
Just remember, that cholesterol keeps fluidity at an optimum level
121
What are the 3 structural determinants of lipid bilayer membrane fluidity?
1. Degree of saturation
2. Tail length
3. Amount of cholesterol
122
What is a terpene?
A member of a broad class of compounds built from isoprene units (C5H8) with the general formula (C5H8)n
Is formally a simply hydrocarbon (compound of H and C)
123
In what two forms can terpenes come in?
Linear and cyclic
124
How are terpenes classified?
Classified by the number of isoprene units they contain
125
What is a monoterpene?
2 isoprene units
126
What is a sesquiterpene?
3 isoprene units
127
What is a diterpene?
4 isoprene units
128
What is a Squalene or triterpene?
6 isoprene units
129
What is squalene (Triterpene) and why is it an important compound?
Contains 6 isoprene units
Important because it is biosynthetically used in the manufacture of steroids.
Also is a component of earwax.
130
What is a terpenoid?
Essentially functionalized terpenes
Natural and synthetically derived species built from isoprene skeleton and functionalized with other elements such as O, S, N, etc.
eg. Vitamin A is a terpenoid
130
What is a terpenoid?
Essentially functionalized terpenes
Natural and synthetically derived species built from isoprene skeleton and functionalized with other elements such as O, S, N, etc.
eg. Vitamin A is a terpenoid
131
How are steroids similar to fats? How do they differ
Similarity: hydrophobicity
Differences: steroids are otherwise unique and unlike fats
132
What structure do steroids have ?
All steroids have the basic tetracyclic ring system which is based on the structure of cholesterol
133
How is cholesterol obtained?
diet
134
How is cholesterol synthesized?
Liver
135
How are lipoproteins produced?
Cholesterol is carried in the blood and packaged with fats and proteins to produce lipoproteins
Cholesterol + fats + Protein = Lipoproteins
136
Name 2 cholesterol derived steroid hormones
1. Estrogen/ Estradiol
2. Testosterone
137
What is an inorganic acid ?
Name an example.
An acid derived from inorganic compounds.
These acids do not contain carbon.
They form hydrogen ions and conjugated base ions when dissolved in water.
Ex. Phosphoric Acid
138
What is pKa?
A number that describes the acidity of a particular molecule
It measures the strength of an acid by how tightly a proton is held by a bronsted acid.
The lower the pKa, the stronger the acid and the greater its ability to donate its protons
139
What happens to phosphoric acid at physiological pH?
Phosphoric acid is significantly dissociated and exists largely in anionic form.
140
What is acid dissociation?
The process in which molecules separate or split into other things such as atoms, ions, radicals
141
What is phosphate also known as?
orthophosphate
142
When 2 orthophosphates bind, what bond is created and what does it form?
They bind via anhydride linkage and form pyrophosphate
143
What is an example of a high energy phosphate bond?
The P-O-P (anhydride) bond in pyrophosphate, meaning hydrolysis of pyrophosphate is extremely favourable
144
Why do anhydride bonds store so much energy? (3 reasons)
1. When phosphates link together, their negative charges repel each other strongly
2. Orthophosphate has more resonance forms (Theres another way of drawing its Lewis dot structure that satisfies the octet rule)
3. Orthophosphate has more favourable interaction with biological solvent (H2O) than linked phosphates
145
What analogy are linked phosphates given?
Linked phosphates are like compressed springs, waiting to fly open and provide energy for an enzyme to catalyze reaction
146
What are the building blocks of nucleic acids?
Nucleotides
147
What are the three components of nucleotides?
1. a Ribose (or deoxyribose) sugar group
2. A Purine or pyrimidine base joined to carbon #1 of the ribose ring
3. 1, 2, or 3 phosphate units joined to carbon #5 of the ribose ring
148
What is ATP? Why is it important
Adenosine Triphosphate
An important nucleotide, the central role in cellular metabolism and is an RNA precursor
ATP is the universal short term energy storage molecule
149
How does ATP store and release energy?
Energy is extracted from oxidation of foodstuff and immediately stored in the phosphoanhydride bonds of ATP
Energy is later used to power cellular processes and synthesize glucose or fats (longer term energy storage molecules)
This applies to all living organisms (bacteria, humans)
150
Why is ATP known as a "high energy" structure at neutral pH?
It exhibits a large decrease in free energy when it undergoes hydrolytic reactions
151
Which of the following best describes the secondary structure of a protein?
a) Various folded polypeptide chains joining together to form a larger unit
b) The amino acid sequence of the chain
c) The polypeptide chain folding upon itself due to hydrophobic/ hydrophilic interactions
d) Peptide bonds hydrogen bonding to one another to create a sheet like structure
D: Because a) is quaternary, b) is primary, c) is tertiary
152
Phenylketonuria (PKU) is an autosomal recessive disorder that results from a deficiency of the enzyme phenylalanine hydroxylase. This enzyme normally converts phenylalanine into tyrosine. PKU results in intellectual disability, growth retardation, fair skin, eczema and a distinct musty body odour. Which of the following is most likely true?
a) Treatment should include a decrease in tyrosine in the diet
b) The musty body odour is likely caused by a disorder in aromatic amino acid metabolism
c) Patients with PKU should increase the amount of phenylalanine in their diet.
d) PKU can be acquired by consuming too much aspartame (an artificial sweetener that contains high levels of phenylalanine)
a) wrong: A defect in phenylalanine hydroxylase would result in a build up of phenylalanine leading to an excess of phenylalanine byproducts like phenylacetate, phenyllactate, and phenylpyruvate, and a decrease in tyrosine. Patients with PKU should increase the amount of tyrosine in their diet (it becomes an essential amino acid in this condition; choice A is wrong.
b) Correct: Phenylalanine and its derivatives are aromatic amino acids and high levels of these compounds lead to the distinct musty body odour
c) Wrong: Patients have a build up of phenylalanine and they should eliminate phenylalanine from their diet
d) Wrong: As stated in the question, PKU is an autosomal recessive disorder (genetically acquired) and thus it is not by consuming to much phenylalanine
153
A genetic regulator is found to contain a lysine residue that is important for its binding to DNA. If a mutation were to occur such that a different amino acid replaces the lysine at that location, which of the following resulting amino acids would likely be the least harmful to its ability to bind DNA?
a) Glycine
b) Glutamate
c) Aspartate
d) Arginine
Answer: D
Knowing the structures of amino acids is unlikely to be important but know which amino acids are basic (Arginine, histidine, lysine) and which are acidic (Glutamate, aspartate).
Since lysine is basic it is therefore best at binding the negatively charged DNA.
You can assume that a mutation resulting in another basic amino acid (lysine --> arginine) would cause the least change in its ability to bind DNA. Therefore a mutation from lysine to arginine would cause the least harm.
b+c) Wrong: A mutation from lysine to glutamate or aspartate would likely cause the most harm to its ability to bind DNA.
a) Wrong: Glycine is a neutral amino acid.
154
Increasing the amount of cholesterol in a plasma membrane would lead to an increase in:
a) Membrane permeability
b) Atherosclerotic plaques
c) Membrane fluidity at low temperatures but a decrease in membrane fluidity at high temperatures
d) Membrane fluidity at high and low temperatures
Answer: C
Plasma membranes can be up to 50% composed of sterols which help stabilize the membrane at both spectrums of the temperature
At low temps, cholesterol increases fluidity because the ring structure of cholesterol doesn't allow for tight phospholipid tail packing.
At high temperatures, cholesterol decreases membrane fluidity (The OH group of cholesterol prevents phospholipid dispersion
a) Wrong: Cholesterol decreases permeability of membranes by filling in the holes between the fatty acid tails
b) Wrong: The formation of atherosclerotic plaques is due to high levels of blood cholesterol not membrane cholesterol
155
Glycogen is to humans as ___ is to plants
Starch
Glycogen serves as n energy storage carbohydrate in animals and is composed of thousands of glucose units
Starch is the same as glycogen, except that the branches are a little different, and serves the same purpose in plants
156
A human space explorer crash lands on a planet where the native inhabitants are entirely unable to digest glycogen but are able to digest cellulose. consequently, they make their clothing out of glycogen based material. The starving space explorer eats one of the native inhabitants’ shirts and the natives are amazed. Based on this information, which of the following is/are true ?
I. The explorer cann digest a- glycosidic linkages
II. The native inhabitants can digest a-glycosidic linkages
III. The native inhabitants can digest starch
A) I only
b) I and III only
c) II and III only
d) I, II, and III
A
Item I is true: humans can digest a- glycosidic linkages, such as those found in glycogen. If the natives’ shirts are made of glycogen, our space explorer should have no trouble consuming and digesting them (choice C can be eliminated).
Item II is false: Cellulose contains B-glycosidic linkages. If the natives can digest cellulose but not glycogen, then they cannot digest a- glycosidic linkages (choice D can be eliminated)
Item III is false: starch also contains a a-glycosidic linkages. If the natives cannot digest glycogen, then they likely cannot digest starch either (choice B can be eliminated, and choice A is true)
