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Flashcards in Biochemistry Deck (556)
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91

Define substrate

Molecule on which enzyme performs a reaction

92

What is lysozyme?

An enzyme found in tissue fluids/secretions, tears, saliva, nasal mucus that protects against bacteria by causing cell to lyse and lose cell content

93

What is peptidoglycan?

Polysaccharide formed from polymerisation of monosaccharides N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

94

Describe the structure of bacterial cell wall

Made of peptidoglycan - retains high internal osmotic pressure
Layers of repeating peptidoglycan units with peptide chain bound to NAM
Penta-glycine cross-links between last residue of chain and second last of opposite chain - added strength

95

Describe alpha and beta linkages

Alpha: H atoms of the C atoms in bond in cis formation
Beta: H atoms of C atoms in bond in trans formation

96

Describe the lysis of the cell wall

Lysozyme hydrolyses B1:4 bond (between C1 and C4) in glycan between NAG and NAM
Occurs rapidly at close to pH 7

97

What else will lysozyme hydrolyse?

Oligo NAG (min 6 residues)
Fit into groove round enzyme that closes on the strand

98

What are serine proteases?

Enzymes with hyperactive serine residue at active site and appropriately spaced histidine and aspartate resides

99

Name some serine proteases

Trypsin
Chymotrypsin
Elastase
Thrombin

100

How do serine proteases work?

Catalyse by breaking peptide bond using charge relay system

101

Describe the charge relay system

Flow of electrons between 3 AAs making serine hyperactive
AAs are far apart in primary structure but close in 3D structure due to protein folding

102

Why are proteolytic enzymes secreted as zymogens?

As are hazardous to body so secreted as zymogens that are activated in gut lumen

103

Give examples of proteolytic enzymes

Pepsinogen
Tryspinogen
Chymotrypsinogen

104

Describe the activation of pepsinogen

Chief cells in gastric gland produce pepsinogen
Parietal cells produce HCl hydrolyses some peptide bonds to produce pepsin
Left over pepsinogen is hydrolysed again to produce more pepsin

105

Describe the activation of trypsinogen

Pancreatic duct secretes pancreatic zymogens
Some zymogens converted to trypsinogen which is converted to trypsin by enterokinase (produced by brush border)
Trypsin activates more enzymes such as chymotrypsinogen

106

Describe the activation of chymotrypsinogen

Inactive state: single polypeptide, 245AAs with 5 SS bridges, three portions of AS caN'T come together
Trypsin cuts bond between Arg15 and next residue - pi-chymotrypsin left
Cut at Leu13 removes dipeptide bond
Cut at Tyr146 and Asn148 removes another - a-chymotrypsin
3 parts held by SS bridges
Active site resides come together
Charge transfer complex with hyperactive serine formed

107

How is specificity of enzymes dictated?

Pocket close to AS into which an AA side chain may fit thus size and nature of pocket are important

108

Describe the specificity of trypsin, chymotrypsin and elastase

Trypsin: cuts basic R groups; Asp at bottom of pocket
Chymotrypsin: cuts hydrophobic R; hydrophobic pocket
Elastase: cuts small R; small pocket

109

Describe the steady state hypothesis

In enzyme catalysed reaction, enzyme and substrate form complex which is a transient state (either breaking down or reversing)
Initially conc. of complex will increase but will become steady as when broken down enzyme will form new complex

110

What is Vmax?

The velocity a reaction approaches as substrate conc. is increased

111

What is Michaelis constant (Km)?

Substrate conc. at 1/2Vmax
Dependent on substrate and enzyme
Measures affinity of enzyme for substrate - a high Km confers low affinity

112

What is the relationship between substrate conc. and velocity?

Michaelis-Menten equation
v = Vmax[S]/(Km+[S])

When [s]=Km v = 1/2Vmax
When [s]>>Km v ~ Vmax

113

What is the importance of Km?

Can use to compare 1 enzyme's affinity for different substrates or
Multiple enzymes affinities' for 1 substrate

114

Describe hexokinase

Phosphorylates glucose
Widespread in tissue
Low Km = high affinity
Always active

115

Describe glucokinase

Found in liver and pancreas
High Km = low affinity only active at high glucose conc.
At high conc. glucose stored in liver as glycogen, insulin produced by pancreas

116

Explain the influencing factors of enzymes

Optimum temp. and pH range above optimum will denature and activity will suddenly drop
Increasing temp. increases energy of collisions, internal energy of reactants and number of collisions

117

What are lipids?

Diverse class of insoluble compounds that don't form polymers
Are hydrophobic (mostly hydrocarbons) and soluble in organic solvents (ether, acetone, chloroform)

118

Give examples of lipids

Fats, oils, fatty acids, triacylglycerols, glycolipids, phospholipids, steroids

119

What is the main function of fatty acids?

Catabolised generating ATP or used to synthesise triglycerides and phospholipids

120

Main function of triglycerides?

Energy storage, protection, insulation