BIOL2171: Biochemistry and Nutrition - Sem1 (W7-W12)

Comment on the characteristics of glycerol.

• Glycerol is a 3 carbon alcohol. Major ‘acceptor; for fatty acids. 

• The carboxyl group of fatty acids is made more reactive by the addition of coenzyme A, which contains a highly reactive thio group - fatty acyl CoA. 

• Fatty acyl CoA’s react with the alcohols on glycerol to generate di and triglycerides. 

Diglyceride and triglyceride functions 🙂

Comment on the phospholipid precursor.

• Phosphatidic acid contains two acyl chains and a phosphate moiety. 

• Phosphatidic acid is a minor component of membranes but plays a vital role in synthesis and metabolism. 

• All phosphoglycerolipids are amphipathic - both hydrophobic and hydrophilic. 

• Saturated fatty acids are localised to C1 while unsaturated chains slot into C2. 

• The X group is the site of the head group attachment. 

Comment on phospholipid structure.

• Phospholipids are composed of fatty acids attached at C1 and C2 in glycerol. 

• Phosphate and a head group (X) attach at C3. 

• Classified according to the head group and acyl chain composition. 

Characteristics of sphingolipid structure?

• The backbone of sphingolipids is sphingosine, which is a long chain amino alcohol (blue). 

• The C1, C2, C3 molecules of sphingosine are analogous to the glycerol moiety in phospholipids. 

• A fatty acid chain (usually 16, 18, 22, 24 carbons) is attached via amide linkage to the NH₂ on C2 (red). 

• The polar head group (X) is attached at C1. 

• Sphingolipids are synthesised de novo from fatty acyl-CoA and serine. 

• Sphingolipids are structurally similar to phosphatidylcholine. 

Types of sphingolipid 🙂

ABO blood groups 🙂

Ganglioside variation.

A - allele encodes a glycosyltransferase that adds N-acetylgalactosamine. 

• B - allele encodes a glycosyltransferase that adds galactose. 

• Individuals with A or B only contain anti-B or anti-A antibodies. 

• O = universal donor. 

• AB = universal acceptor. 

Basic characteristics of cholesterol?

• 27 carbon compound. 

• Built from carbon units of acetyl-CoA.

• All cells are capable of cholesterol biosynthesis. ~25% is formed in the liver. 

• Process occurs in cytosol and transferred to the endoplasmic reticulum. 

• Highly hydrophobic molecule - planar topology. 

• Its amphiphilicity arises from the hydroxyl moiety. 

• Transported around body by lipoproteins. 

Structure of cells determined by…

…physical and chemical properties of constituents. 

• Micelles and liposomes form spontaneously in aqueous solutions. 

• Cholesterol intercalates into either structure - the ‘poly-filla’ of bilayers. 

Importance of compartmentalisation?

• Separating of competing metabolic reactions (ie. beta oxidation vs. fatty acid synthesis). 

• Membrane proteins relay signals. 

• Membranes are themselves dynamic structures. 

Original view of biomembranes?

What are the properties of fluid mosaic membranes?

1. Generally impermeable to polar molecules or ions (hence why signal transduction is important). 

2. Membranes are flexible and adapt as required by cells or organelles (cells encounter shear forces). 

3. Durable since they may encounter significant shear forces. 

4. Membrane proteins are not simply structural - they carry out or regulate biological functions. 

5. Membrane lipids provide optimal conditions for the function of enzymes. 

Other properties of membranes impact protein function 🙂

Issues with fluid mosaic model?

Comment on membrane lipid components.

• PC and PE comprise the major glycerophospholipids. 

• Relative proportions of SM and PC are similar. 

• Extensive glycosylation of lipids (nb. Red blood cells). 

• PS composition is relatively high - signalling. 

• Cholesterol composition is extensive. 

Comment on membrane fatty acid composition.

• C16 and C18 fatty acids are the dominant species. 

• Values will differ significantly in tissues.

• Unsaturated FAs make up almost 50%. 

• Omega symbol refers to distance of final double bond from methyl carbon. 

Comment on membrane lipids being asymmetric.

• The leaflets of bilayers are not identical. 

• Head groups and lipid chains of PLs/SLs have distinct biophysical properties. 

• Provision of membrane curvature. 

• Facilitates association/integration of membrane proteins. 

• Provides correct localisation of signalling molecules. 

• Asymmetry has a cost - ie. mediated by protein energy dependent processes. 

• Disruption of asymmetry used in cell processes - endo/exocytosis and signalling. 

• Asymmetry mediated by ‘flippases’, ‘floppases’, and ‘scramblases’. 

Comment on intraleaflet asymmetry.

• Biomembranes contain distinct mini-domains. 

• Known as lipid rafts - controversial. 

• Rich in cholesterol and SLs - long acyl length. 

• Display a lipid ordered phase. 

• Dynamic, small (10-3000nm) and short-lived). 

• Frequently contain caveolae proteins - structural role?

• Often contain GPI-anchored proteins. 

• Advantage in localising proteins in related processes - ie. signalling.

What is the purpose of membrane protein aggregation?

• Aggregation is useful for function and processing. 

• Used in signalling pathways - co-localisation. 

• Useful for internalisation - lysosomal degradation. 

Lateral motion of proteins is not…

…random!

• Integral membrane protein lateral movements are described as:

  • Transient confinement by obstacle clustered. 

  • Transient confinement by the cytoskeleton (fences). 

  • Directed motion by direct attachment to the cytoskeleton. 

  • Free and random diffusion in the membrane plane. 

Outdated observations ^^ 

Updated model for biomembranes 🙂

Protein structure types?

How do the different protein structures form?

• Chain flexibility. 

• Properties of R-groups in amino acids. 

• Interaction with solvent. 

• Stability - energy driven. 

How are the different protein structures stabiliised?

• Covalent bonds - 1^o sequence only!

• Hydrogen bonds. 

• Ionic interactions. 

• Van der Waals interactions. 

• Hydrophobic effects. 

Comment on primary protein structure.

• Covalent backbone of the polypeptide; sequence of amino acids.

• Covalent bonds - strong, direction, atoms share electrons to have full electron shell (stability). 

• Primary protein structure - sequence of a chain of amino acids. 

• The C-N linkage in a peptide bond (covalent bond) takes on some double bond characteristics, shorter, restricts rotation. 

  • Cα, C, O, and N are planar. 

What forces act as ‘protein glue’?

1. Hydrogen bond = interaction between an electronegative atom and an electropositive hydrogen.

2. Ionic bond = electrostatic interaction between charged or dipolar species. 

3. Van der Waals = attractive or repulsive forces that arise from temporary fluctuations in electron cloud densities between ‘neighbour’ groups. 

4. Hydrophobic effect = a more favourable arrangement is an interaction between hydrophobic regions effectively shielding them from water. 

 

Comment on alpha helix structure.

• Characterised by 3.6 amino acid residues per turn of the helix and a 1.5Å rise along the helix per residue. 

• Held together by hydrogen bonds between the C=O and N-H groups of amino acids (between n and n+4). 

• R-groups are NOT involved in stabilisation of structure. 

Comment on beta sheet structure.

• 𝛽-sheets are composed of several 𝛽-strands, frequently 5-10 residues long. 

• The 𝛽-strands align into sheets such that hydrogen bonds are formed between the C=O and N-H groups of the peptide backbone. 

• Side chains of amino acids in 𝛽-strands are successively above and below the plane of the sheet. 

• 𝛽-sheets may be composed of strands that run parallel or antiparallel. 

What are protein loop regions?

• Peptide chains of various lengths required to link the α-helices and 𝛽-sheets of the polypeptide chain. 

• Surface localised loops are in intimate contact with the solvent environment. 

• Surface localised amino acids form H-bonds and electrostatic interactions with the solvent via the peptide backbone and side chains. 

• The sharp turns connecting anti-parallel 𝛽-sheets are usually composed of four amino acids which cause a 180^o turn. 

Tertiary structure of proteins?

• Definition = organisation of segments of secondary structure into defined motifs. 

  • Much can be deduced about a protein based on its AA sequence. 

• The tertiary motifs form structural or functional units of the protein. 

• Large proteins frequently comprise repeats of simple tertiary motifs. 

• Certain motifs display conserved AA residues. 

1. A structural state achieved without breaking covalent bonds. 

2. Achieved through the combination of many low energy interactions. 

   1. Hydrogen bonds. 

   2. Ionic interactions. 

   3. Van der Waals interactions (weak attractive or repulsive forces). 

   4. Hydrophobic forces (exclusion of water). 

3. Energetically stable. 

Tertiary protein structure 🙂

What are alpha domain structures?

• Composed of helix-loop-helix clusters in various repeats. 

•  Interacting helices are arranged so that side-chains of one helix slot into the groove of a neighbour. 

• These domains are held together by interactions (H-bonds, hydrophobic interactions etc.) between the side-chains of amino acids on the helices. 

What are beta domain structures?

• 𝛽-sheet arrangement is organised into barrels and saddles. 

• Both are generated since 𝛽-sheets to form a right-handed twist. 

• Barrel or saddle? Depends on the nature of H-bonding between strands:

  • Flat rectangular → saddles. 

  • Flat staggered → barrels. 

• The shape is formed from the sum of energies in the ‘battle’ between sheet twisting and stretching of hydrogen bonds. 

Comment on quaternary protein structure.

Definition = the assembly of multiple polypeptide chains (sub-units) into a complete functional protein unit. 

Function of membrane proteins? Provide broad definition.

• Broad definition = perceive a signal (or substrate) and translate that inside the cell. 

• Metabolism: receptors and transporters. 

  

Significance of ATP-ADP translocase?

• Each day we use our own body weight in ATP to fuel our cells. 

• ATP production mostly in mitochondria - need high cytosolic concentrations. 

• Inner mitochondrial membrane carrier protein - ATP-ADP translocase. 

• Comprises 10% of inner mitochondrial membrane protein. 

• 1:1 stoichiometric exchange of ADP_(in) and ATP_(out). 

• Electrogenic transport (ADP^3- vs. ATP^4-).

  • Transport is influenced by the difference in charge across membrane. 

What does ATP-ADP translocase look like?

• 6 transmembrane helices (labelled H1-H6). 

• Cytoplasmic open structure obtained with carboxyatractyloside (CAT). Matrix structure obtained with bongkrekic acid. 

• Helices form a barrel (conical). 

• Cavity - max diameter 20Å, depth of 30Å. 

• Bilayer thickness = 35-40Å. 

• Short α-helical connections between odd-even TM helices. 

• Connector helices are ‘parallel’ to bilayer. 

• Connector helices (interact with bilayer or protein?).

• Cavity lining is hydrophilic. 

• Lipid lining residues are hydrophobic. 

Structural characteristics of ATP-ADP translocase?

• Each monomer (6TM) has a 3x2TM arrangement. 

• Cationic cluster of arginines (R) and lysines (K) at ‘foot’ of the cavity. 

• A second cationic region at ‘entrance’ of cavity. 

• Charge and aromatic distribution within cavity is asymmetric. 

• Series of tyrosine residues in ‘centre’ of cavity. 

• Only binds adenine nucleotides not complexed with Mg²⁺.

• Cationic groups interact with phosphate moieties. 

• Tyrosine intercalation with adenosine moiety. 

• Thought to work as a dimer but structure proves that monomer can explain the transport via an alternating access model. 

• Conformational changes on binding force rearrangement of cavity. 

Comment on ATP-ADP translocase lipid binding.

• Cardiolipins bound to ATP-ADP translocase:

  • Structural stability - conformation change. 

  • Prevent misfolding/aggregation. 

  • Prevent oxidative damage. 

What is energy and how is it measured?

• Physical unit = Watt. 

  • 1 Watt = 1J/second (rate). 

• Also given energy content of food in kilocalories and kilojoules - describes how much energy is associated with the food sources. 

  • 1 kilocalorie = 4.18 kilojoule. 

• The currency of cellular energy is ATP. 

Why is ATP the currency of cellular energy?

• High phosphoryl transferase molecule. Facilitates transfer of energy into a system. 

• ATP is an inherently unstable molecule. Much more favorable to have ADP + P_i instead of ATP.

Comment on the favourability of ATP synthesis.

• Incredibly energetically unfavourable. 

  • 𝚫G = ~30kJ/mol (~7kcal/mol). 

  • 𝚫G (inside cell) = ~50kJ/mol. 

•  Where does this energy come from?

  • Glycolysis. 

  • Substrate level phosphorylation. 

• NB: cell is dead if ATP/ADP ratio = 1. Usually a ratio of 1000 in a cell. 

Comment on the favourability of making ATP in glycolysis.

• 1,3-BPG and PEP have high phosphoryl-transferase potential. 

  • Both molecules have a transferable phosphate group. More favourable to release/yield the phosphate group than retain it. 

What are the characteristics of the creatinine kinase system?

• Substrate level phosphorylation. 

• Used by muscles to maintain ATP levels for work. 

• Creatine phosphate is a high-energy compound. 

• Energy from its breakdown is used to ‘power’ ATP synthesis. 

• ATP levels (cytosol) maintained at 1.5-2mM. 

• Rapid response for ATP generation (creatine phosphate P-Cr). 

• Used (initially) during strenuous exercise. 

• P-Cr replenished during rest.

Characteristics of substrate-level phosphorylation and cellular usage?

• Intense muscle contraction uses substrate level phosphorylation. 

• Initial route is via phospho-creatine. 

• Glycolysis becomes the main provider of ATP. 

• Both systems are severely limited (ie. availability of PCr and NAD⁺). 

  • Limited because of reliance on precursor molecules. 

• Each high energy phosphate donor needs to be synthesised in the body. 

• Also an inefficient process - 1mol ATP per mol P_i donor. 

• We need lots of ATP!

  • Cells have devised complex, but highly efficient system with a large capacity (enables prolonged work - OxPhos). 

ATP synthesis requires what?

• High energy intermediates. 

• Transport of intermediates into the mitochondrial matrix. 

• Electron transport chain. 

• Proton transport - protein motive force.

• ATP synthase. 

• Translocase to deliver ATP to cytosol. 

Metabolite funnelling to mitochondria 🙂

• Cytosol:

  • Carbohydrates provide pyruvate (glycolysis). 

• Mitochondria:

  • Pyruvate transported into matrix - metabolised to acetyl-CoA. 

  • FAs enter mitochondria via carnitine shuttle system. 

  • 𝛽-oxidation occurs in matrix - produces acetyl-CoA. 

  • Entry to TCA cycle is via acetyl-CoA (catabolic oxidation). 

  • TCA ‘energy products’ are NADH and FADH₂. 

Most energy transductions involve flow of…

…electrons between molecules (Redox reactions). 

• Flow is ‘downward’ - ie. to molecules with greater affinity for the electrons. 

  • O₂ is the final acceptor in the ETC. reduced to H₂O and expelled through expiration. 

• NADH and FADH₂ can donate electrons to other electron acceptors. 

How do electrons move around?

• Electrons are incorporated into numerous biological molecules.

• NADH, NADPH, FADH₂. 

• Ubiquinones. 

• Transition metal compounds are useful (ie. Fe²⁺/Fe³⁺). 

• Within proteins - iron-sulphur clusters, porphyrins. 

Comment on nicotinamide cofactors as electron acceptors.

• Nicotinamide Adenine Dinucleotide. 

• NAD⁺ may accept 2 electrons (H^-). 

• Electrons passed over as a hydride ion. 

• Hydride is equivalent to a proton + 2e^-. 

• Nicotinamide ring accepts the electrons. 

• NADP⁺ is phosphorylated - indicated by the arrow. 

  • NADP⁺ is often involved in synthesis reactions. 

Comment on flavin cofactors as electron acceptors.

• Flavin Adenine Dinucleotide. 

• FAD may accept 2 electrons. 

• Accepts H^- and a proton (hence, H₂).

• Isoalloxazine ring accepts the electrons. 

Comment on ubiquinone as an electron acceptor.

• Co-enzyme Q or ubiquinone. 

• A quinone derivative with an isoprenoid tail. 

• Isoprenoid in mammals is n=10. 

• Uptake of a single electron produces a free-radical (unpaired e^-). 

• A second reduction produces ubiquinol. 

• Referred to as a membrane embedded electron carrier. 

Comment on porphyrin electron acceptors.

• Haem → contains organic and inorganic components. 

  • Also known as a prosthetic group (found in proteins).

• Organic component is protoporphyrin. 

• Protoporphyrin comprises four pyrole rings (5-membered containing a nitrogen). 

• Pyrole groups connected by methene bridges. 

• Inorganic component is iron (Fe). 

• Fe may exist in the ferrous (Fe²⁺) or ferric (Fe³⁺) states (capicty to accept and ∴ transfer electrons). 

• Fe is coordinated by the four nitrogen atoms of the pyrole rings. 

• Further coordination possible above and below plane of molecule. 

• Found in numerous proteins. 

  • Ie. myoglobin, cytochrome, endothelial nitric oxide synthase, catalase. 

Comment on iron-sulfur clusters as electron carriers.

• The clusters ‘trap’ iron within an active site of a protein. 

• The coordination of iron is achieved by cysteine residues (ie. -SH containing). 

• Iron atoms switch between ferrous or ferric states. 

• Many different conformations achievable. Cu can also do this too (sometimes).

Describe what happens in complex I of NADH:ubiquinone oxidoreductase.

• NADH oxidised to NAD⁺.

• 2e^- transferred to flavin mononucleotide (FMN) - prosthetic group (associated with a protein). 

• 2e^- transferred to iron-sulfur complexes (redox gradient) to be accepted by ubiquinone (becomes ubiquinol - Q). 

• The process of electron transfer induces conformational changes in the protein. Facilitates movement of electrons from matrix to intermembrane space. 

• Translocation of 4 protons. 

  • Establishes both charge and pH difference across the membrane. 

Topic 11 (oxidative phosphorylation) lecture 1 summary 🙂

• ATP contains high energy phosphate bonds. 

• High energy cost in synthesising ATP. 

• Substrate level phosphorylation - rapid, short-term, inefficient. 

• Oxidative phosphorylation is efficient and O₂ requiring. 

• OxPhos occurs in the mitochondria. 

• Reduced cofactors provide energetic electrons to the ETC. 

Energy current is associated with…

…flow of electrons. Ie. reduction potential E^o.

• Electrons pass to compounds with higher affinity. 

  • Ie. more positive E^o value. 

Electrons ‘roll’ towards…

…O₂.

Electron transport chain 🙂

What is complex 1 of the ETC?

NADH:ubiquinone oxidoreductase. 

What is complex 2 of the ETC?

Succinate dehydrogenase. 

What is complex 3 of the ETC?

Ubiquinol-cytochrome C oxidoreductase. 

What is complex 4 of the ETC?

Cytochrome C oxidase. 

ETC in drawn form 🙂

Describe what the ETC facilitates.

• The ETC converts energy from redox reactions to produce a proton gradient. 

• The ETC comprises integral and peripheral membrane proteins at the inner mitochondrial membrane. 

• Four membrane bound multiprotein complexes (I-IV). 

• Complexes I, III, IV transport H⁺ into the intermembrane space.

• NADH donates electrons to complex I. 

• Complex II is part of the TCA cycle and does not transport H⁺.

• Electrons enter complex II from succinate to FAD.

• Mobile, membranous ‘carriers’ move electrons between complexes.

• Ubiquinone (UQ) moves electrons from complexes I and II to III.

• Cytochrome C moves electrons from complex III to IV. 

Comment on oxidative phosphorylation (in brief).

• Reduction of O₂ to H₂O using electrons from NADH to FADH₂. 

• Occurs on the inner mitochondrial membrane. 

• Explained by the chemi-osmotic theory.

• Involves a series of ‘downhill’ electron transfers (ie. exergonic). 

• Energy used to drive protons through the inner membrane. 

• Transmembrane chemical + electrical difference of the proton gradient drives ATP synthesis. 

Proton motive force 🙂

Comment on the proof of the proton motive force theory.

• F₁F₀-ATPase was known to be crucial in the process.

•  Enzyme purified and reconstituted. 

• Co-reconstitution with bacteriorhodopsin (BR).

• BR is a light driven proton pump. 

• Addition of ADP+P_i resulted in no ATP synthesis. 

• Addition of light+ADP+P_i resulted in ATP synthesis.

• Proton gradient required for ATP synthesis.

What is a common uncoupling agent?

2,4-dinitrophenol (DNP).

Give the characteristics of F₁F₀-ATPase.

• Multiple subunit protein (heteromeric). 

• Nanomachine. 

• Found in all mitochondria. 

• May be isolated into membrane and matrix components. 

• Stalks of mitochondria. 

What occurs in ATP-synthase (F₁F₀-ATPase)?

• Tight coupling of ATP synthesis to proton movement (activity of the ETC).

• ATP synthase converts proton motion into rotation. 

Energetics of ATP synthesis 🙂

The binding change mechanism of F₁F₀-ATPase 🙂

Comment on the notion of ATP synthesis being efficient but poorly located.

• ATP synthesis → mitochondrial matrix. 

• ATP requirement → cytosolic (main). 

• But ATP does not cross membranes. 

  • ATP synthesis requires ADP in matrix. 

• Transport protein exchanges ATP and ADP.

  • Aka adenine nucleotide translocase. 

  • NB: changes on ATP/ADP. Exchange is exergonic. Charge due to H⁺ is dissipated. 

Comment on the process of NADH synthesis in the cytosol.

NADH generated during glycolysis. 

• MHD = malate dehydrogenase / AST = aspartate transaminase. 

• 1 malate, 2 oxaloacetate, 3 aspartate, 4 glutamate, 5 2-oxoglutarate. 

Overview of processes leading to ATP synthesis 🙂

What and when of metabolism 🙂

Oxidative phosphorylation - lecture summary 🙂

• ETC passes electrons along energy gradient. 

• Passage of electrons produces proton gradient.

• ATP synthase uses chemical and osmotic gradient. 

• Uncouplers demonstrate the link between proton gradient and ATP synthesis. 

• ATP-synthase relies on subunit rotation. 

• Nucleotide binding sites exist in multiple conformations. 

• The ‘binding change’ mechanism underlies ATP synthesis.

• ADP entry into mitochondria sets the pace of OxPhos. 

• The ATP-ADP translocase dissipates part of the proton gradient. 

• Process of ATP generation depends on nature of exertion. 

Comment on the fate of amino acids in the broader context of metabolic pathways.

• Amino acids stored in protein. No distinction between stored protein and protein being used. 

• Amino acids eventually join TCA cycle, or become acetyl-CoA, or become pyruvate.

• Amino acids contain nitrogen → no TCA intermediates contain amines. Needs to be removed via urea cycle (first converted to ammonia). 

Pellagra-like symptoms in Hartnup disorder 🙂

• 50% NADH from tryptophan, 50% from nicotinic acid. 

• Need supplementation of nicotinic acid (vitamin). 

Protein digestion schematic 🙂

• Pepsin functions at very acidic pH. 

• Protein fragments enter intestine → digested by pancreatic enzymes. 

• Oligopeptides(2-10AA) encounter brush border enzymes → digested into AAs, dipeptides, tripeptides. 

  • Then taken up by transporters. 

• ACE = amino converting enzyme. 

What is an important feature of chymotrypsin?

It is a catalytic triad - gaps in peptide → held together by disulphide bridges. 

• Three critical residues (dispersed): histidine, aspartate, serine. 

Chymotrypsin action 🙂

• Serine becomes nucleophilic in step 1. 

• Leads to covalent intermediate formation. 

• Splitting of peptide chain → release. 

• Addition of water for hydrolysis. 

• Second tetrahedral intermediate forms. 

• Second part of peptide released. 

• Catalytic chain restored. 

• Readily accept another peptide. 

Interorgan transport of amino acids 😉

• Every villus in intestine has blood vessel. Allows for transportation of AAs to liver via portal vein. 

Comment on interorgan transport of amino acids.

• Protein fragments that aren’t metabolised by liver pass into bloodstream as AAs. 

• Thereafter can go to brain, muscles, kidney. 

• All AAs eventually end up in kidneys where they are reabsorbed and transported back to liver via bloodstream. 

Organisation of the nephron 🙂

Describe characteristics of reabsorption of metabolites along the nephron.

• Very rapid reabsorption of metabolites. Hence why kidneys require so much energy per day. 

• Urine is depleted of nutrients prior to excretion. 

Are lipid membranes permeable to amino acids?

No. Rapid transport of amino acid through lipid membrane requires a transporter.

What are two classes of transporters?

• Primary active.

• Secondary active.

• Primary active transporters are driven by the direct hydrolysis of ATP. 

• Secondary active transporters follow concentration gradients of substrates.

• Topological reaction.

Three common types of transporters?

• Uniporters → typically secondary passive.

• Symporters → secondary active.

• Anitporters → secondary active.

What biological example involves all three transporter types?

AA/Na⁺ pump.

• All three transporter types. 

  • One primary. 

  • Two secondary. 

• AA and Na⁺ enter cell via symporter. 

Transporters are…

…enzymes.

• Transporters have very similar (if not identical) principles to enzymes. 

Summary of transporters 🙂

• Transporters are similar to enzymes, but bind the substrate (not a transition state). 

• They do not convert the substrate but translocate it.

• The Na⁺ gradient maintained by the Na,K-ATPase is used to drive absorption of nutrients. 

Disorders of epithelial transport?

• Cystinuria = high concentrations of arginine, lysine, cystine in the urine (kidney stones). 

• Hartnup disorder = high concentrations of neutral amino acids in the urine (pellagra-like symptoms). 

• Iminoglycinuria = high concentrations of glycine and proline in the urine. 

• Dicarboxylic aminoaciduria = high concentrations of glutamate and aspartate in the urine. 

• Lysinuric protein intolerance = large amounts of lysine in the urine after protein-rich diet (hyperammonemia, osteoporosis, sparse hair, enlarged liver and spleen). 

Amino acid transport in epithelial cells 🙂

What is the minimum protein intake per day?

~32g/day.

• Only excess amino acid gets metabolised. 

Amino acid metabolism overview 🙂

• Urea is a true waste product. 

• Ammonia is very toxic for humans. 

Amino acids are similar to…

…central metabolites.

• Note the similarity in the functional groups. Also note the disparity of keto groups and amine groups. 

How is nitrogen disposed of in amino acid metabolism?

As ammonia, via transamination.

What is the first step in amino acid metabolism (via transaminase), starting with glutamic acid or another amino acid?

Combining with pyruvic acid to form alpha-ketoglutaric acid and alanine. Reaction is catalysed by alanine transaminase (ALT).