CHEM 30B   Dr. R. Rinehart

CHAPTERS 23 & 22.7-22.9: 
LIPID & AMINO ACID METABOLISM

 I.  Fat digestion, absorption, and transport 
            A.  Digestion: requires emulsification by bile salts and hydrolysis by pancreatic lipase
            B.  Products: monoglycerides, diglycerides, fatty acids, glycerol
            C.  Absorbed as mixed micelles; triglycerides resynthesized in mucosal cells and packaged
                    as chylomicra; released into lymphatic circulation; eventually enter blood;
                     blood levels of TG peak ~4 hr after meal, remain elevated > 10 hrs        

D.  Plasma lipoproteins: classified by behavior when centrifuged [also by electrophoresis]                       

             E.  Fat mobilization: epinephrine and other hormones stimulate  “hormone-sensitive lipase” in
                        adipose tissue; the resulting glycerol and “free” (unesterified) fatty acids enter the blood,
                        where the fatty acids are carried by serum albumin to muscle, heart, liver, kidney, etc.
                        Glycerol taken up by cells is converted to DHAP, which can then be catabolized by
                        glycolysis or used to yield glucose via gluconeogenesis. 

II.  Fatty acid oxidation [fats yield 9 kcal/g] 
            A.  Activation: further metabolism requires attachment to CoA -- this process
                        requires the equivalent of 2 ATP per fatty acid. This is a cytoplasmic reaction, but further
                        degradation of the fatty acid occurs in the mitochondrial matrix.  Transport of the fatty
                        acyl group across the mitochondrial inner membrane involves a carrier called carnitine,
                         a derivative of lysine, and two membrane-bound molecules of carnitine acyltransferase. 

            B.  Beta-oxidation is the major catabolic pathway for fatty acids. It is a pseudo-cyclic or “spiral”
                        pathway in which a sequence of four reactions is repeated until the entire molecule has
                         been split into acetyl-CoA. The pathway occurs in the mitochondrial matrix 

1.  Dehydrogenation of RCH₂CH₂CH₂ CO~SCoA with FAD to give FADH₂ and
            RCH₂CH=CHCO~SCoA  (a,b-unsaturated fatty acyl CoA). This reaction is
            analogous to the succinate à fumarate reaction of the Krebs cycle. 

2.  Hydration of the a,b double bond to give a b-hydroxy fatty acyl CoA
            RCH₂CH=CHCO~SCoA  + H₂O  à  RCH₂CHOHCH₂CO~SCoA. This reaction
            is analogous to the fumarate à malate reaction of the Krebs cycle. 

3.  Dehydrogenation of the b 2^o alcohol  with to give a b-ketoacyl CoA
          RCH₂CHOHCH₂CO~SCoA +  NAD⁺  à   RCH₂COCH₂CO~SCoA +  NADH + H⁺
            Analogous to malate à oxaloacetate. 

4.  Cleavage of the first two carbons with HSCoA, yielding acetyl-CoA and a shorter 
        fatty acyl CoA ready to go through the cycle again.
        RCH₂COCH₂CO~SCoA + HSCoA  à  RCH₂CO~SCoA +  CH₃CO~SCoA 

 The reactions of b-oxidation. Each cycle shortens the chain by 2C
Initial activation: RCO₂H + HSCoA + ATP à RCOSCoA + AMP + PP_i
   pyrophosphatase hydrolyzes PP_i to 2 P_i, so net energy loss for this reaction is 2 ~Ⓟ
In the last cycle, butyryl-CoA à 2 AcCoA. Hence, C_2n à n AcCoA in (n-1) cycles
Each AcCoA gives 12 ATP from Krebs cycle and oxidative phosphorylation
Each cycle yields 1 NADH and 1 FADH₂  or 5 ATP/cycle

overall process including Krebs cycle and oxidative phosphorylation:
C_2nH_4nO₂  +   (3n-1) O₂ à 2n CO₂ + 2n H₂O , with net formation of (17n-7) ATP

                        5.  For a typical fatty acid containing 2n carbon atoms:
                                    n molecules of AcCoA are produced; 12n ATP after they go through the Krebs cycle;
                                    (n-1) cycles are required; each cycle produces one FADH₂ and one NADH
                                    FADH₂  yields 2 ATP and NADH yields 3 ATP  after electron transport
                               The net energy yield for our C_2n fatty acid is thus:
                                    12n  [AcCoA/Krebs+ETS]  +  5(n-1)  [ETS per cycle]   -  2 [activation rxn]
                                    or   17n – 7  ATP  [other books using lower ATP from ETS may state this as 14n-6] 

            C.  Ketogenesis:  under conditions of starvation, diabetes mellitus, or carbohydrate deprivation,
                        b-oxidation and amino acid oxidation increase.  In the liver, excess AcCoA  is converted
                        to acetoacetate,  a b-ketoacid which can be reduced with NADH to b-hydroxybutyrate;
                        Both of these molecules are water-soluble fuels for aerobic tissues (brain, heart, muscle)
                        However, they are also acids, and overproduction, as in diabetes, leads to “ketoacidosis”.
                        Excess acetoacetate gives rise to acetone via spontaneous decarboxylation; it is volatile
                        enough to be expelled by the lungs and can be detected in the exhalation of uncontrolled
                        diabetics. The three compounds (acetoacetate, b-hydroxybutyrate, acetone) are 
                        collectively known as  “ketone bodies” . 
                                    [KB] > 20 mg%  =  ketonemia;   > 70 mg% produces ketonuria
                                    ketosis  =  ketonemia + ketonuria + acetone breath
                                    ketoacidosis = ketosis with a plasma pH below 7.35; causes loss of minerals,
                                                dehydration, coma, and death. 

            D.  Ethanol metabolism [I should have covered this when we did aerobic metabolism].
                        An aerobic process, occurs in the liver; net energy yield: 16 ATP/mole EtOH, 7 kcal/g

                    Ethanol catabolism increases the cellular NADH/NAD⁺ ratio, thus inhibiting glycolysis and
                     b-oxidation. As little as 6 weeks of heavy drinking causes fatty infiltration of the liver, which is
                     reversible in the early stages, but which can progress to irreversible cirrhosis of the liver. 
                    Prolonged heavy drinking also induces a "microsomal ethanol-oxidizing system" [MEOS],
                    which, although it does lead to an increased capacity to metabolize ethanol, causes other
                    alterations in liver metabolism that are also deleterious.

III.  Fatty acid synthesis: occurs in adipose tissue and liver when  carbohydrates are plentiful.
            A.  Occurs in cytosol; uses a multienzyme fatty acid synthase complex with acyl carrier protein
            B.  2C from AcCoA leave mitochondrial matrix as citrate and AcCoA is regenerated in cytosol
            C.  Energy from ATP and reducing power from NADPH (a modified form of NADH produced in
                     the “pentose pathway”) and the vitamin biotin and CO₂ are also required
            D.  The actual reactions look pretty much like a reversal of  b-oxidation
            E.  Newly-synthesized fatty acids are converted to triglycerides. 

IV.   Amino acid metabolism 
            A.   Protein digestion and absorption
                        stomach: pepsin;  intestine: trypsin, chymotrypsin, carboxypeptidases, aminopeptidases,
                          elastase, di- and tri-peptidases. Absorption is a complex process requiring a Na⁺ gradient and
                        an additional 3 ATP/AA;  as a result, the body will net 2.8 kcal/g instead of the theoretical 4-4.5 kcal/g

            B.  Amino acid pool: from diet, recovery of AA from degraded proteins, and synthesis (nonessential)
                          ~75% of pool used for protein synthesis;  protein turnover:  continuous synthesis &
                        hydrolysis of body proteins; different proteins have different turnover rates.

            C.  Specialized products from amino acids:
                        TYR:  dopamine, norepinephrine, epinephrine, thyroxine, melanin
                        TRP:   serotonin, melatonin
                        HIS:   histamine
                        SER:  ethanolamine, cephalins
                        CYS:  taurine, b-mercaptoethylamine in HSCoA
                        LYS:  carnitine
                        GLY, TRP, MET, ARG, etc:  
                                    feed the “one-carbon pool” used in all sorts of processes including
                                    synthesis of  thymine, choline, carnitine, creatine, ……
                        GLU:  GABA, a neurotransmitter
                        GLN:  NH₃ for urine buffering, N donor for purine & pyrimidine synthesis
                        ASP:   converted to NMDA, a neurotransmitter

            D.   Catabolism: 20 AA with >1 pathway each; we’ll just hit the high points;  first, remove the
                        a-amino group by transamination and deamination and convert the nitrogen to urea;
                        then, transform the carbon skeleton to useful goodies like Krebs intermediates, glucose
                        and ketone bodies. Most steps occur in mitochondrial matrix of the liver

            E.   Transamination:  requires pyridoxal phosphate (a vit. B₆ derivative) and a-KG or OAA (from
                        the Krebs cycle), yielding an a-keto carbon skeleton and GLU or ASP.

            E.   (Oxidative) deamination:  GLU + NAD⁺  à   aKG + NADH +  NH₄⁺

            F.   The urea cycle converts CO₂, NH₄⁺ and the a-NH₂ group of ASP to urea, a neutral water-
                    soluble molecule excreted in the urine. Net energy requirement 4 ATP / urea made
                        Works in conjunction with the Krebs cycle

            G.  Carbon skeleton fates:
                        1.   If catabolism yields AcCoA and/or AcAcCoA, the amino acid is ketogenic

             ILE, LEU*, TRP, LYS, PHE, TYR are ketogenic  [* = exclusively ketogenic]

2.  If  catabolism yields Krebs intermediates and/ or pyruvate or other C₃ compounds,
            the amino acid is glucogenic [† = exclusively glucogenic]

            ALA†, GLY†, CYS†, SER†, THR, TRP, ASP†, ASN†, GLU†, GLN†, HIS†,
         PRO†, ARG†, ILE, MET†, VAL†, PHE, TYR, LYS are glucogenic

            H.  Biosynthesis of nonessential amino acids: usually glycolysis intermediates or Krebs intermediates
                        provide the C-skeleton and transamination delivers the N;  PHE à  TYR; defect in this
                        process causes phenylketonuria (PKU), a very serious inborn error of metabolism.

            I.   There are plenty of other, but much rarer, hereditary disorders of amino acid metabolism that
                        your book doesn’t mention!

 © Ronald W. Rinehart, 2002-2007  Structures drawn with MDL IsisDraw^®