Anti-CCP antibodies are more specific than RF for diagnosing rheumatoid arthritis and may better predict erosivedisease.

Smoking is a risk factor for RA, especially RF+ RA men and heavy smokers.

We developed a mass spectrometric method to determine the p K a values of individual histidine residues in proteins. The method is based on the fact that the imidazole C 2-proton undergoes pH-dependent hydrogen-deuterium exchange reaction, of which the rate constant ( k phi) reflects the p K a for the ionization of imidazole to imidazolium. The experimental procedure consists of the following: (1) protein incubation in D 2O solvent at various pH values, (2) protein digestion by proteolytic enzyme(s), during which all the rapidly exchanging deuterons such as those in amide and hydroxyl groups are back-exchanged for protons, and (3) measurement of the mass spectrum of each histidine-containing peptide by LC/ESI-MS. The k phi of the H-D exchange reaction is obtained from the mass spectrum reflecting the extent of deuterium incorporation. The p K a value is then determined from a plot of k phi versus pH, which gives a typical sigmoidal curve. Unambiguous assignment of the p K a values to individual histidine residues can be achieved simultaneously based on the observed molecular mass of the peptide. The p K a values of three of four histidine residues (His12, -105, and -119) in RNase A were successfully determined by this method and were in good agreement with those determined by (1)H NMR and hydrogen-tritium exchange methods. The method uses subnanomole quantities of protein, allowing measurement at a much lower concentration than that of 1 mM required for the conventional NMR approach that is currently almost exclusively the method of choice.

Differential scanning calorimetry (DSC) analyses of a series of collagen model peptides suggest that 4-hydroxyproline (Hyp) and 4-fluoroproline (fPro) have different effects on the stability of the collagen triple helices according to the sequence of amino acids and stereochemistry at the 4 positions of these imino acids. The thermodynamic parameters indicate that the enhanced stabilities are classified into two different types: the enthalpy term is primarily responsible for the enhanced stability of the triple helix of (Pro-Hyp(R)-Gly)(10), whereas the entropy term dominates the enhanced stability of (Pro-fPro(R)-Gly)(10). The difference between the molecular volumes observed in solution and intrinsic molecular volumes calculated from the crystal structure indicates the different hydration states of these peptides. (Pro-Hyp(R)-Gly)(10) is highly hydrated compared to (Pro-Pro-Gly)(10), which contributes to the larger enthalpy. In contrast, the volume of (Pro-fPro(R)-Gly)(10) shows a smaller degree of hydration than that of (Pro-Pro-Gly)(10). The entropic cost of forming the triple helix of the fPro-containing peptides is compensated by a decrease in an ordered structure of water molecules surrounding the peptide molecule, although the contribution of enthalpy originating from the hydration is reduced. These arguments about the different contribution of entropic and enthalpic terms were successfully applied to interpret the stability of the triple helix of (fPro(S)-Pro-Gly)(10) as well.

X-ray analysis has been carried out on a crystal of the collagen model peptide (Hyp(R)-Hyp(R)-Gly)10 [where Hyp(R) is 4(R)-hydroxyproline] with 1.5 A resolution. The triple-helical structure of (Hyp(R)-Hyp(R)-Gly)10 has the same helical parameters and Rich and Crick II hydrogen bond patterns as those of other collagen model peptides. However, our full-length crystal structure revealed that almost all consecutive Hyp(R) residues take the up-up pucker in contrast to putative down-up puckering propensities of other collagen model peptides. The unique feature of thermodynamic parameters associated with the conformational transition of this peptide from triple helix to single coil is that both enthalpy and entropy changes of the transition are much smaller than those of other model peptides such as (Pro-Pro-Gly)10 and (Pro-Hyp(R)-Gly)10. To corroborate the precise structural information including main- and side-chain dihedral angles and intra- and interwater bridge networks, we estimated the degrees of hydration by comparing molecular volumes observed experimentally in solution to those calculated ones from the crystal structure. The results showed that the degree of hydration of (Hyp(R)-Hyp(R)-Gly)10 is comparable to that of (Pro-Hyp(R)-Gly)10 in the triple-helical state, but the former was more highly hydrated than (Pro-Hyp(R)-Gly)10 in the single-coil state. Because hydration reduces the enthalpy due to the formation of a hydrogen bond with a water molecule and diminishes the entropy due to the restriction of water molecules surrounding a peptide molecule, we concluded that the high thermal stability of (Hyp(R)-Hyp(R)-Gly)10 is able to be described by its high hydration in the single-coil state.

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. These reconstitute the water bridge. Based on these features, we suggest here a catalytic mechanism for hCAII: the tautomerization of His 64 can mediate the transfers of both protons and water molecules at a neutral pH with high efficiency, requiring no time-or energy-consuming processes.Carbonic anhydrase (CA) 2 (EC 4.2.1.1) is a ubiquitous enzyme that catalyzes the reversible hydration of carbon dioxide (1). Isozymes of carbonic anhydrase regulate or function in such diverse physiological processes as pH regulation, ion transport, water-electrolyte balance, bicarbonate secretion-absorption, bone resorption, maintenance of intraocular pressure, renal acidification, and brain development (2). Nonfunctioning CA is implicated in such diseases as osteopetrosis syndrome, glaucoma, respiratory acidosis, epilepsy, and Méni-ère syndrome. Diseases due to CA deficiency include those affecting bones, the brain, and the kidneys. Consequently determining the detailed structure/function relationships or mechanisms responsible for its catalytic properties is mandatory for developing inhibitors or replacement therapies.CA is present in at least three gene families (␣, ␤, and ␥), which has made it a popular model for the study of the evolution of gene families and protein folding, and for transgenic and gene target studies (2). Among the three families, the ␣ family is the best characterized, with 11 known isozymes identified in mammals. Earnhardt and co-workers have summarized maximal k cat and k cat /K m values for CO 2 hydration by isozyme I-VII (3). The human isozyme II (hCAII) has a remarkably high turnover rate or catalytic efficiency (k cat /K m ϭ 1.5 ϫ 10 8 M Ϫ1 s Ϫ1 ) that is very close to the frequency with which the enzyme and substrate molecules collide with each other in solution.It is widely accepted that the hydration of CO 2 catalyzed by hCAII proceeds through several chemical steps as shown in Scheme 1 (1, 4, 5): the direct nucleophilic attack of the zinc-bound hydroxide ion on the carbonyl carbon of substrate CO 2 (structures 1-2), the formation of a zinc-bound bicarbonate intermediate (structures 2-3), the isomerization of the bicarbonate ion (structures 3-4), the exchange of the product bicarbonate ion with a H 2 O (structures 4 -5), and the regeneration of the zinc-bound hydroxide ion by the transfer of a proton to bulk solvent (structures 1-5). The proton transfer step (structures 1-5) consists of two substeps: 1) an intra-molecular transfer of protons to another residue in the enzyme and 2) a release of protons to the outside of the enzyme with the aid of a base. The intra-molecular proton transfer is the rate-limiting step of the maximal turnover rate (10 6 s Ϫ1 ) at high concentrations of a base, whereas the proton release into the medium is rate-limiting at low buffer concentrations.

A high-throughput method for sequencing of N termini of proteins by using postsource decay (PSD) of matrix-assisted laser desorption/ionization mass spectrometry has been developed. After a protein blotted on the PVDF membrane was successively reduced, S-alkylated, and guanidinated, its N-amino group was coupled to biotinylcysteic acid. The protein was then extracted from the membrane and digested with trypsin. The derivatized N-terminal fragment was then specifically isolated from the tryptic digest with avidin resins, and its de novo sequencing was successfully performed by PSD utilizing a sulfonic acid group introduced to the N terminus.